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Hird K, Campeciño JO, Lehnert N, Hegg EL. Recent mechanistic developments for cytochrome c nitrite reductase, the key enzyme in the dissimilatory nitrate reduction to ammonium pathway. J Inorg Biochem 2024; 256:112542. [PMID: 38631103 DOI: 10.1016/j.jinorgbio.2024.112542] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2024] [Revised: 03/19/2024] [Accepted: 03/23/2024] [Indexed: 04/19/2024]
Abstract
Cytochrome c nitrite reductase, NrfA, is a soluble, periplasmic pentaheme cytochrome responsible for the reduction of nitrite to ammonium in the Dissimilatory Nitrate Reduction to Ammonium (DNRA) pathway, a vital reaction in the global nitrogen cycle. NrfA catalyzes this six-electron and eight-proton reduction of nitrite at a single active site with the help of its quinol oxidase partners. In this review, we summarize the latest progress in elucidating the reaction mechanism of ammonia production, including new findings about the active site architecture of NrfA, as well as recent results that elucidate electron transfer and storage in the pentaheme scaffold of this enzyme.
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Affiliation(s)
- Krystina Hird
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, MI, USA
| | - Julius O Campeciño
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, MI, USA
| | - Nicolai Lehnert
- Department of Chemistry, University of Michigan, Ann Arbor, MI, USA
| | - Eric L Hegg
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, MI, USA.
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2
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Manickas EC, LaLonde AB, Hu MY, Alp EE, Lehnert N. Stabilization of a Heme-HNO Model Complex Using a Bulky Bis-Picket Fence Porphyrin and Reactivity Studies with NO. J Am Chem Soc 2023; 145:23014-23026. [PMID: 37824502 DOI: 10.1021/jacs.3c05333] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/14/2023]
Abstract
Nitroxyl, HNO/NO-, the one-electron reduced form of NO, is suggested to take part in distinct signaling pathways in mammals and is also a key intermediate in various heme-catalyzed NOx interconversions in the nitrogen cycle. Cytochrome P450nor (Cyt P450nor) is a heme-containing enzyme that performs NO reduction to N2O in fungal denitrification. The reactive intermediate in this enzyme, termed "Intermediate I", is proposed to be an Fe-NHO/Fe-NHOH type species, but it is difficult to study its electronic structure and exact protonation state due to its instability. Here, we utilize a bulky bis-picket fence porphyrin to obtain the first stable heme-HNO model complex, [Fe(3,5-Me-BAFP)(MI)(NHO)], as a model for Intermediate I, and more generally HNO adducts of heme proteins. Due to the steric hindrance of the bis-picket fence porphyrin, [Fe(3,5-Me-BAFP)(MI)(NHO)] is stable (τ1/2 = 56 min at -30 °C), can be isolated as a solid, and is available for thorough spectroscopic characterization. In particular, we were able to solve a conundrum in the literature and provide the first full vibrational characterization of a heme-HNO complex using IR and nuclear resonance vibrational spectroscopy (NRVS). Reactivity studies of [Fe(3,5-Me-BAFP)(MI)(NHO)] with NO gas show a 91 ± 10% yield for N2O formation, demonstrating that heme-HNO complexes are catalytically competent intermediates for NO reduction to N2O in Cyt P450nor. The implications of these results for the mechanism of Cyt P450nor are further discussed.
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Affiliation(s)
- Elizabeth C Manickas
- Department of Chemistry and Department of Biophysics, The University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Ashley B LaLonde
- Department of Chemistry and Department of Biophysics, The University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Michael Y Hu
- Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, Illinois 60439, United States
| | - E Ercan Alp
- Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, Illinois 60439, United States
| | - Nicolai Lehnert
- Department of Chemistry and Department of Biophysics, The University of Michigan, Ann Arbor, Michigan 48109-1055, United States
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3
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Lehnert N, Kim E, Dong HT, Harland JB, Hunt AP, Manickas EC, Oakley KM, Pham J, Reed GC, Alfaro VS. The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity. Chem Rev 2021; 121:14682-14905. [PMID: 34902255 DOI: 10.1021/acs.chemrev.1c00253] [Citation(s) in RCA: 96] [Impact Index Per Article: 32.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023]
Abstract
Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.
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Affiliation(s)
- Nicolai Lehnert
- Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Eunsuk Kim
- Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
| | - Hai T Dong
- Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Jill B Harland
- Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Andrew P Hunt
- Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Elizabeth C Manickas
- Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Kady M Oakley
- Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
| | - John Pham
- Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
| | - Garrett C Reed
- Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
| | - Victor Sosa Alfaro
- Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
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4
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Kroneck PMH. Nature's nitrite-to-ammonia expressway, with no stop at dinitrogen. J Biol Inorg Chem 2021; 27:1-21. [PMID: 34865208 PMCID: PMC8840924 DOI: 10.1007/s00775-021-01921-4] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2021] [Accepted: 11/22/2021] [Indexed: 12/26/2022]
Abstract
Since the characterization of cytochrome c552 as a multiheme nitrite reductase, research on this enzyme has gained major interest. Today, it is known as pentaheme cytochrome c nitrite reductase (NrfA). Part of the NH4+ produced from NO2- is released as NH3 leading to nitrogen loss, similar to denitrification which generates NO, N2O, and N2. NH4+ can also be used for assimilatory purposes, thus NrfA contributes to nitrogen retention. It catalyses the six-electron reduction of NO2- to NH4+, hosting four His/His ligated c-type hemes for electron transfer and one structurally differentiated active site heme. Catalysis occurs at the distal side of a Fe(III) heme c proximally coordinated by lysine of a unique CXXCK motif (Sulfurospirillum deleyianum, Wolinella succinogenes) or, presumably, by the canonical histidine in Campylobacter jejeuni. Replacement of Lys by His in NrfA of W. succinogenes led to a significant loss of enzyme activity. NrfA forms homodimers as shown by high resolution X-ray crystallography, and there exist at least two distinct electron transfer systems to the enzyme. In γ-proteobacteria (Escherichia coli) NrfA is linked to the menaquinol pool in the cytoplasmic membrane through a pentaheme electron carrier (NrfB), in δ- and ε-proteobacteria (S. deleyianum, W. succinogenes), the NrfA dimer interacts with a tetraheme cytochrome c (NrfH). Both form a membrane-associated respiratory complex on the extracellular side of the cytoplasmic membrane to optimize electron transfer efficiency. This minireview traces important steps in understanding the nature of pentaheme cytochrome c nitrite reductases, and discusses their structural and functional features.
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Affiliation(s)
- Peter M H Kroneck
- Department of Biology, University of Konstanz, Universitätsstrasse 10, 78457, Konstanz, Germany.
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Shahid S, Ali M, Legaspi-Humiston D, Wilcoxen J, Pacheco AA. A Kinetic Investigation of the Early Steps in Cytochrome c Nitrite Reductase (ccNiR)-Catalyzed Reduction of Nitrite. Biochemistry 2021; 60:2098-2115. [PMID: 34143605 DOI: 10.1021/acs.biochem.1c00172] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The decaheme enzyme cytochrome c nitrite reductase (ccNiR) catalyzes reduction of nitrite to ammonium in a six-electron, eight-proton process. With a strong reductant as the electron source, ammonium is the sole product. However, intermediates accumulate when weaker reductants are employed, facilitating study of the ccNiR mechanism. Herein, the early stages of Shewanella oneidensis ccNiR-catalyzed nitrite reduction were investigated by using the weak reductants N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and ferrocyanide. In stopped-flow experiments, reduction of nitrite-loaded ccNiR by TMPD generated a transient intermediate, identified as FeH1II(NO2-), where FeH1 represents the ccNiR active site. FeH1II(NO2-) accumulated rapidly and was then more slowly converted to the two-electron-reduced moiety {FeH1NO}7; ccNiR was not reduced beyond the {FeH1NO}7 state. The midpoint potentials for sequential reduction of FeH1III(NO2-) to FeH1II(NO2-) and then to {FeH1NO}7 were estimated to be 130 and 370 mV versus the standard hydrogen electrode, respectively. FeH1II(NO2-) does not accumulate at equilibrium because its reduction to {FeH1NO}7 is so much easier than the reduction of FeH1III(NO2-) to FeH1II(NO2-). With weak reductants, free NO• was released from nitrite-loaded ccNiR. The release of NO• from {FeH1NO}7 is exceedingly slow (k ∼ 0.001 s-1), but it is somewhat faster (k ∼ 0.050 s-1) while FeH1III(NO2-) is being reduced to {FeH1NO}7; then, the release of NO• from the undetectable transient {FeH1NO}6 can compete with reduction of {FeH1NO}6 to {FeH1NO}7. CcNiR appears to be optimized to capture nitrite and minimize the release of free NO•. Nitrite capture is achieved by reducing bound nitrite with even weak electron donors, while NO• release is minimized by stabilizing the substitutionally inert {FeH1NO}7 over the more labile {FeH1NO}6.
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Affiliation(s)
- Shahid Shahid
- Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Mahbbat Ali
- Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Desiree Legaspi-Humiston
- Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Jarett Wilcoxen
- Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - A Andrew Pacheco
- Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
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Sosa Alfaro V, Campeciño J, Tracy M, Elliott SJ, Hegg EL, Lehnert N. Elucidating Electron Storage and Distribution within the Pentaheme Scaffold of Cytochrome c Nitrite Reductase (NrfA). Biochemistry 2021; 60:1853-1867. [PMID: 34061493 DOI: 10.1021/acs.biochem.0c00977] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Cytochrome c nitrite reductases (CcNIR or NrfA) play important roles in the global nitrogen cycle by conserving the usable nitrogen in the soil. Here, the electron storage and distribution properties within the pentaheme scaffold of Geobacter lovleyi NrfA were investigated via electron paramagnetic resonance (EPR) spectroscopy coupled with chemical titration experiments. Initially, a chemical reduction method was established to sequentially add electrons to the fully oxidized protein, 1 equiv at a time. The step-by-step reduction of the hemes was then followed using ultraviolet-visible absorption and EPR spectroscopy. EPR spectral simulations were used to elucidate the sequence of heme reduction within the pentaheme scaffold of NrfA and identify the signals of all five hemes in the EPR spectra. Electrochemical experiments ascertain the reduction potentials for each heme, observed in a narrow range from +10 mV (heme 5) to -226 mV (heme 3) (vs the standard hydrogen electrode). On the basis of quantitative analysis and simulation of the EPR data, we demonstrate that hemes 4 and 5 are reduced first (before the active site heme 1) and serve the purpose of an electron storage unit within the protein. To probe the role of the central heme 3, an H108M NrfA variant was generated where the reduction potential of heme 3 is shifted positively (from -226 to +48 mV). The H108M mutation significantly impacts the distribution of electrons within the pentaheme scaffold and the reduction potentials of the hemes, reducing the catalytic activity of the enzyme to 1% compared to that of the wild type. We propose that this is due to heme 3's important role as an electron gateway in the wild-type enzyme.
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Affiliation(s)
- Victor Sosa Alfaro
- Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
| | - Julius Campeciño
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, Michigan 48824, United States
| | - Matthew Tracy
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
| | - Sean J Elliott
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
| | - Eric L Hegg
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, Michigan 48824, United States
| | - Nicolai Lehnert
- Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
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Campeciño J, Lagishetty S, Wawrzak Z, Sosa Alfaro V, Lehnert N, Reguera G, Hu J, Hegg EL. Cytochrome c nitrite reductase from the bacterium Geobacter lovleyi represents a new NrfA subclass. J Biol Chem 2020; 295:11455-11465. [PMID: 32518164 PMCID: PMC7450111 DOI: 10.1074/jbc.ra120.013981] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2020] [Revised: 06/07/2020] [Indexed: 01/07/2023] Open
Abstract
Cytochrome c nitrite reductase (NrfA) catalyzes the reduction of nitrite to ammonium in the dissimilatory nitrate reduction to ammonium (DNRA) pathway, a process that competes with denitrification, conserves nitrogen, and minimizes nutrient loss in soils. The environmental bacterium Geobacter lovleyi has recently been recognized as a key driver of DNRA in nature, but its enzymatic pathway is still uncharacterized. To address this limitation, here we overexpressed, purified, and characterized G. lovleyi NrfA. We observed that the enzyme crystallizes as a dimer but remains monomeric in solution. Importantly, its crystal structure at 2.55-Å resolution revealed the presence of an arginine residue in the region otherwise occupied by calcium in canonical NrfA enzymes. The presence of EDTA did not affect the activity of G. lovleyi NrfA, and site-directed mutagenesis of this arginine reduced enzymatic activity to <3% of the WT levels. Phylogenetic analysis revealed four separate emergences of Arg-containing NrfA enzymes. Thus, the Ca2+-independent, Arg-containing NrfA from G. lovleyi represents a new subclass of cytochrome c nitrite reductase. Most genera from the exclusive clades of Arg-containing NrfA proteins are also represented in clades containing Ca2+-dependent enzymes, suggesting convergent evolution.
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Affiliation(s)
- Julius Campeciño
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, Michigan, USA
| | - Satyanarayana Lagishetty
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, Michigan, USA
| | - Zdzislaw Wawrzak
- Synchrotron Research Center, Life Science Collaborative Access Team, Northwestern University, Argonne, Illinois, USA
| | - Victor Sosa Alfaro
- Department of Chemistry and Biophysics, The University of Michigan, Ann Arbor, Michigan, USA
| | - Nicolai Lehnert
- Department of Chemistry and Biophysics, The University of Michigan, Ann Arbor, Michigan, USA
| | - Gemma Reguera
- Department of Microbiology & Molecular Genetics, Michigan State University, East Lansing, Michigan, USA
| | - Jian Hu
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, Michigan, USA,Department of Chemistry, Michigan State University, East Lansing, Michigan, USA
| | - Eric L. Hegg
- Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, Michigan, USA,For correspondence: Eric L. Hegg,
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Ali M, Stein N, Mao Y, Shahid S, Schmidt M, Bennett B, Pacheco AA. Trapping of a Putative Intermediate in the Cytochrome c Nitrite Reductase (ccNiR)-Catalyzed Reduction of Nitrite: Implications for the ccNiR Reaction Mechanism. J Am Chem Soc 2019; 141:13358-13371. [DOI: 10.1021/jacs.9b03036] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Mahbbat Ali
- Department of Chemistry and Biochemistry, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Natalia Stein
- Department of Chemistry and Biochemistry, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Yingxi Mao
- Department of Chemistry and Biochemistry, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Shahid Shahid
- Department of Chemistry and Biochemistry, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Marius Schmidt
- Department of Physics, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Brian Bennett
- Department of Physics, Marquette University, Milwaukee, Wisconsin 53233, United States
| | - A. Andrew Pacheco
- Department of Chemistry and Biochemistry, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States
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9
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McGarry JM, Pacheco AA. Upon further analysis, neither cytochrome c 554 from Nitrosomonas europaea nor its F156A variant display NO reductase activity, though both proteins bind nitric oxide reversibly. J Biol Inorg Chem 2018; 23:861-878. [PMID: 29946979 DOI: 10.1007/s00775-018-1582-4] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Accepted: 06/15/2018] [Indexed: 01/02/2023]
Abstract
A re-investigation of the interaction with NO of the small tetraheme protein cytochrome c554 (C554) from Nitrosomonas europaea has shown that the 5-coordinate heme II of the two- or four-electron-reduced protein will nitrosylate reversibly. The process is first order in C554, first order in NO, and second-order overall. The rate constant for NO binding to the heme is 3000 ± 140 M-1s-1, while that for dissociation is 0.034 ± 0.009 s-1; the degree of protein reduction does not appear to significantly influence the nitrosylation rate. In contrast to a previous report (Upadhyay AK, et al. J Am Chem Soc 128:4330, 2006), this study found no evidence of C554-catalyzed NO reduction, either with [Formula: see text] or with [Formula: see text] Some sub-stoichiometric oxidation of the lowest potential heme IV was detected when [Formula: see text] was exposed to an excess of NO, but this is believed to arise from partial intramolecular electron transfer that generates {Fe(NO)}8 at heme II. The vacant heme II coordination site of C554 is crowded by three non-bonding hydrophobic amino acids. After replacing one of these (Phe156) with the smaller alanine, the nitrosylation rate for F156A2- and F156A4- was about 400× faster than for the wild type, though the rate of the reverse denitrosylation process was almost unchanged. Unlike in the wild-type C554, the 6-coordinate low-spin hemes of F156A4- oxidized over the course of several minutes after exposure to NO. Concomitant formation of N2O could explain this heme oxidation, though alternative explanations are equally plausible given the available data.
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Affiliation(s)
- Jennifer M McGarry
- Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, 3210 N. Cramer Street, Milwaukee, WI, 53211, USA
| | - A Andrew Pacheco
- Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, 3210 N. Cramer Street, Milwaukee, WI, 53211, USA.
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Stein N, Love D, Judd ET, Elliott SJ, Bennett B, Pacheco AA. Correlations between the Electronic Properties of Shewanella oneidensis Cytochrome c Nitrite Reductase (ccNiR) and Its Structure: Effects of Heme Oxidation State and Active Site Ligation. Biochemistry 2015; 54:3749-58. [PMID: 26042961 DOI: 10.1021/acs.biochem.5b00330] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The electrochemical properties of Shewanella oneidensis cytochrome c nitrite reductase (ccNiR), a homodimer that contains five hemes per protomer, were investigated by UV-visible and electron paramagnetic resonance (EPR) spectropotentiometries. Global analysis of the UV-vis spectropotentiometric results yielded highly reproducible values for the heme midpoint potentials. These midpoint potential values were then assigned to specific hemes in each protomer (as defined in previous X-ray diffraction studies) by comparing the EPR and UV-vis spectropotentiometric results, taking advantage of the high sensitivity of EPR spectra to the structural microenvironment of paramagnetic centers. Addition of the strong-field ligand cyanide led to a 70 mV positive shift of the active site's midpoint potential, as the cyanide bound to the initially five-coordinate high-spin heme and triggered a high-spin to low-spin transition. With cyanide present, three of the remaining hemes gave rise to distinctive and readily assignable EPR spectral changes upon reduction, while a fourth was EPR-silent. At high applied potentials, interpretation of the EPR spectra in the absence of cyanide was complicated by a magnetic interaction that appears to involve three of five hemes in each protomer. At lower applied potentials, the spectra recorded in the presence and absence of cyanide were similar, which aided global assignment of the signals. The midpoint potential of the EPR-silent heme could be assigned by default, but the assignment was also confirmed by UV-vis spectropotentiometric analysis of the H268M mutant of ccNiR, in which one of the EPR-silent heme's histidine axial ligands was replaced with a methionine.
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Affiliation(s)
- Natalia Stein
- †Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
| | - Daniel Love
- †Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
| | | | | | - Brian Bennett
- §Department of Biophysics, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, United States
| | - A Andrew Pacheco
- †Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
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