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Rogers CJ, Asthana D, Brookfield A, Chiesa A, Timco GA, Collison D, Natrajan LS, Carretta S, Winpenny REP, Bowen AM. Modelling Conformational Flexibility in a Spectrally Addressable Molecular Multi‐Qubit Model System. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202207947] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Ciarán J. Rogers
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
| | - Deepak Asthana
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
- Department of Chemistry Ashoka University Sonipat Haryana India
| | - Adam Brookfield
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
| | - Alessandro Chiesa
- Dipartimento di Scienze Matematiche Fisiche e Informatiche Università di Parma 43124 Parma Italy
- INFN–Sezione di Milano-Bicocca Gruppo Collegato di Parma I-43124 Parma Italy
- UdR Parma INSTM I-43124 Parma Italy
| | - Grigore A. Timco
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
| | - David Collison
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
| | - Louise S. Natrajan
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
| | - Stefano Carretta
- Dipartimento di Scienze Matematiche Fisiche e Informatiche Università di Parma 43124 Parma Italy
- INFN–Sezione di Milano-Bicocca Gruppo Collegato di Parma I-43124 Parma Italy
- UdR Parma INSTM I-43124 Parma Italy
| | - Richard E. P. Winpenny
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
| | - Alice M. Bowen
- National Research Facility for Electron Paramagnetic Resonance Spectroscopy Department of Chemistry and Photon Science Institute The University of Manchester Oxford Road Manchester M13 9PL UK
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2
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Rogers CJ, Asthana D, Brookfield A, Chiesa A, Timco GA, Collison D, Natrajan LS, Carretta S, Winpenny REP, Bowen AM. Modelling Conformational Flexibility in a Spectrally Addressable Molecular Multi-Qubit Model System. Angew Chem Int Ed Engl 2022; 61:e202207947. [PMID: 36222278 PMCID: PMC9828767 DOI: 10.1002/anie.202207947] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2022] [Indexed: 11/11/2022]
Abstract
Dipolar coupled multi-spin systems have the potential to be used as molecular qubits. Herein we report the synthesis of a molecular multi-qubit model system with three individually addressable, weakly interacting, spin 1 / 2 ${{ 1/2 }}$ centres of differing g-values. We use pulsed Electron Paramagnetic Resonance (EPR) techniques to characterise and separately address the individual electron spin qubits; CuII , Cr7 Ni ring and a nitroxide, to determine the strength of the inter-qubit dipolar interaction. Orientation selective Relaxation-Induced Dipolar Modulation Enhancement (os-RIDME) detecting across the CuII spectrum revealed a strongly correlated CuII -Cr7 Ni ring relationship; detecting on the nitroxide resonance measured both the nitroxide and CuII or nitroxide and Cr7 Ni ring correlations, with switchability of the interaction based on differing relaxation dynamics, indicating a handle for implementing EPR-based quantum information processing (QIP) algorithms.
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Affiliation(s)
- Ciarán J. Rogers
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK
| | - Deepak Asthana
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK,Department of ChemistryAshoka UniversitySonipatHaryanaIndia
| | - Adam Brookfield
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK
| | - Alessandro Chiesa
- Dipartimento di Scienze Matematiche Fisiche e InformaticheUniversità di Parma43124ParmaItaly,INFN–Sezione di Milano-BicoccaGruppo Collegato di ParmaI-43124ParmaItaly,UdR ParmaINSTMI-43124ParmaItaly
| | - Grigore A. Timco
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK
| | - David Collison
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK
| | - Louise S. Natrajan
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK
| | - Stefano Carretta
- Dipartimento di Scienze Matematiche Fisiche e InformaticheUniversità di Parma43124ParmaItaly,INFN–Sezione di Milano-BicoccaGruppo Collegato di ParmaI-43124ParmaItaly,UdR ParmaINSTMI-43124ParmaItaly
| | - Richard E. P. Winpenny
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK
| | - Alice M. Bowen
- National Research Facility for Electron Paramagnetic Resonance SpectroscopyDepartment of Chemistry and Photon Science InstituteThe University of ManchesterOxford RoadManchesterM13 9PLUK
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3
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Binding site for coenzyme A revealed in the structure of pyruvate:ferredoxin oxidoreductase from Moorella thermoacetica. Proc Natl Acad Sci U S A 2018; 115:3846-3851. [PMID: 29581263 DOI: 10.1073/pnas.1722329115] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Pyruvate:ferredoxin oxidoreductase (PFOR) is a microbial enzyme that uses thiamine pyrophosphate (TPP), three [4Fe-4S] clusters, and coenzyme A (CoA) in the reversible oxidation of pyruvate to generate acetyl-CoA and carbon dioxide. The two electrons that are generated as a result of pyruvate decarboxylation are used in the reduction of low potential ferredoxins, which provide reducing equivalents for central metabolism, including the Wood-Ljungdahl pathway. PFOR is a member of the 2-oxoacid:ferredoxin oxidoreductase (OFOR) superfamily, which plays major roles in both microbial redox reactions and carbon dioxide fixation. Here, we present a set of crystallographic snapshots of the best-studied member of this superfamily, the PFOR from Moorella thermoacetica (MtPFOR). These snapshots include the native structure, those of lactyl-TPP and acetyl-TPP reaction intermediates, and the first of an OFOR with CoA bound. These structural data reveal the binding site of CoA as domain III, the function of which in OFORs was previously unknown, and establish sequence motifs for CoA binding in the OFOR superfamily. MtPFOR structures further show that domain III undergoes a conformational change upon CoA binding that seals off the active site and positions the thiolate of CoA directly adjacent to the TPP cofactor. These structural findings provide a molecular basis for the experimental observation that CoA binding accelerates catalysis by 105-fold.
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4
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Pierce E, Mansoorabadi SO, Can M, Reed GH, Ragsdale SW. Properties of Intermediates in the Catalytic Cycle of Oxalate Oxidoreductase and Its Suicide Inactivation by Pyruvate. Biochemistry 2017; 56:2824-2835. [PMID: 28514140 PMCID: PMC5463272 DOI: 10.1021/acs.biochem.7b00222] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Oxalate:ferredoxin oxidoreductase (OOR) is an unusual member of the thiamine pyrophosphate (TPP)-dependent 2-oxoacid:ferredoxin oxidoreductase (OFOR) family in that it catalyzes the coenzyme A (CoA)-independent conversion of oxalate into 2 equivalents of carbon dioxide. This reaction is surprising because binding of CoA to the acyl-TPP intermediate of other OFORs results in formation of a CoA ester, and in the case of pyruvate:ferredoxin oxidoreductase (PFOR), CoA binding generates the central metabolic intermediate acetyl-CoA and promotes a 105-fold acceleration of the rate of electron transfer. Here we describe kinetic, spectroscopic, and computational results to show that CoA has no effect on catalysis by OOR and describe the chemical rationale for why this cofactor is unnecessary in this enzymatic transformation. Our results demonstrate that, like PFOR, OOR binds pyruvate and catalyzes decarboxylation to form the same hydroxyethylidine-TPP (HE-TPP) intermediate and one-electron transfer to generate the HE-TPP radical. However, in OOR, this intermediate remains stranded at the active site as a covalent inhibitor. These and other results indicate that, like other OFOR family members, OOR generates an oxalate-derived adduct with TPP (oxalyl-TPP) that undergoes decarboxylation and one-electron transfer to form a radical intermediate remaining bound to TPP (dihydroxymethylidene-TPP). However, unlike in PFOR, where CoA binding drives formation of the product, in OOR, proton transfer and a conformational change in the "switch loop" alter the redox potential of the radical intermediate sufficiently to promote the transfer of an electron into the iron-sulfur cluster network, leading directly to a second decarboxylation and completing the catalytic cycle.
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Affiliation(s)
- Elizabeth Pierce
- Department of Biological Chemistry, University of Michigan , Ann Arbor, Michigan 48109-0606, United States
| | - Steven O Mansoorabadi
- Department of Chemistry and Biochemistry, Auburn University , 179 Chemistry Building, Auburn, Alabama 36849, United States
| | - Mehmet Can
- Department of Biological Chemistry, University of Michigan , Ann Arbor, Michigan 48109-0606, United States
| | - George H Reed
- Department of Biochemistry, University of Wisconsin-Madison , 440 Henry Mall, Madison, Wisconsin 53726, United States
| | - Stephen W Ragsdale
- Department of Biological Chemistry, University of Michigan , Ann Arbor, Michigan 48109-0606, United States
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Keller K, Zalibera M, Qi M, Koch V, Wegner J, Hintz H, Godt A, Jeschke G, Savitsky A, Yulikov M. EPR characterization of Mn(ii) complexes for distance determination with pulsed dipolar spectroscopy. Phys Chem Chem Phys 2016; 18:25120-25135. [DOI: 10.1039/c6cp04884f] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
EPR properties of four Mn(ii) complexes and Tikhonov regularization-based analysis of RIDME data containing dipolar overtones are presented.
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Affiliation(s)
- Katharina Keller
- Laboratory of Physical Chemistry
- Department of Chemistry and Applied Bioscience
- ETH Zurich
- 8093 Zurich
- Switzerland
| | - Michal Zalibera
- Max Planck Institut for Chemical Energy Conversion
- D-45470 Mülheim an der Ruhr
- Germany
- Institute of Physical Chemistry and Chemical Physics
- Slovak University of Technology in Bratislava
| | - Mian Qi
- Faculty of Chemistry and Center for Molecular Materials (CM2)
- Bielefeld University
- 33615 Bielefeld
- Germany
| | - Vanessa Koch
- Faculty of Chemistry and Center for Molecular Materials (CM2)
- Bielefeld University
- 33615 Bielefeld
- Germany
| | - Julia Wegner
- Faculty of Chemistry and Center for Molecular Materials (CM2)
- Bielefeld University
- 33615 Bielefeld
- Germany
| | - Henrik Hintz
- Faculty of Chemistry and Center for Molecular Materials (CM2)
- Bielefeld University
- 33615 Bielefeld
- Germany
| | - Adelheid Godt
- Faculty of Chemistry and Center for Molecular Materials (CM2)
- Bielefeld University
- 33615 Bielefeld
- Germany
| | - Gunnar Jeschke
- Laboratory of Physical Chemistry
- Department of Chemistry and Applied Bioscience
- ETH Zurich
- 8093 Zurich
- Switzerland
| | - Anton Savitsky
- Max Planck Institut for Chemical Energy Conversion
- D-45470 Mülheim an der Ruhr
- Germany
| | - Maxim Yulikov
- Laboratory of Physical Chemistry
- Department of Chemistry and Applied Bioscience
- ETH Zurich
- 8093 Zurich
- Switzerland
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6
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One-carbon chemistry of oxalate oxidoreductase captured by X-ray crystallography. Proc Natl Acad Sci U S A 2015; 113:320-5. [PMID: 26712008 DOI: 10.1073/pnas.1518537113] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Thiamine pyrophosphate (TPP)-dependent oxalate oxidoreductase (OOR) metabolizes oxalate, generating two molecules of CO2 and two low-potential electrons, thus providing both the carbon and reducing equivalents for operation of the Wood-Ljungdahl pathway of acetogenesis. Here we present structures of OOR in which two different reaction intermediate bound states have been trapped: the covalent adducts between TPP and oxalate and between TPP and CO2. These structures, along with the previously determined structure of substrate-free OOR, allow us to visualize how active site rearrangements can drive catalysis. Our results suggest that OOR operates via a bait-and-switch mechanism, attracting substrate into the active site through the presence of positively charged and polar residues, and then altering the electrostatic environment through loop and side chain movements to drive catalysis. This simple but elegant mechanism explains how oxalate, a molecule that humans and most animals cannot break down, can be used for growth by acetogenic bacteria.
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7
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Gibson M, Brignole EJ, Pierce E, Can M, Ragsdale SW, Drennan CL. The Structure of an Oxalate Oxidoreductase Provides Insight into Microbial 2-Oxoacid Metabolism. Biochemistry 2015; 54:4112-20. [PMID: 26061898 PMCID: PMC4498597 DOI: 10.1021/acs.biochem.5b00521] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Thiamine pyrophosphate (TPP), a derivative of vitamin B1, is a versatile and ubiquitous cofactor. When coupled with [4Fe-4S] clusters in microbial 2-oxoacid:ferredoxin oxidoreductases (OFORs), TPP is involved in catalyzing low-potential redox reactions that are important for the synthesis of key metabolites and the reduction of N2, H(+), and CO2. We have determined the high-resolution (2.27 Å) crystal structure of the TPP-dependent oxalate oxidoreductase (OOR), an enzyme that allows microbes to grow on oxalate, a widely occurring dicarboxylic acid that is found in soil and freshwater and is responsible for kidney stone disease in humans. OOR catalyzes the anaerobic oxidation of oxalate, harvesting the low-potential electrons for use in anaerobic reduction and fixation of CO2. We compare the OOR structure to that of the only other structurally characterized OFOR family member, pyruvate:ferredoxin oxidoreductase. This side-by-side structural analysis highlights the key similarities and differences that are relevant for the chemistry of this entire class of TPP-utilizing enzymes.
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Affiliation(s)
- Marcus
I. Gibson
- Department
of Chemistry, Massachusetts Institute of
Technology, 77 Massachusetts
Avenue, Cambridge, Massachusetts 02139, United States
| | - Edward J. Brignole
- Department
of Chemistry, Massachusetts Institute of
Technology, 77 Massachusetts
Avenue, Cambridge, Massachusetts 02139, United States,Howard
Hughes Medical Institute, Massachusetts
Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
| | - Elizabeth Pierce
- Department
of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
| | - Mehmet Can
- Department
of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
| | - Stephen W. Ragsdale
- Department
of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
| | - Catherine L. Drennan
- Department
of Chemistry, Massachusetts Institute of
Technology, 77 Massachusetts
Avenue, Cambridge, Massachusetts 02139, United States,Howard
Hughes Medical Institute, Massachusetts
Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States,Department
of Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States,E-mail: . Telephone: (617) 253-5622
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9
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Razzaghi S, Qi M, Nalepa AI, Godt A, Jeschke G, Savitsky A, Yulikov M. RIDME Spectroscopy with Gd(III) Centers. J Phys Chem Lett 2014; 5:3970-5. [PMID: 26276479 DOI: 10.1021/jz502129t] [Citation(s) in RCA: 65] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/15/2023]
Abstract
The relaxation induced dipolar modulation enhancement (RIDME) technique is applied at W-band microwave frequencies around 94 GHz to a pair of Gd(III) complexes that are connected by a rodlike spacer, and the extraction of the interspin distance distribution is discussed. A dipolar pattern derived from RIDME experimental data is a superposition of Pake-like dipolar patterns corresponding to the fundamental dipolar interaction and higher harmonics thereof. Intriguingly, the relative weights of the stretched patterns do not depend significantly on mixing time. As much larger modulation depths can be achieved than in double electron-electron resonance distance measurements at the same frequency, Gd(III)-Gd(III) RIDME may become attractive for structural characterization of biomacromolecules and biomolecular complexes.
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Affiliation(s)
- Sahand Razzaghi
- †Laboratory of Physical Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland
| | - Mian Qi
- ‡Faculty of Chemistry and Center for Molecular Materials, Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany
| | - Anna I Nalepa
- §Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany
| | - Adelheid Godt
- ‡Faculty of Chemistry and Center for Molecular Materials, Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany
| | - Gunnar Jeschke
- †Laboratory of Physical Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland
| | - Anton Savitsky
- §Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany
| | - Maxim Yulikov
- †Laboratory of Physical Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland
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10
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Roy A, Sarrou I, Vaughn MD, Astashkin AV, Ghirlanda G. De Novo Design of an Artificial Bis[4Fe-4S] Binding Protein. Biochemistry 2013; 52:7586-94. [DOI: 10.1021/bi401199s] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Anindya Roy
- Department
of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States
| | - Iosifina Sarrou
- Department
of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States
| | - Michael D. Vaughn
- Department
of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States
| | - Andrei V. Astashkin
- Department
of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States
| | - Giovanna Ghirlanda
- Department
of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States
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11
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Hirsh DJ, McCracken J, Biczo R, Gesuelli KA. Saturation-Recovery EPR with Nitroxyl Radical–Dy(III) Spin Pairs: Distances and Orientations. J Phys Chem B 2013; 117:11960-77. [DOI: 10.1021/jp406159m] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Donald J. Hirsh
- Department of Chemistry, The College of New Jersey, Ewing, New Jersey 08628, United States
| | - Joselle McCracken
- Department of Chemistry, The College of New Jersey, Ewing, New Jersey 08628, United States
| | - Ryan Biczo
- Department of Chemistry, The College of New Jersey, Ewing, New Jersey 08628, United States
| | - Kelly-Ann Gesuelli
- Department of Chemistry, The College of New Jersey, Ewing, New Jersey 08628, United States
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12
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Ezhevskaya M, Bordignon E, Polyhach Y, Moens L, Dewilde S, Jeschke G, Van Doorslaer S. Distance determination between low-spin ferric haem and nitroxide spin label using DEER: the neuroglobin case. Mol Phys 2013. [DOI: 10.1080/00268976.2013.813592] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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13
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Astashkin AV, Rajapakshe A, Cornelison MJ, Johnson-Winters K, Enemark JH. Determination of the distance between the Mo(V) and Fe(III) heme centers of wild type human sulfite oxidase by pulsed EPR spectroscopy. J Phys Chem B 2012; 116:1942-50. [PMID: 22229742 DOI: 10.1021/jp210578f] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Intramolecular electron transfer (IET) between the molybdenum and heme centers of vertebrate sulfite oxidase (SO) is proposed to be a key step in the catalytic cycle of the enzyme. However, the X-ray crystallographic distance between these centers, R(MoFe) = 32.3 Å, appears to be too long for the rapid IET rates observed in liquid solution. The Mo and heme domains are linked by a flexible tether, and it has been proposed that dynamic interdomain motion brings the two metal centers closer together and thereby facilitates rapid IET. To date, there have been no direct distance measurements for SO in solution that would support or contradict this model. In this work, pulsed electron-electron double resonance (ELDOR) and relaxation induced dipolar modulation enhancement (RIDME) techniques were used to obtain information about R(MoFe) in the Mo(V)Fe(III) state of wild type recombinant human SO in frozen glassy solution. Surprisingly, the data obtained suggest a fixed structure with R(MoFe) = 32 Å, similar to that determined by X-ray crystallography for chicken SO, although the orientation of the R(MoFe) radius-vector with respect to the heme center was found to be somewhat different. The implications of these findings for the flexible tether model are discussed.
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Affiliation(s)
- Andrei V Astashkin
- Department of Chemistry & Biochemistry, University of Arizona, Tucson, Arizona 85721, USA.
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14
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Reed GH, Ragsdale SW, Mansoorabadi SO. Radical reactions of thiamin pyrophosphate in 2-oxoacid oxidoreductases. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2011; 1824:1291-8. [PMID: 22178227 DOI: 10.1016/j.bbapap.2011.11.010] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/04/2011] [Accepted: 11/28/2011] [Indexed: 11/27/2022]
Abstract
Thiamin pyrophosphate (TPP) is essential in carbohydrate metabolism in all forms of life. TPP-dependent decarboxylation reactions of 2-oxo-acid substrates result in enamine adducts between the thiazolium moiety of the coenzyme and decarboxylated substrate. These central enamine intermediates experience different fates from protonation in pyruvate decarboxylase to oxidation by the 2-oxoacid dehydrogenase complexes, the pyruvate oxidases, and 2-oxoacid oxidoreductases. Virtually all of the TPP-dependent enzymes, including pyruvate decarboxylase, can be assayed by 1-electron redox reactions linked to ferricyanide. Oxidation of the enamines is thought to occur via a 2-electron process in the 2-oxoacid dehydrogenase complexes, wherein acyl group transfer is associated with reduction of the disulfide of the lipoamide moiety. However, discrete 1-electron steps occur in the oxidoreductases, where one or more [4Fe-4S] clusters mediate the electron transfer reactions to external electron acceptors. These radical intermediates can be detected in the absence of the acyl-group acceptor, coenzyme A (CoASH). The π-electron system of the thiazolium ring stabilizes the radical. The extensively delocalized character of the radical is evidenced by quantitative analysis of nuclear hyperfine splitting tensors as detected by electron paramagnetic resonance (EPR) spectroscopy and by electronic structure calculations. The second electron transfer step is markedly accelerated by the presence of CoASH. While details of the second electron transfer step and its facilitation by CoASH remain elusive, expected redox properties of potential intermediates limit possible scenarios. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.
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Affiliation(s)
- George H Reed
- Department of Biochemistry, University of Wisconsin-Madison, 1710 University Avenue, Madison, WI 53726, USA.
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15
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Song Y, Meade TJ, Astashkin A, Klein E, Enemark J, Raitsimring A. Pulsed dipolar spectroscopy distance measurements in biomacromolecules labeled with Gd(III) markers. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2011; 210:59-68. [PMID: 21388847 PMCID: PMC3081411 DOI: 10.1016/j.jmr.2011.02.010] [Citation(s) in RCA: 66] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2011] [Revised: 02/03/2011] [Accepted: 02/08/2011] [Indexed: 05/11/2023]
Abstract
This work demonstrates the feasibility of using Gd(III) tags for long-range Double Electron Electron Resonance (DEER) distance measurements in biomacromolecules. Double-stranded 14- base pair Gd(III)-DNA conjugates were synthesized and investigated at K(a) band. For the longest Gd(III) tag the average distance and average deviation between Gd(III) ions determined from the DEER time domains was about 59±12Å. This result demonstrates that DEER measurements with Gd(III) tags can be routinely carried out for distances of at least 60Å, and analysis indicates that distance measurements up to 100Å are possible. Compared with commonly used nitroxide labels, Gd(III)-based labels will be most beneficial for the detection of distance variations in large biomacromolecules, with an emphasis on large scale changes in shape or distance. Tracking the folding/unfolding and domain interactions of proteins and the conformational changes in DNA are examples of such applications.
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Affiliation(s)
- Y. Song
- Departments of Chemistry; Molecular Biosciences; Neurobiology & Physiology; and Radiology, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
| | - T. J. Meade
- Departments of Chemistry; Molecular Biosciences; Neurobiology & Physiology; and Radiology, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
| | - A.V. Astashkin
- Department of Chemistry and Biochemistry, University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041, USA
| | - E.L. Klein
- Department of Chemistry and Biochemistry, University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041, USA
| | - J.H. Enemark
- Department of Chemistry and Biochemistry, University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041, USA
| | - A. Raitsimring
- Department of Chemistry and Biochemistry, University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041, USA
- Corresponding Author: Arnold Raitsimring, Department of Chemistry and Biochemistry, University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041, USA.
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Astashkin AV. Integrated refocused virtual ESEEM: detection of nuclear transition spectra without dead time and blind spots. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2011; 209:69-74. [PMID: 21273102 DOI: 10.1016/j.jmr.2011.01.001] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/28/2010] [Revised: 01/04/2011] [Accepted: 01/04/2011] [Indexed: 05/30/2023]
Abstract
General expressions describing the refocused stimulated (RS) and refocused virtual (RV) electron spin echo envelope modulations (ESEEM) generated with the same basic four-pulse sequence are derived. It is shown that integration of the 3D time domain trace over the two "low-resolution" time intervals (those between the first and second and between the third and fourth microwave pulses) results in a dead time-free 1D ESEEM trace in the "high-resolution" dimension (i.e., the time interval between the second and third microwave pulses) that only contains harmonics with the fundamental frequencies of nuclear transitions. The practical implementation of the integrated RS ESEEM requires pulse swapping, which leads to unrecoverable distortions in the ESEEM traces and the resulting spectra. The integrated RV ESEEM is free from such distortions and represents a robust practical technique for obtaining dead time- and blind spots-free spectra of nuclear transitions, without homonuclear combination lines. As an application example, the integrated RV ESEEM was used to obtain the spectrum of a strongly-coupled proton of the OH ligand of the Mo(V) active center of the low-pH form of the molybdoenzyme sulfite oxidase.
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Affiliation(s)
- Andrei V Astashkin
- University of Arizona, Department of Chemistry and Biochemistry, Tucson, AZ 85721, USA.
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Pierce E, Becker DF, Ragsdale SW. Identification and characterization of oxalate oxidoreductase, a novel thiamine pyrophosphate-dependent 2-oxoacid oxidoreductase that enables anaerobic growth on oxalate. J Biol Chem 2010; 285:40515-24. [PMID: 20956531 PMCID: PMC3003350 DOI: 10.1074/jbc.m110.155739] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2010] [Revised: 10/15/2010] [Indexed: 11/06/2022] Open
Abstract
Moorella thermoacetica is an anaerobic acetogen, a class of bacteria that is found in the soil, the animal gastrointestinal tract, and the rumen. This organism engages the Wood-Ljungdahl pathway of anaerobic CO(2) fixation for heterotrophic or autotrophic growth. This paper describes a novel enzyme, oxalate oxidoreductase (OOR), that enables M. thermoacetica to grow on oxalate, which is produced in soil and is a common component of kidney stones. Exposure to oxalate leads to the induction of three proteins that are subunits of OOR, which oxidizes oxalate coupled to the production of two electrons and CO(2) or bicarbonate. Like other members of the 2-oxoacid:ferredoxin oxidoreductase family, OOR contains thiamine pyrophosphate and three [Fe(4)S(4)] clusters. However, unlike previously characterized members of this family, OOR does not use coenzyme A as a substrate. Oxalate is oxidized with a k(cat) of 0.09 s(-1) and a K(m) of 58 μM at pH 8. OOR also oxidizes a few other 2-oxoacids (which do not induce OOR) also without any requirement for CoA. The enzyme transfers its reducing equivalents to a broad range of electron acceptors, including ferredoxin and the nickel-dependent carbon monoxide dehydrogenase. In conjunction with the well characterized Wood-Ljungdahl pathway, OOR should be sufficient for oxalate metabolism by M. thermoacetica, and it constitutes a novel pathway for oxalate metabolism.
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Affiliation(s)
- Elizabeth Pierce
- From the Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109-0606 and
| | - Donald F. Becker
- the Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664
| | - Stephen W. Ragsdale
- From the Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109-0606 and
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Astashkin AV, Elmore BO, Fan W, Guillemette JG, Feng C. Pulsed EPR determination of the distance between heme iron and FMN centers in a human inducible nitric oxide synthase. J Am Chem Soc 2010; 132:12059-67. [PMID: 20695464 DOI: 10.1021/ja104461p] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Mammalian nitric oxide synthase (NOS) is a homodimeric flavo-hemoprotein that catalyzes the oxidation of L-arginine to nitric oxide (NO). Regulation of NO biosynthesis by NOS is primarily through control of interdomain electron transfer (IET) processes in NOS catalysis. The IET from the flavin mononucleotide (FMN) to heme domains is essential in the delivery of electrons required for O(2) activation in the heme domain and the subsequent NO synthesis by NOS. The NOS output state for NO production is an IET-competent complex of the FMN-binding domain and heme domain, and thereby it facilitates the IET from the FMN to the catalytic heme site. The structure of the functional output state has not yet been determined. In the absence of crystal structure data for NOS holoenzyme, it is important to experimentally determine the Fe...FMN distance to provide a key calibration for computational docking studies and for the IET kinetics studies. Here we used the relaxation-induced dipolar modulation enhancement (RIDME) technique to measure the electron spin echo envelope modulation caused by the dipole interactions between paramagnetic FMN and heme iron centers in the [Fe(III)][FMNH(*)] (FMNH(*): FMN semiquinone) form of a human inducible NOS (iNOS) bidomain oxygenase/FMN construct. The FMNH(*)...Fe distance has been directly determined from the RIDME spectrum. This distance (18.8 +/- 0.1 A) is in excellent agreement with the IET rate constant measured by laser flash photolysis [Feng, C. J.; Dupont, A.; Nahm, N.; Spratt, D.; Hazzard, J. T.; Weinberg, J.; Guillemette, J.; Tollin, G.; Ghosh, D. K. J. Biol. Inorg. Chem. 2009, 14, 133-142].
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Affiliation(s)
- Andrei V Astashkin
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, USA
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Yang Z, Kise D, Saxena S. An Approach towards the Measurement of Nanometer Range Distances Based on Cu2+ Ions and ESR. J Phys Chem B 2010; 114:6165-74. [DOI: 10.1021/jp911637s] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Zhongyu Yang
- Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
| | - Drew Kise
- Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
| | - Sunil Saxena
- Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
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20
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Milikisyants S, Scarpelli F, Finiguerra MG, Ubbink M, Huber M. A pulsed EPR method to determine distances between paramagnetic centers with strong spectral anisotropy and radicals: the dead-time free RIDME sequence. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2009; 201:48-56. [PMID: 19758831 DOI: 10.1016/j.jmr.2009.08.008] [Citation(s) in RCA: 140] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/22/2009] [Revised: 07/15/2009] [Accepted: 08/07/2009] [Indexed: 05/09/2023]
Abstract
Methods to determine distances between paramagnetic metal centers and radicals are scarce. This is unfortunate because paramagnetic metal centers are frequent in biological systems and so far have not been employed much as distance markers. Successful pulse sequences that directly target the dipolar interactions cannot be applied to paramagnetic metal centers with fast relaxation rates and large g-anisotropy, if no echos can be detected and the excitation bandwidth is not sufficient to cover a sufficiently large part of the spectrum. The RIDME method Kulik et al. (2002) [20] circumvents this problem by making use of the T(1)-induced spin-flip of the transition-metal ion. Designed to measure distance between such a fast relaxing metal center and a radical, it suffers from a dead time problem. We show that this is severe because the anisotropy of the metal center broadens the dipolar curves, which therefore, only can be analyzed if the full curve is known. Here, we introduce five-pulse RIDME (5p-RIDME) that is intrinsically dead-time free. Proper functioning of the sequence is demonstrated on a nitroxide biradical. The distance between a low-spin Fe(III) center and a spin label in spin-labeled cytochrome f shows the complete dipolar trace of a transition-metal ion center and a spin label, yielding the distance expected from the structure.
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Affiliation(s)
- Sergey Milikisyants
- Department of Molecular Physics, Huygens Laboratory, Leiden University, 2300 RA Leiden, The Netherlands
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22
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Bode BE, Plackmeyer J, Bolte M, Prisner TF, Schiemann O. PELDOR on an exchange coupled nitroxide copper(II) spin pair. J Organomet Chem 2009. [DOI: 10.1016/j.jorganchem.2008.11.029] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
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Ragsdale SW, Pierce E. Acetogenesis and the Wood-Ljungdahl pathway of CO(2) fixation. BIOCHIMICA ET BIOPHYSICA ACTA 2008; 1784:1873-98. [PMID: 18801467 PMCID: PMC2646786 DOI: 10.1016/j.bbapap.2008.08.012] [Citation(s) in RCA: 704] [Impact Index Per Article: 44.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/02/2008] [Revised: 08/12/2008] [Accepted: 08/13/2008] [Indexed: 01/04/2023]
Abstract
Conceptually, the simplest way to synthesize an organic molecule is to construct it one carbon at a time. The Wood-Ljungdahl pathway of CO(2) fixation involves this type of stepwise process. The biochemical events that underlie the condensation of two one-carbon units to form the two-carbon compound, acetate, have intrigued chemists, biochemists, and microbiologists for many decades. We begin this review with a description of the biology of acetogenesis. Then, we provide a short history of the important discoveries that have led to the identification of the key components and steps of this usual mechanism of CO and CO(2) fixation. In this historical perspective, we have included reflections that hopefully will sketch the landscape of the controversies, hypotheses, and opinions that led to the key experiments and discoveries. We then describe the properties of the genes and enzymes involved in the pathway and conclude with a section describing some major questions that remain unanswered.
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Affiliation(s)
- Stephen W Ragsdale
- Department of Biological Chemistry, MSRB III, 5301, 1150 W. Medical Center Drive, University of Michigan, Ann Arbor, MI 48109-0606, USA.
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Abstract
The biochemistry of acetogenesis is reviewed. The microbes that catalyze the reactions that are central to acetogenesis are described and the focus is on the enzymology of the process. These microbes play a key role in the global carbon cycle, producing over 10 trillion kilograms of acetic acid annually. Acetogens have the ability to anaerobically convert carbon dioxide and CO into acetyl-CoA by the Wood-Ljungdahl pathway, which is linked to energy conservation. They also can convert the six carbons of glucose stoichiometrically into 3 mol of acetate using this pathway. Acetogens and other anaerobic microbes (e.g., sulfate reducers and methanogens) use the Wood-Ljungdahl pathway for cell carbon synthesis. Important enzymes in this pathway that are covered in this review are pyruvate ferredoxin oxidoreductase, CO dehydrogenase/acetyl-CoA synthase, a corrinoid iron-sulfur protein, a methyltransferase, and the enzymes involved in the conversion of carbon dioxide to methyl-tetrahydrofolate.
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Affiliation(s)
- Stephen W Ragsdale
- Department of Biological Chemistry, University of Michigan, Ann Arbor, MI, USA.
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25
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Kluger R, Tittmann K. Thiamin diphosphate catalysis: enzymic and nonenzymic covalent intermediates. Chem Rev 2008; 108:1797-833. [PMID: 18491870 DOI: 10.1021/cr068444m] [Citation(s) in RCA: 216] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Ronald Kluger
- Davenport Laboratories, Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6 Canada.
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26
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Bode BE, Plackmeyer J, Prisner TF, Schiemann O. PELDOR Measurements on a Nitroxide-Labeled Cu(II) Porphyrin: Orientation Selection, Spin-Density Distribution, and Conformational Flexibility. J Phys Chem A 2008; 112:5064-73. [DOI: 10.1021/jp710504k] [Citation(s) in RCA: 115] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Bela E. Bode
- Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, Max-von-Laue-Strasse 7, J. W. Goethe-University, 60438 Frankfurt am Main, Germany
| | - Jörn Plackmeyer
- Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, Max-von-Laue-Strasse 7, J. W. Goethe-University, 60438 Frankfurt am Main, Germany
| | - Thomas F. Prisner
- Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, Max-von-Laue-Strasse 7, J. W. Goethe-University, 60438 Frankfurt am Main, Germany
| | - Olav Schiemann
- Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, Max-von-Laue-Strasse 7, J. W. Goethe-University, 60438 Frankfurt am Main, Germany
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Yang Z, Becker J, Saxena S. On Cu(II)-Cu(II) distance measurements using pulsed electron electron double resonance. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2007; 188:337-43. [PMID: 17825593 DOI: 10.1016/j.jmr.2007.08.006] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/16/2007] [Revised: 08/10/2007] [Accepted: 08/10/2007] [Indexed: 05/17/2023]
Abstract
The effects of orientational selectivity on the 4-pulse electron electron double resonance (PELDOR) ESR spectra of coupled Cu(II)-Cu(II) spins are presented. The data were collected at four magnetic fields on a poly-proline peptide containing two Cu(II) centers. The Cu(II)-PELDOR spectra of this peptide do not change appreciably with magnetic field at X-band. The data were analyzed by adapting the theory of Maryasov, Tsvetkov, and Raap [A.G. Maryasov, Y.D. Tsvetkov, J. Raap, Weakly coupled radical pairs in solids:ELDOR in ESE structure studies, Appl. Magn. Reson. 14 (1998) 101-113]. Simulations indicate that orientational effects are important for Cu(II)-PELDOR. Based on simulations, the field-independence of the PELDOR data for this peptide is likely due to two effects. First, for this peptide, the Cu(II) g-tensor(s) are in a very specific orientation with respect to the interspin vector. Second, the flexibility of the peptide washes out the orientation effects. These effects reduce the suitability of the poly-proline based peptide as a good model system to experimentally probe orientational effects in such experiments. An average Cu(II)-Cu(II) distance of 2.1-2.2 nm was determined, which is consistent with earlier double quantum coherence ESR results.
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Affiliation(s)
- Zhongyu Yang
- Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
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28
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Affiliation(s)
- Stephen W Ragsdale
- Department of Biochemistry, Beadle Center, University of Nebraska, Lincoln, Nebraska 68588-0664, USA.
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29
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Denysenkov VP, Prisner TF, Stubbe J, Bennati M. High-field pulsed electron-electron double resonance spectroscopy to determine the orientation of the tyrosyl radicals in ribonucleotide reductase. Proc Natl Acad Sci U S A 2006; 103:13386-90. [PMID: 16938868 PMCID: PMC1569173 DOI: 10.1073/pnas.0605851103] [Citation(s) in RCA: 116] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Class I ribonucleotide reductases (RNRs) are composed of two subunits, R1 and R2. The R2 subunit contains the essential diferric cluster-tyrosyl radical (Y.) cofactor, and R1 is the site of the conversion of nucleoside diphosphates to 2'-deoxynucleoside diphosphates. It has been proposed that the function of the tyrosyl radical in R2 is to generate a transient thiyl radical (C439.) in R1 over a distance of 35 A, which in turn initiates the reduction process. EPR distance measurements provide a tool with which to study the mechanism of radical initiation in class I RNRs. These types of experiments at low magnetic fields and frequencies (0.3 T, 9 GHz) give insight into interradical distances and populations. We present a pulsed electron-electron double resonance (PELDOR) experiment at high EPR frequency (180-GHz electron Larmor frequency) that detects the dipolar interaction between the Y.s in each protomer of RNR R2 from Escherichia coli. We observe a correlation between the orientation-dependent dipolar interaction and their resolved g-tensors. This information has allowed us to define the relative orientation of two radicals embedded in the active homodimeric protein in solution. This experiment demonstrates that high-field PELDOR spectroscopy is a powerful tool with which to study the assembly of proteins that contain multiple paramagnetic centers.
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Affiliation(s)
- V. P. Denysenkov
- *Institute for Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, J. W. Goethe University, 60438 Frankfurt am Main, Germany; and
| | - T. F. Prisner
- *Institute for Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, J. W. Goethe University, 60438 Frankfurt am Main, Germany; and
| | - J. Stubbe
- Departments of Chemistry and Biology, Massachusetts Institute of Technology, Cambridge, MA 02139
- To whom correspondence may be addressed. E-mail:
or
| | - M. Bennati
- *Institute for Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, J. W. Goethe University, 60438 Frankfurt am Main, Germany; and
- To whom correspondence may be addressed. E-mail:
or
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