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Kirk ML, Kc K. Molybdenum and Tungsten Cofactors and the Reactions They Catalyze. Met Ions Life Sci 2020; 20:/books/9783110589757/9783110589757-015/9783110589757-015.xml. [PMID: 32851830 PMCID: PMC8176780 DOI: 10.1515/9783110589757-015] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2023]
Abstract
The last 20 years have seen a dramatic increase in our mechanistic understanding of the reactions catalyzed by pyranopterin Mo and W enzymes. These enzymes possess a unique cofactor (Moco) that contains a novel ligand in bioinorganic chemistry, the pyranopterin ene-1,2-dithiolate. A synopsis of Moco biosynthesis and structure is presented, along with our current understanding of the role Moco plays in enzymatic catalysis. Oxygen atom transfer (OAT) reactivity is discussed in terms of breaking strong metal-oxo bonds and the mechanism of OAT catalyzed by enzymes of the sulfite oxidase (SO) family that possess dioxo Mo(VI) active sites. OAT reactivity is also discussed in members of the dimethyl sulfoxide (DMSO) reductase family, which possess des-oxo Mo(IV) sites. Finally, we reveal what is known about hydride transfer reactivity in xanthine oxidase (XO) family enzymes and the formate dehydrogenases. The formal hydride transfer reactivity catalyzed by xanthine oxidase family enzymes is complex and cleaves substrate C-H bonds using a mechanism that is distinct from monooxygenases. The chapter primarily highlights developments in the field that have occurred since ~2000, which have contributed to our collective structural and mechanistic understanding of the three canonical pyranopterin Mo enzymes families: XO, SO, and DMSO reductase.
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Djeghader A, Rossotti M, Abdulkarim S, Biaso F, Gerbaud G, Nitschke W, Schoepp-Cothenet B, Soulimane T, Grimaldi S. Structural evidence for a reaction intermediate mimic in the active site of a sulfite dehydrogenase. Chem Commun (Camb) 2020; 56:9850-9853. [DOI: 10.1039/d0cc03634j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We provide structural and spectroscopic evidence for a molybdenum–phosphate adduct mimicking a proposed reaction intermediate in the active site of a prokaryotic sulfite oxidizing enzyme.
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Affiliation(s)
- Ahmed Djeghader
- Department of Chemical Sciences and Bernal Institute
- University of Limerick
- Ireland
| | | | - Saleh Abdulkarim
- Department of Chemical Sciences and Bernal Institute
- University of Limerick
- Ireland
| | | | | | | | | | - Tewfik Soulimane
- Department of Chemical Sciences and Bernal Institute
- University of Limerick
- Ireland
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3
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Mechanism of nitrite-dependent NO synthesis by human sulfite oxidase. Biochem J 2019; 476:1805-1815. [DOI: 10.1042/bcj20190143] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2019] [Revised: 05/21/2019] [Accepted: 06/05/2019] [Indexed: 02/07/2023]
Abstract
AbstractIn addition to nitric oxide (NO) synthases, molybdenum-dependent enzymes have been reported to reduce nitrite to produce NO. Here, we report the stoichiometric reduction in nitrite to NO by human sulfite oxidase (SO), a mitochondrial intermembrane space enzyme primarily involved in cysteine catabolism. Kinetic and spectroscopic studies provide evidence for direct nitrite coordination at the molybdenum center followed by an inner shell electron transfer mechanism. In the presence of the physiological electron acceptor cytochrome c, we were able to close the catalytic cycle of sulfite-dependent nitrite reduction thus leading to steady-state NO synthesis, a finding that strongly supports a physiological relevance of SO-dependent NO formation. By engineering SO variants with reduced intramolecular electron transfer rate, we were able to increase NO generation efficacy by one order of magnitude, providing a mechanistic tool to tune NO synthesis by SO.
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Del Barrio M, Sensi M, Fradale L, Bruschi M, Greco C, de Gioia L, Bertini L, Fourmond V, Léger C. Interaction of the H-Cluster of FeFe Hydrogenase with Halides. J Am Chem Soc 2018; 140:5485-5492. [PMID: 29590528 DOI: 10.1021/jacs.8b01414] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
FeFe hydrogenases catalyze H2 oxidation and production using an "H-cluster", where two Fe ions are bound by an aza-dithiolate (adt) ligand. Various hypotheses have been proposed (by us and others) to explain that the enzyme reversibly inactivates under oxidizing, anaerobic conditions: intramolecular binding of the N atom of adt, formation of the so-called "Hox/inact" state or nonproductive binding of H2 to isomers of the H-cluster. Here, we show that none of the above explains the new finding that the anaerobic, oxidative, H2-dependent reversible inactivation is strictly dependent on the presence of Cl- or Br-. We provide experimental evidence that chloride uncompetitively inhibits the enzyme: it reversibly binds to catalytic intermediates of H2 oxidation (but not to the resting "Hox" state), after which oxidation locks the active site into a stable, saturated, inactive form, the structure of which is proposed here based on DFT calculations. The halides interact with the amine group of the H-cluster but do not directly bind to iron. It should be possible to stabilize the inhibited state in amounts compatible with spectroscopic investigations to explore further this unexpected reactivity of the H-cluster of hydrogenase.
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Affiliation(s)
- Melisa Del Barrio
- Aix Marseille Université , CNRS, Bioénergétique et Ingénierie des Protéines , Marseille , France
| | - Matteo Sensi
- Aix Marseille Université , CNRS, Bioénergétique et Ingénierie des Protéines , Marseille , France
| | - Laura Fradale
- Aix Marseille Université , CNRS, Bioénergétique et Ingénierie des Protéines , Marseille , France
| | - Maurizio Bruschi
- Department of Earth and Environmental Sciences , University of Milano-Bicocca , 20126 Milan , Italy
| | - Claudio Greco
- Department of Earth and Environmental Sciences , University of Milano-Bicocca , 20126 Milan , Italy
| | - Luca de Gioia
- Department of Biotechnologies and Biosciences , University of Milano-Bicocca , 20126 Milan , Italy
| | - Luca Bertini
- Department of Biotechnologies and Biosciences , University of Milano-Bicocca , 20126 Milan , Italy
| | - Vincent Fourmond
- Aix Marseille Université , CNRS, Bioénergétique et Ingénierie des Protéines , Marseille , France
| | - Christophe Léger
- Aix Marseille Université , CNRS, Bioénergétique et Ingénierie des Protéines , Marseille , France
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Enemark JH. Consensus structures of the Mo(v) sites of sulfite-oxidizing enzymes derived from variable frequency pulsed EPR spectroscopy, isotopic labelling and DFT calculations. Dalton Trans 2017. [PMID: 28640289 DOI: 10.1039/c7dt01731f] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Sulfite-oxidizing enzymes from eukaryotes and prokaryotes have five-coordinate distorted square-pyramidal coordination about the molybdenum atom. The paramagnetic Mo(v) state is easily generated, and over the years four distinct CW EPR spectra have been identified, depending upon enzyme source and the reaction conditions, namely high and low pH (hpH and lpH), phosphate inhibited (Pi) and sulfite (or blocked). Extensive studies of these paramagnetic forms of sulfite-oxidizing enzymes using variable frequency pulsed electron spin echo (ESE) spectroscopy, isotopic labeling and density functional theory (DFT) calculations have led to the consensus structures that are described here. Errors in some of the previously proposed structures are corrected.
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Affiliation(s)
- John H Enemark
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85711, USA.
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Maia LB, Moura I, Moura JJ. EPR Spectroscopy on Mononuclear Molybdenum-Containing Enzymes. FUTURE DIRECTIONS IN METALLOPROTEIN AND METALLOENZYME RESEARCH 2017. [DOI: 10.1007/978-3-319-59100-1_4] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
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Kappler U, Enemark JH. Sulfite-oxidizing enzymes. J Biol Inorg Chem 2014; 20:253-64. [DOI: 10.1007/s00775-014-1197-3] [Citation(s) in RCA: 45] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2014] [Accepted: 09/15/2014] [Indexed: 11/24/2022]
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Ha Y, Tenderholt AL, Holm RH, Hedman B, Hodgson KO, Solomon EI. Sulfur K-edge X-ray absorption spectroscopy and density functional theory calculations on monooxo Mo(IV) and bisoxo Mo(VI) bis-dithiolenes: insights into the mechanism of oxo transfer in sulfite oxidase and its relation to the mechanism of DMSO reductase. J Am Chem Soc 2014; 136:9094-105. [PMID: 24884723 PMCID: PMC4073832 DOI: 10.1021/ja503316p] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2014] [Indexed: 12/25/2022]
Abstract
Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two complexes [Mo(IV)O(bdt)2](2-) and [Mo(VI)O2(bdt)2](2-) (bdt = benzene-1,2-dithiolate(2-)) that relate to the reduced and oxidized forms of sulfite oxidase (SO). These are compared with those of previously studied dimethyl sulfoxide reductase (DMSOr) models. DFT calculations supported by the data are extended to evaluate the reaction coordinate for oxo transfer to a phosphite ester substrate. Three possible transition states are found with the one at lowest energy, stabilized by a P-S interaction, in good agreement with experimental kinetics data. Comparison of both oxo transfer reactions shows that in DMSOr, where the oxo is transferred from the substrate to the metal ion, the oxo transfer induces electron transfer, while in SO, where the oxo transfer is from the metal site to the substrate, the electron transfer initiates oxo transfer. This difference in reactivity is related to the difference in frontier molecular orbitals (FMO) of the metal-oxo and substrate-oxo bonds. Finally, these experimentally related calculations are extended to oxo transfer by sulfite oxidase. The presence of only one dithiolene at the enzyme active site selectively activates the equatorial oxo for transfer, and allows facile structural reorganization during turnover.
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Affiliation(s)
- Yang Ha
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
| | - Adam L. Tenderholt
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
| | - Richard H. Holm
- Department
of Chemistry and Chemical Biology, Harvard
University, Cambridge, Massachusetts 02138, United States
| | - Britt Hedman
- Stanford
Synchrotron Radiation Lightsource, SLAC, Stanford University, Menlo Park, California 94025, United States
| | - Keith O. Hodgson
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
- Stanford
Synchrotron Radiation Lightsource, SLAC, Stanford University, Menlo Park, California 94025, United States
| | - Edward I. Solomon
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
- Stanford
Synchrotron Radiation Lightsource, SLAC, Stanford University, Menlo Park, California 94025, United States
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Affiliation(s)
- Russ Hille
- Department of Biochemistry, University of California, Riverside, Riverside, California 92521, United States
| | - James Hall
- Department of Biochemistry, University of California, Riverside, Riverside, California 92521, United States
| | - Partha Basu
- Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
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Abstract
A perspective is provided of recent advances in our understanding of molybdenum-containing enzymes other than nitrogenase, a large and diverse group of enzymes that usually (but not always) catalyze oxygen atom transfer to or from a substrate, utilizing a Mo=O group as donor or acceptor. An emphasis is placed on the diversity of protein structure and reaction catalyzed by each of the three major families of these enzymes.
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Affiliation(s)
- Russ Hille
- Department of Biochemistry, University of California, 1643 Boyce Hall, Riverside, CA 92521, USA.
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Klein EL, Astashkin AV, Raitsimring AM, Enemark JH. Applications of pulsed EPR spectroscopy to structural studies of sulfite oxidizing enzymes(). Coord Chem Rev 2013; 257:110-118. [PMID: 23440026 DOI: 10.1016/j.ccr.2012.05.038] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
Abstract
Sulfite oxidizing enzymes (SOEs), including sulfite oxidase (SO) and bacterial sulfite dehydrogenase (SDH), catalyze the oxidation of sulfite (SO(3) (2-)) to sulfate (SO(4) (2-)). The active sites of SO and SDH are nearly identical, each having a 5-coordinate, pseudo-square-pyramidal Mo with an axial oxo ligand and three equatorial sulfur donor atoms. One sulfur is from a conserved Cys residue and two are from a pyranopterindithiolene (molybdopterin, MPT) cofactor. The identity of the remaining equatorial ligand, which is solvent-exposed, varies during the catalytic cycle. Numerous in vitro studies, particularly those involving electron paramagnetic resonance (EPR) spectroscopy of the Mo(V) states of SOEs, have shown that the identity and orientation of this exchangeable equatorial ligand depends on the buffer pH, the presence and concentration of certain anions in the buffer, as well as specific point mutations in the protein. Until very recently, however, EPR has not been a practical technique for directly probing specific structures in which the solvent-exposed, exchangeable ligand is an O, OH(-), H(2)O, SO(3) (2-), or SO(4) (2-) group, because the primary O and S isotopes ((16)O and (32)S) are magnetically silent (I = 0). This review focuses on the recent advances in the use of isotopic labeling, variable-frequency high resolution pulsed EPR spectroscopy, synthetic model compounds, and DFT calculations to elucidate the roles of various anions, point mutations, and steric factors in the formation, stabilization, and transformation of SOE active site structures.
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Affiliation(s)
- Eric L Klein
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721-0041, USA
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Rajapakshe A, Tollin G, Enemark JH. Kinetic and thermodynamic effects of mutations of human sulfite oxidase. Chem Biodivers 2012; 9:1621-34. [PMID: 22976958 PMCID: PMC3517162 DOI: 10.1002/cbdv.201200010] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Affiliation(s)
- Asha Rajapakshe
- Department of Chemistry and Biochemistry, The University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041 U.S.A
| | - Gordon Tollin
- Department of Chemistry and Biochemistry, The University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041 U.S.A
| | - John H. Enemark
- Department of Chemistry and Biochemistry, The University of Arizona, 1306 E. University Blvd., Tucson, Arizona 85721-0041 U.S.A
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Klein EL, Raitsimring AM, Astashkin AV, Rajapakshe A, Johnson-Winters K, Arnold AR, Potapov A, Goldfarb D, Enemark JH. Identity of the exchangeable sulfur-containing ligand at the Mo(V) center of R160Q human sulfite oxidase. Inorg Chem 2012; 51:1408-18. [PMID: 22225516 DOI: 10.1021/ic201643t] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
In our previous study of the fatal R160Q mutant of human sulfite oxidase (hSO) at low pH (Astashkin et al. J. Am. Chem. Soc.2008, 130, 8471-8480), a new Mo(V) species, denoted "species 1", was observed at low pH values. Species 1 was ascribed to a six-coordinate Mo(V) center with an exchangeable terminal oxo ligand and an equatorial sulfate group on the basis of pulsed EPR spectroscopy and (33)S and (17)O labeling. Here we report new results for species 1 of R160Q, based on substitution of the sulfur-containing ligand by a phosphate group, pulsed EPR spectroscopy in K(a)- and W-bands, and extensive density functional theory (DFT) calculations applied to large, more realistic molecular models of the enzyme active site. The combined results unambiguously show that species 1 has an equatorial sulfite as the only exchangeable ligand. The two types of (17)O signals that are observed arise from the coordinated and remote oxygen atoms of the sulfite ligand. A typical five-coordinate Mo(V) site is compatible with the observed and calculated EPR parameters.
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Affiliation(s)
- Eric L Klein
- Department of Chemistry and Biochemistry, 1306 East University Boulevard, University of Arizona, Tucson, Arizona 85721-0041, United States
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14
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Intramolecular electron transfer in sulfite-oxidizing enzymes: probing the role of aromatic amino acids. J Biol Inorg Chem 2011; 17:345-52. [DOI: 10.1007/s00775-011-0856-x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2011] [Accepted: 10/13/2011] [Indexed: 10/15/2022]
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Pushie MJ, Doonan CJ, Wilson HL, Rajagopalan KV, George GN. Nature of Halide Binding to the Molybdenum Site of Sulfite Oxidase. Inorg Chem 2011; 50:9406-13. [DOI: 10.1021/ic201030u] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- M. Jake Pushie
- Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada
| | - Christian J. Doonan
- Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada
- School of Chemistry and Physics, University of Adelaide, Adelaide, South Australia 5005, Australia
| | - Heather L. Wilson
- School of Medicine, Duke University, Durham, North Carolina 27710, United States
| | - K. V. Rajagopalan
- School of Medicine, Duke University, Durham, North Carolina 27710, United States
| | - Graham. N. George
- Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada
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17
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Abstract
Recent progress in our understanding of the structural and catalytic properties of molybdenum-containing enzymes in eukaryotes is reviewed, along with aspects of the biosynthesis of the cofactor and its insertion into apoprotein.
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Affiliation(s)
- Russ Hille
- Department of Biochemistry, University of California, Riverside, CA 92521
| | - Takeshi Nishino
- Department of Biochemistry and Molecular Biology, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-ku, Tokyo, Japan and Department of Biochemistry, University of California, Riverside, CA 92521
| | - Florian Bittner
- Department of Plant Biology, Technical University of Braunschweig, 38023 Braunschweig, Germany
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Drew SC, Reijerse E, Quentmeier A, Rother D, Friedrich CG, Lubitz W. Spectroscopic Characterization of the Molybdenum Cofactor of the Sulfane Dehydrogenase SoxCD from Paracoccus pantotrophus. Inorg Chem 2011; 50:409-11. [DOI: 10.1021/ic102201f] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Simon C. Drew
- Max-Planck-Institut für Bioanorganische Chemie, 45470 Mülheim an der Ruhr, Germany,
| | - Eduard Reijerse
- Max-Planck-Institut für Bioanorganische Chemie, 45470 Mülheim an der Ruhr, Germany,
| | - Armin Quentmeier
- Fakultät Bio- und Chemieingenieurwesen, Technische Universität Dortmund, 44221 Dortmund, Germany
| | - Dagmar Rother
- Fakultät Bio- und Chemieingenieurwesen, Technische Universität Dortmund, 44221 Dortmund, Germany
| | - Cornelius G. Friedrich
- Fakultät Bio- und Chemieingenieurwesen, Technische Universität Dortmund, 44221 Dortmund, Germany
| | - Wolfgang Lubitz
- Max-Planck-Institut für Bioanorganische Chemie, 45470 Mülheim an der Ruhr, Germany,
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Enemark JH, Raitsimring AM, Astashkin AV, Klein EL. Implications for the mechanism of sulfite oxidizing enzymes from pulsed EPR spectroscopy and DFT calculations for "difficult" nuclei. Faraday Discuss 2011; 148:249-67; discussion 299-314. [PMID: 21322488 PMCID: PMC3079391 DOI: 10.1039/c004404k] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The catalytic mechanisms of sulfite oxidizing enzymes (SOEs) have been investigated by multi-frequency pulsed EPR measurements of "difficult" magnetic nuclei (35.37Cl, 33S, 17O) associated with the Mo(v) center. Extensive DFT calculations have been used to relate the experimental magnetic resonance parameters of these nuclei to specific active site structures. This combined spectroscopic and computational approach has provided new insights concerning the structure/function relationships of the active sites of SOEs, including: (i) the exchange of oxo ligands; (ii) the nature of the blocked forms; and (iii) the role of Cl- in low pH forms.
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Affiliation(s)
- John H Enemark
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721-0041, USA.
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Johnson-Winters K, Nordstrom AR, Davis AC, Tollin G, Enemark JH. Effects of large-scale amino acid substitution in the polypeptide tether connecting the heme and molybdenum domains on catalysis in human sulfite oxidase. Metallomics 2010; 2:766-70. [PMID: 21072368 DOI: 10.1039/c0mt00021c] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Sulfite oxidase (SO) is a molybdenum-cofactor-dependent enzyme that catalyzes the oxidation of sulfite to sulfate, the final step in the catabolism of the sulfur-containing amino acids, cysteine and methionine. The catalytic mechanism of vertebrate SO involves intramolecular electron transfer (IET) from molybdenum to the integral b-type heme of SO and then to exogenous cytochrome c. However, the crystal structure of chicken sulfite oxidase (CSO) has shown that there is a 32 Å distance between the Fe and Mo atoms of the respective heme and molybdenum domains, which are connected by a flexible polypeptide tether. This distance is too long to be consistent with the measured IET rates. Previous studies have shown that IET is viscosity dependent (Feng et al., Biochemistry, 2002, 41, 5816) and also dependent upon the flexibility and length of the tether (Johnson-Winters et al., Biochemistry, 2010, 49, 1290). Since IET in CSO is more rapid than in human sulfite oxidase (HSO) (Feng et al., Biochemistry, 2003, 42, 12235) the tether sequence of HSO has been mutated into that of CSO, and the resultant chimeric HSO enzyme investigated by laser flash photolysis and steady-state kinetics in order to study the specificity of the tether sequence of SO on the kinetic properties. Surprisingly, the IET kinetics of the chimeric HSO protein with the CSO tether sequence are slower than wildtype HSO. This observation raises the possibility that the composition of the non-conserved tether sequence of animal SOs may be optimized for individual species.
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Affiliation(s)
- Kayunta Johnson-Winters
- Department of Chemistry and Biochemistry, The University of Arizona, 1306 E. University Blvd., Tucson, AZ 85721, USA
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Johnson-Winters K, Tollin G, Enemark JH. Elucidating the catalytic mechanism of sulfite oxidizing enzymes using structural, spectroscopic, and kinetic analyses. Biochemistry 2010; 49:7242-54. [PMID: 20666399 DOI: 10.1021/bi1008485] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Sulfite oxidizing enzymes (SOEs) are molybdenum cofactor-dependent enzymes that are found in plants, animals, and bacteria. Sulfite oxidase (SO) is found in animals and plants, while sulfite dehydrogenase (SDH) is found in bacteria. In animals, SO catalyzes the oxidation of toxic sulfite to sulfate as the final step in the catabolism of the sulfur-containing amino acids, methionine and cysteine. In humans, sulfite oxidase deficiency is an inherited recessive disorder that produces severe neonatal neurological problems that lead to early death. Plant SO (PSO) also plays an important role in sulfite detoxification and in addition serves as an intermediate enzyme in the assimilatory reduction of sulfate. In vertebrates, the proposed catalytic mechanism of SO involves two intramolecular one-electron transfer (IET) steps from the molybdenum cofactor to the iron of the integral b-type heme. A similar mechanism is proposed for SDH, involving its molybdenum cofactor and c-type heme. However, PSO, which lacks an integral heme cofactor, uses molecular oxygen as its electron acceptor. Here we review recent results for SOEs from kinetic measurements, computational studies, electron paramagnetic resonance (EPR) spectroscopy, electrochemical measurements, and site-directed mutagenesis on active site residues of SOEs and of the flexible polypepetide tether that connects the heme and molybdenum domains of human SO. Rapid kinetic studies of PSO are also discussed.
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Affiliation(s)
- Kayunta Johnson-Winters
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, USA
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