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Ding Z, Sun C, Yi SM, Gennis RB, Dikanov SA. The Ubiquinol Binding Site of Cytochrome bo3 from Escherichia coli Accommodates Menaquinone and Stabilizes a Functional Menasemiquinone. Biochemistry 2019; 58:4559-4569. [PMID: 31644263 DOI: 10.1021/acs.biochem.9b00750] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Cytochrome bo3, one of three terminal oxygen reductases in the aerobic respiratory chain of Escherichia coli, has been well characterized as a ubiquinol oxidase. The ability of cytochrome bo3 to catalyze the two-electron oxidation of ubiquinol-8 requires the enzyme to stabilize the one-electron oxidized ubisemiquinone species that is a transient intermediate in the reaction. Cytochrome bo3 has been shown recently to also utilize demethylmenaquinol-8 as a substrate that, along with menaquinol-8, replaces ubiquinol-8 when E. coli is grown under microaerobic or anaerobic conditions. In this work, we show that its steady-state turnover with 2,3-dimethyl-1,4-naphthoquinol, a water-soluble menaquinol analogue, is just as efficient as with ubiquinol-1. Using pulsed electron paramagnetic resonance spectroscopy, we demonstrate that the same residues in cytochrome bo3 that stabilize the semiquinone state of ubiquinone also stabilize the semiquinone state of menaquinone, with the hydrogen bond strengths and the distribution of unpaired spin density accommodated for the different substrate. Catalytic function with menaquinol is more tolerant of mutations at the active site than with ubiquinol. A mutation of one of the stabilizing residues (R71H in subunit I) that eliminates the ubiquinol oxidase activity of cytochrome bo3 does not abolish activity with soluble menaquinol analogues.
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Affiliation(s)
- Ziqiao Ding
- Department of Biochemistry , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
| | - Chang Sun
- Department of Biochemistry , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
| | - Sophia M Yi
- Department of Biochemistry , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
| | - Robert B Gennis
- Department of Biochemistry , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States.,Center for Biophysics and Computational Biology , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
| | - Sergei A Dikanov
- Department of Veterinary Clinical Medicine , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
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2
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Choi SK, Schurig-Briccio L, Ding Z, Hong S, Sun C, Gennis RB. Location of the Substrate Binding Site of the Cytochrome bo 3 Ubiquinol Oxidase from Escherichia coli. J Am Chem Soc 2017; 139:8346-8354. [PMID: 28538096 DOI: 10.1021/jacs.7b03883] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Cytochrome bo3 is a respiratory proton-pumping oxygen reductase that is a member of the heme-copper superfamily that utilizes ubiquinol-8 (Q8H2) as a substrate. The current consensus model has Q8H2 oxidized at a low affinity site (QL), passing electrons to a tightly bound quinone cofactor at a high affinity site (QH site) that stabilizes the one-electron reduced ubisemiquinone, facilitating the transfer of electrons to the redox active metal centers where O2 is reduced to water. The current work shows that the Q8 bound to the QH site is more dynamic than previously thought. In addition, mutations of residues at the QH site that do not abolish activity have been re-examined and shown to have properties expected of mutations at the substrate binding site (QL): an increase in the KM of the substrate ubiquinol-1 (up to 4-fold) and an increase in the apparent Ki of the inhibitor HQNO (up to 8-fold). The data suggest that there is only one binding site for ubiquinol in cyt bo3 and that site corresponds to the QH site.
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Affiliation(s)
- Sylvia K Choi
- Center for Biophysics and Quantitative Biology, University of Illinois , Urbana, Illinois 61801, United States.,Department of Biochemistry, University of Illinois , Urbana, Illinois 61801, United States
| | - Lici Schurig-Briccio
- Department of Biochemistry, University of Illinois , Urbana, Illinois 61801, United States
| | - Ziqiao Ding
- Department of Biochemistry, University of Illinois , Urbana, Illinois 61801, United States
| | - Sangjin Hong
- Department of Biochemistry, University of Illinois , Urbana, Illinois 61801, United States
| | - Chang Sun
- Department of Biochemistry, University of Illinois , Urbana, Illinois 61801, United States
| | - Robert B Gennis
- Center for Biophysics and Quantitative Biology, University of Illinois , Urbana, Illinois 61801, United States.,Department of Biochemistry, University of Illinois , Urbana, Illinois 61801, United States
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3
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Choi SK, Lin MT, Ouyang H, Gennis RB. Searching for the low affinity ubiquinone binding site in cytochrome bo 3 from Escherichia coli. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2017; 1858:366-370. [PMID: 28235459 DOI: 10.1016/j.bbabio.2017.02.008] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/12/2016] [Revised: 02/11/2017] [Accepted: 02/14/2017] [Indexed: 11/25/2022]
Abstract
The cytochrome bo3 ubiquinol oxidase is one of three respiratory oxygen reductases in the aerobic respiratory chain of Escherichia coli. The generally accepted model of catalysis assumes that cyt bo3 contains two distinct ubiquinol binding sites: (i) a low affinity (QL) site which is the traditional substrate binding site; and (ii) a high affinity (QH) site where a "permanently" bound quinone acts as a cofactor, taking two electrons from the substrate quinol and passing them one-by-one to the heme b component of the enzyme which, in turn, transfers them to the heme o3/CuB active site. Whereas the residues at the QH site are well defined, the location of the QL site remains unknown. The published X-ray structure does not contain quinone, and substantial amounts of the protein are missing as well. A recent bioinformatics study by Bossis et al. [Biochem J. (2014) 461, 305-314] identified a sequence motif G163EFX3GWX2Y173 as the likely QL site in the family of related quinol oxidases. In the current work, this was tested by site-directed mutagenesis. The results show that these residues are not important for catalytic function and do not define the QL substrate binding site.
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Affiliation(s)
- Sylvia K Choi
- Center for Biophysics and Quantitative Biology, University of Illinois, Urbana, IL 61801, USA
| | - Myat T Lin
- Center for Biophysics and Quantitative Biology, University of Illinois, Urbana, IL 61801, USA
| | - Hanlin Ouyang
- Department of Chemistry, University of Illinois, Urbana, IL 61801, USA
| | - Robert B Gennis
- Center for Biophysics and Quantitative Biology, University of Illinois, Urbana, IL 61801, USA; Department of Chemistry, University of Illinois, Urbana, IL 61801, USA; Department of Biochemistry, University of Illinois, Urbana, IL 61801, USA.
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4
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Sun C, Taguchi AT, Vermaas JV, Beal NJ, O'Malley PJ, Tajkhorshid E, Gennis RB, Dikanov SA. Q-Band Electron-Nuclear Double Resonance Reveals Out-of-Plane Hydrogen Bonds Stabilize an Anionic Ubisemiquinone in Cytochrome bo 3 from Escherichia coli. Biochemistry 2016; 55:5714-5725. [PMID: 27622672 DOI: 10.1021/acs.biochem.6b00669] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The respiratory cytochrome bo3 ubiquinol oxidase from Escherichia coli has a high-affinity ubiquinone binding site that stabilizes the one-electron reduced ubisemiquinone (SQH), which is a transient intermediate during the electron-mediated reduction of O2 to water. It is known that SQH is stabilized by two strong hydrogen bonds from R71 and D75 to ubiquinone carbonyl oxygen O1 and weak hydrogen bonds from H98 and Q101 to O4. In this work, SQH was investigated with orientation-selective Q-band (∼34 GHz) pulsed 1H electron-nuclear double resonance (ENDOR) spectroscopy on fully deuterated cytochrome (cyt) bo3 in a H2O solvent so that only exchangeable protons contribute to the observed ENDOR spectra. Simulations of the experimental ENDOR spectra provided the principal values and directions of the hyperfine (hfi) tensors for the two strongly coupled H-bond protons (H1 and H2). For H1, the largest principal component of the proton anisotropic hfi tensor Tz' = 11.8 MHz, whereas for H2, Tz' = 8.6 MHz. Remarkably, the data show that the direction of the H1 H-bond is nearly perpendicular to the quinone plane (∼70° out of plane). The orientation of the second strong hydrogen bond, H2, is out of plane by ∼25°. Equilibrium molecular dynamics simulations on a membrane-embedded model of the cyt bo3 QH site show that these H-bond orientations are plausible but do not distinguish which H-bond, from R71 or D75, is nearly perpendicular to the quinone ring. Density functional theory calculations support the idea that the distances and geometries of the H-bonds to the ubiquinone carbonyl oxygens, along with the measured proton anisotropic hfi couplings, are most compatible with an anionic (deprotonated) ubisemiquinone.
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Affiliation(s)
- Chang Sun
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Alexander T Taguchi
- Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Josh V Vermaas
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Nathan J Beal
- School of Chemistry, The University of Manchester , Manchester M13 9PL, U.K
| | - Patrick J O'Malley
- School of Chemistry, The University of Manchester , Manchester M13 9PL, U.K
| | - Emad Tajkhorshid
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Beckman Institute, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Robert B Gennis
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Sergei A Dikanov
- Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
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5
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Yi SM, Taguchi AT, Samoilova RI, O'Malley PJ, Gennis RB, Dikanov SA. Plasticity in the High Affinity Menaquinone Binding Site of the Cytochrome aa3-600 Menaquinol Oxidase from Bacillus subtilis. Biochemistry 2015. [PMID: 26196462 DOI: 10.1021/acs.biochem.5b00528] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Cytochrome aa3-600 is a terminal oxidase in the electron transport pathway that contributes to the electrochemical membrane potential by actively pumping protons. A notable feature of this enzyme complex is that it uses menaquinol as its electron donor instead of cytochrome c when it reduces dioxygen to water. The enzyme stabilizes a menasemiquinone radical (SQ) at a high affinity site that is important for catalysis. One of the residues that interacts with the semiquinone is Arg70. We have made the R70H mutant and have characterized the menasemiquinone radical by advanced X- and Q-band EPR. The bound SQ of the R70H mutant exhibits a strong isotropic hyperfine coupling (a(14)N ≈ 2.0 MHz) with a hydrogen bonded nitrogen. This nitrogen originates from a histidine side chain, based on its quadrupole coupling constant, e(2)qQ/h = 1.44 MHz, typical for protonated imidazole nitrogens. In the wild-type cyt aa3-600, the SQ is instead hydrogen bonded with Nε from the Arg70 side chain. Analysis of the (1)H 2D electron spin echo envelope modulation (ESEEM) spectra shows that the mutation also changes the number and strength of the hydrogen bonds between the SQ and the surrounding protein. Despite the alterations in the immediate environment of the SQ, the R70H mutant remains catalytically active. These findings are in contrast to the equivalent mutation in the close homologue, cytochrome bo3 ubiquinol oxidase from Escherichia coli, where the R71H mutation eliminates function.
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Affiliation(s)
- Sophia M Yi
- §Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
| | - Alexander T Taguchi
- †Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,‡Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
| | - Rimma I Samoilova
- ⊥V. V. Voevodsky Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk 630090, Russian Federation
| | - Patrick J O'Malley
- ∥School of Chemistry, The University of Manchester, Manchester M13 9PL, U.K
| | - Robert B Gennis
- §Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.,†Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
| | - Sergei A Dikanov
- ‡Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
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6
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Comba P, Dovalil N, Hanson GR, Harmer JR, Noble CJ, Riley MJ, Seibold B. Insights into the Electronic Structure of CuII Bound to an Imidazole Analogue of Westiellamide. Inorg Chem 2014; 53:12323-36. [DOI: 10.1021/ic5014413] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Peter Comba
- Anorganisch-Chemisches
Institut, Universität Heidelberg, D-69120 Heidelberg, Germany
| | - Nina Dovalil
- Anorganisch-Chemisches
Institut, Universität Heidelberg, D-69120 Heidelberg, Germany
| | | | | | | | | | - Bjoern Seibold
- Anorganisch-Chemisches
Institut, Universität Heidelberg, D-69120 Heidelberg, Germany
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7
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A Tightly Bound Quinone Functions in the Ubiquinone Reaction Sites of Quinoprotein Alcohol Dehydrogenase of an Acetic Acid Bacterium,Gluconobacter suboxydans. Biosci Biotechnol Biochem 2014; 72:2723-31. [DOI: 10.1271/bbb.80363] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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8
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Vennam PR, Fisher N, Krzyaniak MD, Kramer DM, Bowman MK. A caged, destabilized, free radical intermediate in the q-cycle. Chembiochem 2013; 14:1745-53. [PMID: 24009094 PMCID: PMC3951126 DOI: 10.1002/cbic.201300265] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2013] [Indexed: 11/12/2022]
Abstract
The Rieske/cytochrome b complexes, also known as cytochrome bc complexes, catalyze a unique oxidant-induced reduction reaction at their quinol oxidase (Qo ) sites, in which substrate hydroquinone reduces two distinct electron transfer chains, one through a series of high-potential electron carriers, the second through low-potential cytochrome b. This reaction is a critical step in energy storage by the Q-cycle. The semiquinone intermediate in this reaction can reduce O2 to produce deleterious superoxide. It is yet unknown how the enzyme controls this reaction, though numerous models have been proposed. In previous work, we trapped a Q-cycle semiquinone anion intermediate, termed SQo , in bacterial cytochrome bc1 by rapid freeze-quenching. In this work, we apply pulsed-EPR techniques to determine the location and properties of SQo in the mitochondrial complex. In contrast to semiquinone intermediates in other enzymes, SQo is not thermodynamically stabilized, and can even be destabilized with respect to solution. It is trapped in Qo at a site that is distinct from previously described inhibitor-binding sites, yet sufficiently close to cytochrome bL to allow rapid electron transfer. The binding site and EPR analyses show that SQo is not stabilized by hydrogen bonds to proteins. The formation of SQo involves "stripping" of both substrate -OH protons during the initial oxidation step, as well as conformational changes of the semiquinone and Qo proteins. The resulting charged radical is kinetically trapped, rather than thermodynamically stabilized (as in most enzymatic semiquinone species), conserving redox energy to drive electron transfer to cytochrome bL while minimizing certain Q-cycle bypass reactions, including oxidation of prereduced cytochrome b and reduction of O2 .
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Affiliation(s)
- Preethi R. Vennam
- Chemistry Department University of Alabama Box 870336, Tuscaloosa, AL 35487, United States
| | - Nicholas Fisher
- Biochemistry and Molecular Biology and the MSU-DOE Plant Research Laboratory Michigan State University East Lansing, MI 48824, United States
| | - Matthew D. Krzyaniak
- Chemistry Department University of Alabama Box 870336, Tuscaloosa, AL 35487, United States
| | - David M. Kramer
- Biochemistry and Molecular Biology and the MSU-DOE Plant Research Laboratory Michigan State University East Lansing, MI 48824, United States
| | - Michael K. Bowman
- Chemistry Department University of Alabama Box 870336, Tuscaloosa, AL 35487, United States
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9
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Dikanov SA. Resolving protein-semiquinone interactions by two-dimensional ESEEM spectroscopy. ELECTRON PARAMAGNETIC RESONANCE 2012. [DOI: 10.1039/9781849734837-00103] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Affiliation(s)
- S. A. Dikanov
- University of Illinois at Urbana-Champaign, Department of Veterinary Clinical Medicine 190 MSB, 506 S. Mathews Ave., Urbana IL 61801 USA
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10
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Flores M, Okamura MY, Niklas J, Pandelia ME, Lubitz W. Pulse Q-band EPR and ENDOR spectroscopies of the photochemically generated monoprotonated benzosemiquinone radical in frozen alcoholic solution. J Phys Chem B 2012; 116:8890-900. [PMID: 22731760 DOI: 10.1021/jp304555u] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Quinones are essential cofactors in many physiological processes, among them proton-coupled electron transfer (PCET) in photosynthesis and respiration. A key intermediate in PCET is the monoprotonated semiquinone radical. In this work we produced the monoprotonated benzosemiquinone (BQH(•)) by UV illumination of BQ dissolved in 2-propanol at cryogenic temperatures and investigated the electronic and geometric structures of BQH(•) in the solid state (80 K) using EPR and ENDOR techniques at 34 GHz. The g-tensor of BQH(•) was found to be similar to that of the anionic semiquinone species (BQ(•-)) in frozen solution. The peaks present in the ENDOR spectrum of BQH(•) were identified and assigned by (1)H/(2)H substitutions. The experiments reconfirmed that the hydroxyl proton (O-H) on BQH(•), which is abstracted from a solvent molecule, mainly originates from the central CH group of 2-propanol. They also showed that the protonation has a strong impact on the electron spin distribution over the quinone. This is reflected in the hyperfine couplings (hfc's) of the ring protons, which dramatically changed with respect to those typically observed for BQ(•-). The hfc tensor of the O-H proton was determined by a detailed orientation-selection ENDOR study and found to be rhombic, resembling those of protons covalently bound to carbon atoms in a π-system (i.e., α-protons). It was found that the O-H bond lies in the quinone plane and is oriented along the direction of the quinone oxygen lone pair orbital. DFT calculations were performed on different structures of BQH(•) coordinated by four, three, or zero 2-propanol molecules. The O-H bond length was found to be around 1.0 Å, typical for a single covalent O-H bond. Good agreement between experimental and DFT results were found. This study provides a detailed picture of the electronic and geometric structures of BQH(•) and should be applicable to other naturally occurring quinones.
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Affiliation(s)
- Marco Flores
- Max-Planck-Institut für Chemische Energiekonversion, Mülheim an der Ruhr, D-45470, Germany.
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11
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Lin MT, Baldansuren A, Hart R, Samoilova RI, Narasimhulu KV, Yap LL, Choi SK, O'Malley PJ, Gennis RB, Dikanov SA. Interactions of intermediate semiquinone with surrounding protein residues at the Q(H) site of wild-type and D75H mutant cytochrome bo3 from Escherichia coli. Biochemistry 2012; 51:3827-38. [PMID: 22497216 DOI: 10.1021/bi300151q] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Selective (15)N isotope labeling of the cytochrome bo(3) ubiquinol oxidase from Escherichia coli with auxotrophs was used to characterize the hyperfine couplings with the side-chain nitrogens from residues R71, H98, and Q101 and peptide nitrogens from residues R71 and H98 around the semiquinone (SQ) at the high-affinity Q(H) site. The two-dimensional ESEEM (HYSCORE) data have directly identified N(ε) of R71 as an H-bond donor carrying the largest amount of unpaired spin density. In addition, weaker hyperfine couplings with the side-chain nitrogens from all residues around the SQ were determined. These hyperfine couplings reflect a distribution of the unpaired spin density over the protein in the SQ state of the Q(H) site and the strength of interaction with different residues. The approach was extended to the virtually inactive D75H mutant, where the intermediate SQ is also stabilized. We found that N(ε) of a histidine residue, presumably H75, carries most of the unpaired spin density instead of N(ε) of R71, as in wild-type bo(3). However, the detailed characterization of the weakly coupled (15)N atoms from selective labeling of R71 and Q101 in D75H was precluded by overlap of the (15)N lines with the much stronger ~1.6 MHz line from the quadrupole triplet of the strongly coupled (14)N(ε) atom of H75. Therefore, a reverse labeling approach, in which the enzyme was uniformly labeled except for selected amino acid types, was applied to probe the contribution of R71 and Q101 to the (15)N signals. Such labeling has shown only weak coupling with all nitrogens of R71 and Q101. We utilize density functional theory-based calculations to model the available information about (1)H, (15)N, and (13)C hyperfine couplings for the Q(H) site and to describe the protein-substrate interactions in both enzymes. In particular, we identify the factors responsible for the asymmetric distribution of the unpaired spin density and ponder the significance of this asymmetry to the quinone's electron transfer function.
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Affiliation(s)
- Myat T Lin
- Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
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12
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MacMillan F, Kacprzak S, Hellwig P, Grimaldi S, Michel H, Kaupp M. Elucidating mechanisms in haemcopperoxidases: The high-affinity QHbinding site in quinol oxidase as studied by DONUT-HYSCOREspectroscopy and density functional theory. Faraday Discuss 2011; 148:315-44; discussion 421-41. [DOI: 10.1039/c005149g] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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13
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Yap LL, Lin MT, Ouyang H, Samoilova RI, Dikanov SA, Gennis RB. The quinone-binding sites of the cytochrome bo3 ubiquinol oxidase from Escherichia coli. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2010; 1797:1924-32. [PMID: 20416270 DOI: 10.1016/j.bbabio.2010.04.011] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/14/2009] [Revised: 03/04/2010] [Accepted: 04/13/2010] [Indexed: 11/18/2022]
Abstract
Cytochrome bo(3) is the major respiratory oxidase located in the cytoplasmic membrane of Escherichia coli when grown under high oxygen tension. The enzyme catalyzes the 2-electron oxidation of ubiquinol-8 and the 4-electron reduction of dioxygen to water. When solubilized and isolated using dodecylmaltoside, the enzyme contains one equivalent of ubiquinone-8, bound at a high affinity site (Q(H)). The quinone bound at the Q(H) site can form a stable semiquinone, and the amino acid residues which hydrogen bond to the semiquinone have been identified. In the current work, it is shown that the tightly bound ubiquinone-8 at the Q(H) site is not displaced by ubiquinol-1 even during enzyme turnover. Furthermore, the presence of high affinity inhibitors, HQNO and aurachin C1-10, does not displace ubiquinone-8 from the Q(H) site. The data clearly support the existence of a second binding site for ubiquinone, the Q(L) site, which can rapidly exchange with the substrate pool. HQNO is shown to bind to a single site on the enzyme and to prevent formation of the stable ubisemiquinone, though without displacing the bound quinone. Inhibition of the steady state kinetics of the enzyme indicates that aurachin C1-10 may compete for binding with quinol at the Q(L) site while, at the same time, preventing formation of the ubisemiquinone at the Q(H) site. It is suggested that the two quinone binding sites may be adjacent to each other or partially overlap.
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Affiliation(s)
- Lai Lai Yap
- Department of Biochemistry, University of Illinois, 600 S. Goodwin Avenue, Urbana, IL 61801, USA
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14
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Arias-Cartin R, Lyubenova S, Ceccaldi P, Prisner T, Magalon A, Guigliarelli B, Grimaldi S. HYSCORE Evidence That Endogenous Mena- and Ubisemiquinone Bind at the Same Q Site (QD) of Escherichia coli Nitrate Reductase A. J Am Chem Soc 2010; 132:5942-3. [DOI: 10.1021/ja1009234] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Rodrigo Arias-Cartin
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036) and Laboratoire de Chimie Bactérienne (UPR9043), CNRS and Aix-Marseille Université, 31 chemin J. Aiguier, 13009 Marseille, France, and Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
| | - Sevdalina Lyubenova
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036) and Laboratoire de Chimie Bactérienne (UPR9043), CNRS and Aix-Marseille Université, 31 chemin J. Aiguier, 13009 Marseille, France, and Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
| | - Pierre Ceccaldi
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036) and Laboratoire de Chimie Bactérienne (UPR9043), CNRS and Aix-Marseille Université, 31 chemin J. Aiguier, 13009 Marseille, France, and Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
| | - Thomas Prisner
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036) and Laboratoire de Chimie Bactérienne (UPR9043), CNRS and Aix-Marseille Université, 31 chemin J. Aiguier, 13009 Marseille, France, and Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
| | - Axel Magalon
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036) and Laboratoire de Chimie Bactérienne (UPR9043), CNRS and Aix-Marseille Université, 31 chemin J. Aiguier, 13009 Marseille, France, and Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
| | - Bruno Guigliarelli
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036) and Laboratoire de Chimie Bactérienne (UPR9043), CNRS and Aix-Marseille Université, 31 chemin J. Aiguier, 13009 Marseille, France, and Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
| | - Stéphane Grimaldi
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036) and Laboratoire de Chimie Bactérienne (UPR9043), CNRS and Aix-Marseille Université, 31 chemin J. Aiguier, 13009 Marseille, France, and Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
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15
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Yi SM, Narasimhulu KV, Samoilova RI, Gennis RB, Dikanov SA. Characterization of the semiquinone radical stabilized by the cytochrome aa3-600 menaquinol oxidase of Bacillus subtilis. J Biol Chem 2010; 285:18241-51. [PMID: 20351111 DOI: 10.1074/jbc.m110.116186] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cytochrome aa(3)-600 is one of the principle respiratory oxidases from Bacillus subtilis and is a member of the heme-copper superfamily of oxygen reductases. This enzyme catalyzes the two-electron oxidation of menaquinol and the four-electron reduction of O(2) to 2H(2)O. Cytochrome aa(3)-600 is of interest because it is a very close homologue of the cytochrome bo(3) ubiquinol oxidase from Escherichia coli, except that it uses menaquinol instead of ubiquinol as a substrate. One question of interest is how the proteins differ in response to the differences in structure and electrochemical properties between ubiquinol and menaquinol. Cytochrome bo(3) has a high affinity binding site for ubiquinol that stabilizes a ubi-semiquinone. This has permitted the use of pulsed EPR techniques to investigate the protein interaction with the ubiquinone. The current work initiates studies to characterize the equivalent site in cytochrome aa(3)-600. Cytochrome aa(3)-600 has been cloned and expressed in a His-tagged form in B. subtilis. After isolation of the enzyme in dodecylmaltoside, it is shown that the pure enzyme contains 1 eq of menaquinone-7 and that the enzyme stabilizes a mena-semiquinone. Pulsed EPR studies have shown that there are both similarities as well as significant differences in the interactions of the mena-semiquinone with cytochrome aa(3)-600 in comparison with the ubi-semiquinone in cytochrome bo(3). Our data indicate weaker hydrogen bonds of the menaquinone in cytochrome aa(3)-600 in comparison with ubiquinone in cytochrome bo(3). In addition, the electronic structure of the semiquinone cyt aa(3)-600 is more shifted toward the anionic form from the neutral state in cyt bo(3).
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Affiliation(s)
- Sophia M Yi
- Departments of Biochemistry, University of Illinois, Urbana, Illinois 61801, USA
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16
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Grimaldi S, Arias-Cartin R, Lanciano P, Lyubenova S, Endeward B, Prisner TF, Magalon A, Guigliarelli B. Direct evidence for nitrogen ligation to the high stability semiquinone intermediate in Escherichia coli nitrate reductase A. J Biol Chem 2009; 285:179-87. [PMID: 19892705 DOI: 10.1074/jbc.m109.060251] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The membrane-bound heterotrimeric nitrate reductase A (NarGHI) catalyzes the oxidation of quinols in the cytoplasmic membrane of Escherichia coli and reduces nitrate to nitrite in the cytoplasm. The enzyme strongly stabilizes a menasemiquinone intermediate at a quinol oxidation site (Q(D)) located in the vicinity of the distal heme b(D). Here molecular details of the interaction between the semiquinone radical and the protein environment have been provided using advanced multifrequency pulsed EPR methods. (14)N and (15)N ESEEM and HYSCORE measurements carried out at X-band ( approximately 9.7 GHz) on the wild-type enzyme or the enzyme uniformly labeled with (15)N nuclei reveal an interaction between the semiquinone and a single nitrogen nucleus. The isotropic hyperfine coupling constant A(iso)((14)N) approximately 0.8 MHz shows that it occurs via an H-bond to one of the quinone carbonyl group. Using (14)N ESEEM and HYSCORE spectroscopies at a lower frequency (S-band, approximately 3.4 GHz), the (14)N nuclear quadrupolar parameters of the interacting nitrogen nucleus (kappa = 0.49, eta = 0.50) were determined and correspond to those of a histidine N(delta), assigned to the heme b(D) ligand His-66 residue. Moreover S-band (15)N ESEEM spectra enabled us to directly measure the anisotropic part of the nitrogen hyperfine interaction (T((15)N) = 0.16 MHz). A distance of approximately 2.2 Abetween the carbonyl oxygen and the nitrogen could then be calculated. Mechanistic implications of these results are discussed in the context of the peculiar properties of the menasemiquinone intermediate stabilized at the Q(D) site of NarGHI.
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Affiliation(s)
- Stéphane Grimaldi
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036), Institut de Microbiologie de laMéditerranée, CNRS and Aix-Marseille Université, 31 chemin Joseph Aiguier, 13402 Marseille Cedex 20, France.
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17
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Lin MT, Samoilova RI, Gennis RB, Dikanov SA. Identification of the nitrogen donor hydrogen bonded with the semiquinone at the Q(H) site of the cytochrome bo3 from Escherichia coli. J Am Chem Soc 2008; 130:15768-9. [PMID: 18983149 PMCID: PMC2645916 DOI: 10.1021/ja805906a] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The selective (15)N isotope labeling was used for the identification of the nitrogen involved in a hydrogen bond formation with the semiquinone in the high-affinity Q(H) site in the cytochrome bo(3) ubiquinol oxidase. This nitrogen produces dominating contribution to X-Band (14)N ESEEM spectra. The 2D ESEEM (HYSCORE) experiments with the Q(H) site SQ in the series of selectively (15)N labeled bo(3) oxidase proteins have directly identified the N(epsilon) of R71 as an H-bond donor. In addition, selective (15)N labeling has allowed us for the first time to determine weak hyperfine couplings with the side-chain nitrogens from all residues around the SQ. Those are reflecting a distribution of the unpaired spin density over the protein in the SQ state of the quinone processing site.
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Affiliation(s)
- Myat T. Lin
- Department of Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
| | - Rimma I. Samoilova
- Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk 630090, Russia
| | - Robert B. Gennis
- Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
| | - Sergei A. Dikanov
- Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
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18
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Hinderberger D, Ebner S, Mayr S, Jaun B, Reiher M, Goenrich M, Thauer RK, Harmer J. Coordination and binding geometry of methyl-coenzyme M in the red1m state of methyl-coenzyme M reductase. J Biol Inorg Chem 2008; 13:1275-89. [PMID: 18712421 DOI: 10.1007/s00775-008-0417-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2008] [Accepted: 07/27/2008] [Indexed: 10/21/2022]
Abstract
Methane formation in methanogenic Archaea is catalyzed by methyl-coenzyme M reductase (MCR) and takes place via the reduction of methyl-coenzyme M (CH3-S-CoM) with coenzyme B (HS-CoB) to methane and the heterodisulfide CoM-S-S-CoB. MCR harbors the nickel porphyrinoid coenzyme F430 as a prosthetic group, which has to be in the Ni(I) oxidation state for the enzyme to be active. To date no intermediates in the catalytic cycle of MCRred1 (red for reduced Ni) have been identified. Here, we report a detailed characterization of MCRred1m ("m" for methyl-coenzyme M), which is the complex of MCRred1a ("a" for absence of substrate) with CH3-S-CoM. Using continuous-wave and pulse electron paramagnetic resonance spectroscopy in combination with selective isotope labeling (13C and 2H) of CH3-S-CoM, it is shown that CH3-S-CoM binds in the active site of MCR such that its thioether sulfur is weakly coordinated to the Ni(I) of F430. The complex is stable until the addition of the second substrate, HS-CoB. Results from EPR spectroscopy, along with quantum mechanical calculations, are used to characterize the electronic and geometric structure of this complex, which can be regarded as the first intermediate in the catalytic mechanism.
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Affiliation(s)
- Dariush Hinderberger
- Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128, Mainz, Germany.
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19
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Mustafa G, Ishikawa Y, Kobayashi K, Migita CT, Elias M, Nakamura S, Tagawa S, Yamada M. Amino Acid Residues Interacting with Both the Bound Quinone and Coenzyme, Pyrroloquinoline Quinone, in Escherichia coli Membrane-bound Glucose Dehydrogenase. J Biol Chem 2008; 283:22215-21. [DOI: 10.1074/jbc.m800911200] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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20
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Harmer J, Finazzo C, Piskorski R, Ebner S, Duin EC, Goenrich M, Thauer RK, Reiher M, Schweiger A, Hinderberger D, Jaun B. A Nickel Hydride Complex in the Active Site of Methyl-Coenzyme M Reductase: Implications for the Catalytic Cycle. J Am Chem Soc 2008; 130:10907-20. [DOI: 10.1021/ja710949e] [Citation(s) in RCA: 52] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Jeffrey Harmer
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Cinzia Finazzo
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Rafal Piskorski
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Sieglinde Ebner
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Evert C. Duin
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Meike Goenrich
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Rudolf K. Thauer
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Markus Reiher
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Arthur Schweiger
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Dariush Hinderberger
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
| | - Bernhard Jaun
- Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, Oxford, United Kingdom, Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Laboratory of Organic Chemistry, ETH Zurich, 8093 Zurich, Switzerland, Max Planck Institute for Terrestrial Microbiology, Department of Biochemistry, Karl-von-Frisch Strasse, 35043 Marburg, Germany, and Department of Chemistry and Biochemistry, Auburn University, Alabama 36849-5312
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21
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Kacprzak S, Kaupp M, MacMillan F. Protein-cofactor interactions and EPR parameters for the Q(H) quinone binding site of quinol oxidase. A density functional study. J Am Chem Soc 2007; 128:5659-71. [PMID: 16637632 DOI: 10.1021/ja053988b] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Recent multifrequency EPR studies of the "high-affinity" quinone binding site of quinol oxidase (Q(H) site) have suggested a very asymmetric hydrogen-bonding environment for the semiquinone radical anion state. Single-sided hydrogen bonding to the O1 carbonyl position was one of the proposals, which contrasts with some previous experimental indications. Here density functional calculations of the EPR parameters (g-tensors, 13C, 1H, and 17O hyperfine tensors) for a wide variety of supermolecular model complexes have been used to provide insight into the detailed relations among structure, environment, and EPR parameters of ubisemiquinone radical anions. A single-sided binding model is not able to account for the experimentally observed low g(x) component of the g-tensor or for the observed magnitude of the asymmetry of the 13C carbonyl HFC tensors. Based on the detailed comparison between computation and experiment, a model with two hydrogen bonds to O1 and one hydrogen bond to O4 is suggested for the Q(H) site, but a model with one more hydrogen bond on each side cannot be excluded. Several general conclusions on the interrelations between EPR parameters and hydrogen bond patterns of ubisemiquinones in proteins are provided.
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Affiliation(s)
- Sylwia Kacprzak
- Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D 97074 Würzburg, Germany
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22
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Cape JL, Bowman MK, Kramer DM. A semiquinone intermediate generated at the Qo site of the cytochrome bc1 complex: importance for the Q-cycle and superoxide production. Proc Natl Acad Sci U S A 2007; 104:7887-92. [PMID: 17470780 PMCID: PMC1876542 DOI: 10.1073/pnas.0702621104] [Citation(s) in RCA: 133] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
The cytochrome bc1 and related complexes are essential energy-conserving components of mitochondrial and bacterial electron transport chains. They orchestrate a complex sequence of electron and proton transfer reactions resulting in the oxidation of quinol, the reduction of a mobile electron carrier, and the translocation of protons across the membrane to store energy in an electrochemical proton gradient. The enzyme can also catalyze substantial rates of superoxide production, with deleterious physiological consequences. Progress on understanding these processes has been hindered by the lack of observable enzymatic intermediates. We report the first direct detection of a semiquinone radical generated by the Q(o) site using continuous wave and pulsed EPR spectroscopy. The radical is a ubisemiquinone anion and is sensitive to both specific inhibitors and mutations within the Q(o) site as well as O2, suggesting that it is the elusive intermediate responsible for superoxide production. Paramagnetic interactions show that the new semiquinone species is buried in the protein, probably in or near the Q(o) site but not strongly interacting with the 2Fe2S cluster. The semiquinone is substoichiometric, even with conditions optimized for its accumulation, consistent with recently proposed models where the semiquinone is destabilized to limit superoxide production. The discovery of this intermediate provides a critical tool to directly probe the elusive chemistry that takes place within the Q(o) site.
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Affiliation(s)
- Jonathan L. Cape
- *Institute of Biological Chemistry, Washington State University, 289 Clark Hall, Pullman, WA 99164-6314; and
| | - Michael K. Bowman
- *Institute of Biological Chemistry, Washington State University, 289 Clark Hall, Pullman, WA 99164-6314; and
- Department of Chemistry, University of Alabama, Shelby Hall 113A, P.O. Box 870336, Tuscaloosa, AL 35487-0336
| | - David M. Kramer
- *Institute of Biological Chemistry, Washington State University, 289 Clark Hall, Pullman, WA 99164-6314; and
- To whom correspondence should be addressed. E-mail:
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23
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Lanciano P, Magalon A, Bertrand P, Guigliarelli B, Grimaldi S. High-Stability Semiquinone Intermediate in Nitrate Reductase A (NarGHI) fromEscherichia coliIs Located in a Quinol Oxidation Site Close to HemebD. Biochemistry 2007; 46:5323-9. [PMID: 17439244 DOI: 10.1021/bi700074y] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Quinol/nitrate oxidoreductase (NarGHI) is the first enzyme involved in respiratory denitrification in prokaryotes. Although this complex in E. coli is known to operate with both ubi and menaquinones, the location and the number of quinol binding sites remain elusive. NarGHI strongly stabilizes a semiquinone radical located within the dihemic anchor subunit NarI. To identify its location and function, we used a combination of mutagenesis, kinetics, EPR, and ENDOR spectroscopies. For the NarGHIH66Y and NarGHIH187Y mutants lacking the distal heme bD, no EPR signal of the semiquinone was observed. In contrast, a semiquinone was detected in the NarGHIH56Y mutant lacking the proximal heme bP. Its thermodynamic properties and spectroscopic characteristics, as revealed by Q-band EPR and ENDOR spectroscopies, are identical to those observed in the native enzyme. The substitution by Ala of the Lys86 residue close to heme bD, which was previously proposed to be in a quinol oxidation site of NarGHI (QD), also leads to the loss of the EPR signal of the semiquinone, although both hemes are present. Enzymatic assays carried out on the NarGHIK86A mutant reveal that the substitution dramatically reduces the rate of oxidation of both mena and ubiquinol analogues. These observations demonstrate that the semiquinone observed in NarI is strongly associated with heme bD and that Lys86 is required for its stabilization. Overall, our results indicate that the semiquinone is located within the quinol oxidation site QD. Details of the possible binding motif of the semiquinone and mechanistic implications are discussed.
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Affiliation(s)
- Pascal Lanciano
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036), Institut de Biologie Structurale et de Microbiologie, CNRS and Université d'Aix-Marseille, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France
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24
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Yap LL, Samoilova RI, Gennis RB, Dikanov SA. Characterization of Mutants That Change the Hydrogen Bonding of the Semiquinone Radical at the QH Site of the Cytochrome bo3 from Escherichia coli. J Biol Chem 2007; 282:8777-85. [PMID: 17267395 DOI: 10.1074/jbc.m611595200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The cytochrome bo3 ubiquinol oxidase catalyzes the two-electron oxidation of ubiquinol in the cytoplasmic membrane of Escherichia coli, and reduces O2 to water. This enzyme has a high affinity quinone binding site (QH), and the quinone bound to this site acts as a cofactor, necessary for rapid electron transfer from substrate ubiquinol, which binds at a separate site (QL), to heme b. Previous pulsed EPR studies have shown that a semiquinone at the QH site formed during the catalytic cycle is a neutral species, with two strong hydrogen bonds to Asp-75 and either Arg-71 or Gln-101. In the current work, pulsed EPR studies have been extended to two mutants at the QH site. The D75E mutation has little influence on the catalytic activity, and the pattern of hydrogen bonding is similar to the wild type. In contrast, the D75H mutant is virtually inactive. Pulsed EPR revealed significant structural changes in this mutant. The hydrogen bond to Arg-71 or Gln-101 that is present in both the wild type and D75E mutant oxidases is missing in the D75H mutant. Instead, the D75H has a single, strong hydrogen bond to a histidine, likely His-75. The D75H mutant stabilizes an anionic form of the semiquinone as a result of the altered hydrogen bond network. Either the redistribution of charge density in the semiquinone species, or the altered hydrogen bonding network is responsible for the loss of catalytic function.
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Affiliation(s)
- Lai Lai Yap
- Department of Biochemistry, University of Illinois, Urbana, Illinois 61801, USA
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25
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White GF, Field S, Marritt S, Oganesyan VS, Gennis RB, Yap LL, Katsonouri A, Thomson AJ. An EPR Spin Label Study of the Quinol Oxidase, E. coli Cytochrome bo3: A Search for Redox Induced Conformational Changes. Biochemistry 2007; 46:2355-63. [PMID: 17288457 DOI: 10.1021/bi062265h] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
A search for conformational changes at the cytosolic entrance to the proton channels of the heme-copper quinol oxidase (QO), cytochrome bo3, E. coli, has been carried out using site directed nitroxide spin labeling (SDSL) of cysteine residues. These were positioned at R134 and R309, on loops that link helices II and III and VI and VII at the entrances to the D and K proton channels, respectively. The motional characteristics of both labels have been determined using X- and W-band EPR spectroscopy at room temperature in selected redox levels in the reaction sequence of QO with oxygen, namely, the mixed valence carbon monoxide form (COMV), the oxidized (O) and super-oxidized (PM) states. The O to PM step is accompanied by the uptake of protons through the K pathway. We find no evidence for changes in the motional characteristics of either label that are expected to be associated with helical motions at the entrances to the channels. Because kinetic studies of mutants show that the redox gating of protons occurs deep within the D channel close to the heme-copper site, the present study implies that no motion is transmitted to the ends of the helices.
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Affiliation(s)
- Gaye F White
- School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, UK
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26
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Flores M, Isaacson R, Abresch E, Calvo R, Lubitz W, Feher G. Protein-cofactor interactions in bacterial reaction centers from Rhodobacter sphaeroides R-26: II. Geometry of the hydrogen bonds to the primary quinone formula by 1H and 2H ENDOR spectroscopy. Biophys J 2006; 92:671-82. [PMID: 17071655 PMCID: PMC1751397 DOI: 10.1529/biophysj.106.092460] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
The geometry of the hydrogen bonds to the two carbonyl oxygens of the semiquinone Q(A)(. -) in the reaction center (RC) from the photosynthetic purple bacterium Rhodobacter sphaeroides R-26 were determined by fitting a spin Hamiltonian to the data derived from (1)H and (2)H ENDOR spectroscopies at 35 GHz and 80 K. The experiments were performed on RCs in which the native Fe(2+) (high spin) was replaced by diamagnetic Zn(2+) to prevent spectral line broadening of the Q(A)(. -) due to magnetic coupling with the iron. The principal components of the hyperfine coupling and nuclear quadrupolar coupling tensors of the hydrogen-bonded protons (deuterons) and their principal directions with respect to the quinone axes were obtained by spectral simulations of ENDOR spectra at different magnetic fields on frozen solutions of deuterated Q(A)(. -) in H(2)O buffer and protonated Q(A)(. -) in D(2)O buffer. Hydrogen-bond lengths were obtained from the nuclear quadrupolar couplings. The two hydrogen bonds were found to be nonequivalent, having different directions and different bond lengths. The H-bond lengths r(OH) are 1.73 +/- 0.03 Angstrom and 1.60 +/- 0.04 Angstrom, from the carbonyl oxygens O(1) and O(4) to the NH group of Ala M260 and the imidazole nitrogen N(delta) of His M219, respectively. The asymmetric hydrogen bonds of Q(A)(. -) affect the spin density distribution in the quinone radical and its electronic structure. It is proposed that the H-bonds play an important role in defining the physical properties of the primary quinone, which affect the electron transfer processes in the RC.
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Affiliation(s)
- M Flores
- Department of Physics, University of California at San Diego, La Jolla, California, USA
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Khenkin AM, Carl P, Baute D, Raitsimring AM, Astashkin AV, Shimon LJ, Goldfarb D, Neumann R. Structural and EPR/ENDOR/ESEEM spectroscopic investigations of a vanadomolybdate Keggin-type polyoxometalate in organic solvent. Inorganica Chim Acta 2006. [DOI: 10.1016/j.ica.2006.01.043] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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28
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Yap LL, Samoilova RI, Gennis RB, Dikanov SA. Characterization of the exchangeable protons in the immediate vicinity of the semiquinone radical at the QH site of the cytochrome bo3 from Escherichia coli. J Biol Chem 2006; 281:16879-16887. [PMID: 16624801 DOI: 10.1074/jbc.m602544200] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The cytochrome bo3 ubiquinol oxidase from Escherichia coli resides in the bacterial cytoplasmic membrane and catalyzes the two-electron oxidation of ubiquinol-8 and four-electron reduction of O2 to water. The one-electron reduced semiquinone forms transiently during the reaction, and the enzyme has been demonstrated to stabilize the semiquinone. Two-dimensional electron spin echo envelope modulation has been applied to explore the exchangeable protons involved in hydrogen bonding to the semiquinone by substitution of 1H2O by 2H2O. Three exchangeable protons possessing different isotropic and anisotropic hyperfine couplings were identified. The strength of the hyperfine interaction with one proton suggests a significant covalent O-H binding of carbonyl oxygen O1 that is a characteristic of a neutral radical, an assignment that is also supported by the unusually large hyperfine coupling to the methyl protons. The second proton with a large anisotropic coupling also forms a strong hydrogen bond with a carbonyl oxygen. This second hydrogen bond, which has a significant out-of-plane character, is from an NH2 or NH nitrogen, probably from an arginine (Arg-71) known to be in the quinone binding site. Assignment of the third exchangeable proton with smaller anisotropic coupling is more ambiguous, but it is clearly not involved in a direct hydrogen bond with either of the carbonyl oxygens. The results support a model that the semiquinone is bound to the protein in a very asymmetric manner by two strong hydrogen bonds from Asp-75 and Arg-71 to the O1 carbonyl, while the O4 carbonyl is not hydrogen-bonded to the protein.
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Affiliation(s)
- Lai Lai Yap
- Department of Biochemistry, University of Illinois, Urbana, Illinois 61801
| | - Rimma I Samoilova
- Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk 630090, Russia
| | - Robert B Gennis
- Department of Biochemistry, University of Illinois, Urbana, Illinois 61801.
| | - Sergei A Dikanov
- Department of Veterinary Clinical Medicine, University of Illinois, Urbana, Illinois 61801.
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Kobayashi K, Mustafa G, Tagawa S, Yamada M. Transient Formation of a Neutral Ubisemiquinone Radical and Subsequent Intramolecular Electron Transfer to Pyrroloquinoline Quinone in the Escherichia coli Membrane-Integrated Glucose Dehydrogenase. Biochemistry 2005; 44:13567-72. [PMID: 16216080 DOI: 10.1021/bi051347n] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The membrane-bound quinoprotein glucose dehydrogenase (mGDH) in Escherichia coli contains pyrroloquinoline quinone (PQQ) and participates in the direct oxidation of D-glucose to D-gluconate by transferring electrons to ubiquinone (UQ). To elucidate the mechanism of ubiquinone reduction by mGDH, we applied a pulse radiolysis technique to mGDH with or without bound UQ8. With the UQ8-bound enzyme, a hydrated electron reacted with mGDH to form a transient species with an absorption maximum at 420 nm, characteristic of formation of a neutral ubisemiquinone radical. Subsequently, the decay of the absorbance at 420 nm was accompanied by an increase in the absorbance at 370 nm. Experiments with the PQQ-free apoenzyme showed no such subsequent absorption changes, although ubisemiquinone was formed. These results indicate that a pathway for an intramolecular electron transfer from ubisemiquinone radical at the UQ8 binding site to PQQ exists in mGDH. The first-order rate constant of this process was calculated to be equal to 1.2 x 10(3) s(-1). These findings are consistent with our proposal that during the catalytic cycle of mGDH the bound UQ8 mediates electron transfer from the reduced PQQ to UQ8 pools.
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Affiliation(s)
- Kazuo Kobayashi
- The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan.
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Grimaldi S, Lanciano P, Bertrand P, Blasco F, Guigliarelli B. Evidence for an EPR-Detectable Semiquinone Intermediate Stabilized in the Membrane-Bound Subunit NarI of Nitrate Reductase A (NarGHI) from Escherichia coli. Biochemistry 2004; 44:1300-8. [PMID: 15667223 DOI: 10.1021/bi048009r] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Nitrate reductase A (NRA, NarGHI) is expressed in Escherichia coli by growing the bacterium in anaerobic conditions in the presence of nitrate. This enzyme reduces nitrate to nitrite and uses menaquinol (or ubiquinol) as the electron donor. The location of quinones in the enzyme, their number, and their role in the electron transfer mechanism are still controversial. In this work, we have investigated the spectroscopic and thermodynamic properties of a semiquinone (SQ) in membrane samples of overexpressed E. coli nitrate reductase poised in appropriate redox conditions. This semiquinone is highly stabilized with respect to free semiquinone. The g-values determined from the numerical simulation of its Q-band (35 GHz) EPR spectrum are equal to 2.0061, 2.0051, 2.0023. The midpoint potential of the Q/QH(2) couple is about -100 mV, and the SQ stability constant is about 100 at pH 7.5. The semiquinone EPR signal disappears completely upon addition of the quinol binding site inhibitor 2-n-nonyl-4-hydroxyquinoline N-oxide (NQNO). A semiquinone radical could also be stabilized in preparations where only the NarI membrane subunit is overexpressed in the absence of the NarGH catalytic dimer. Its thermodynamic and spectroscopic properties show only slight variations with those of the wild-type enzyme. The X-band continuous wave (cw) electron nuclear double resonance (ENDOR) spectra of the radicals display similar proton hyperfine coupling patterns in NarGHI and in NarI, showing that they arise from the same semiquinone species bound to a single site located in the NarI membrane subunit. These results are discussed with regard to the location and the potential function of quinones in the enzyme.
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Affiliation(s)
- Stéphane Grimaldi
- Unité de Bioénergétique et Ingénierie des Protéines (UPR9036), Institut de Biologie Structurale et de Microbiologie, CNRS, and Université de Provence, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France.
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Grimaldi S, Ostermann T, Weiden N, Mogi T, Miyoshi H, Ludwig B, Michel H, Prisner TF, MacMillan F. Asymmetric binding of the high-affinity Q(H)(*)(-) ubisemiquinone in quinol oxidase (bo3) from Escherichia coli studied by multifrequency electron paramagnetic resonance spectroscopy. Biochemistry 2003; 42:5632-9. [PMID: 12741819 DOI: 10.1021/bi034010z] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Ubiquinone-2 (UQ-2) selectively labeled with (13)C (I =(1)/(2)) at either the position 1- or the 4-carbonyl carbon is incorporated into the ubiquinol oxidase bo(3) from Escherichia coli in which the native quinone (UQ-8) has been previously removed. The resulting stabilized anion radical in the high-affinity quinone-binding site (Q(H)(*)(-)) is investigated using multifrequency (9, 34, and 94 GHz) electron paramagnetic resonance (EPR) spectroscopy. The corresponding spectra reveal dramatic differences in (13)C hyperfine couplings indicating a strongly asymmetric spin density distribution over the quinone headgroup. By comparison with previous results on labeled ubisemiquinones in proteins as well as in organic solvents, it is concluded that Q(H)(*)(-) is most probably bound to the protein via a one-sided hydrogen bond or a strongly asymmetric hydrogen-bonding network. This observation is discussed with regard to the function of Q(H) in the enzyme and contrasted with the information available on other protein-bound semiquinone radicals.
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Affiliation(s)
- S Grimaldi
- Institut für Physikalische und Theoretische Chemie, J. W. Goethe Universität Frankfurt, Centre for Biological Magnetic Resonance, Frankfurt am Main, Germany
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32
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Hellwig P, Yano T, Ohnishi T, Gennis RB. Identification of the residues involved in stabilization of the semiquinone radical in the high-affinity ubiquinone binding site in cytochrome bo(3) from Escherichia coli by site-directed mutagenesis and EPR spectroscopy. Biochemistry 2002; 41:10675-9. [PMID: 12186553 DOI: 10.1021/bi012146w] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
During turnover of cytochrome bo(3) from Escherichia coli, a semiquinone radical is stabilized in a high-affinity binding site. To identify binding partners of this radical, site-directed mutants have been designed on the basis of a recently modeled quinone binding site (Abramson et al., 2000). The R71H, H98F, D75H, and I102W mutant enzymes were found to show very little or no quinol oxidase activity. The thermodynamic and EPR spectroscopic properties of semiquinone radicals in these mutants were characterized. For the H98F and the R71H mutants, no EPR signal of the semiquinone radical was observed in the redox potential range from -100 to 250 mV. During potentiometric titration of the D75H mutant enzyme, a semiquinone signal was detected in the same potential range as that of the wild-type enzyme. However, the EPR spectrum of the D75H mutant lacks the characteristic hyperfine structure of the semiquinone radical signal observed in the wild-type oxidase, indicating that D75 or the introduced His, interacts with the semiquinone radical. For the I102W mutant, a free radical signal was observed with a redox midpoint potential downshifted by about 200 mV. On the basis of these observations, it is suggested that R71, D75, and H98 residues are involved in the stabilization of the semiquinone state in the high-affinity binding site. Details of the possible binding motif and mechanistic implications are discussed.
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Affiliation(s)
- Petra Hellwig
- Department of Biochemistry, University of Illinois, 600 South Mathews Street, Urbana, IL 61801, USA.
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Weber A, Schiemann O, Bode B, Prisner TF. PELDOR at S- and X-band frequencies and the separation of exchange coupling from dipolar coupling. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2002; 157:277-285. [PMID: 12323146 DOI: 10.1006/jmre.2002.2596] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
A pulsed electron double resonance (PELDOR) setup working at S-band frequencies is introduced and its performance compared with an X-band setup. Furthermore, to verify experimentally that it is possible to disentangle the dipolar coupling nu(Dip) from the exchange coupling J by PELDOR we synthesized and investigated four bisnitroxide radicals. They exhibit in pairs the same distances r(AB) between the nitroxide moieties but only one of each pair possesses a non-zero J. The experimental values for r(AB) match the ones from molecular modeling very well for the molecules without exchange coupling. For one bisnitroxide it was possible to separate nu(Dip) from J and to ascertain the magnitude and sign of J to +11 MHz (antiferromagnetic spin-spin coupling).
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Affiliation(s)
- Axel Weber
- Institut für Physikalische und Theoretische Chemie, Johann Wolfgang Goethe-Universität, Marie-Curie-Strasse 11, 60439 Frankfurt/Main, Germany
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35
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Lakshmi KV, Brudvig GW. Pulsed electron paramagnetic resonance methods for macromolecular structure determination. Curr Opin Struct Biol 2001; 11:523-31. [PMID: 11785751 DOI: 10.1016/s0959-440x(00)00242-6] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Pulsed electron paramagnetic resonance (EPR) distance measurement techniques target macromolecular structure elucidation at both the local and global level. Recent developments in pulse microwave technology and high-field EPR have led to the development of a variety of pulsed EPR distance measurement techniques. These methods have emerged as powerful tools for the determination of structure/function relationships in macromolecular systems. In this review article, we discuss recent applications of long-range and short-range EPR distance measurements.
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Affiliation(s)
- K V Lakshmi
- Department of Chemistry, Yale University, New Haven, CT 06520, USA.
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36
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Abstract
Pulsed electron paramagnetic resonance (EPR) methods such as ESEEM, PELDOR, relaxation time measurements, transient EPR, high-field/high-frequency EPR, and pulsed ENDOR, have been used successfully to investigate the local structure and dynamics of paramagnetic centers in biological samples. These methods allow different contributions to the EPR spectra to be distinguished and can help unravel complicated EPR spectra consisting of overlapping resonance lines, as are often found in disordered protein samples. The basic principles, specific potentials, technical requirements, and limitations of these advanced EPR techniques will be reviewed together with recent applications to metal centers, organic radicals, and spin labels in proteins.
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Affiliation(s)
- T Prisner
- Institute for Physical and Theoretical Chemistry, J. W. Goethe-University Frankfurt, Marie-Curie-Strasse 11, Frankfurt am Main, D-60439 Germany.
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