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de Lichtenberg C, Rapatskiy L, Reus M, Heyno E, Schnegg A, Nowaczyk MM, Lubitz W, Messinger J, Cox N. Assignment of the slowly exchanging substrate water of nature's water-splitting cofactor. Proc Natl Acad Sci U S A 2024; 121:e2319374121. [PMID: 38437550 PMCID: PMC10945779 DOI: 10.1073/pnas.2319374121] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2023] [Accepted: 02/12/2024] [Indexed: 03/06/2024] Open
Abstract
Identifying the two substrate water sites of nature's water-splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O-O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time-Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time-Resolved 17O-Electron-electron Double resonance detected NMR (TR-17O-EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen-isotope label into the substrate sites after addition of H218O to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with H217O. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and WS are both enhanced by Ca2+→Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being WS. This strongly restricts options for the mechanism of water oxidation.
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Affiliation(s)
- Casper de Lichtenberg
- Department of Chemistry- Ångström Laboratorium, Uppsala University, UppsalaS-75120, Sweden
- Department of Chemistry, Chemical Biological Centre, Umeå University, UmeåS-90187, Sweden
| | - Leonid Rapatskiy
- Max Planck Institute for Chemical Energy Conversion, Mülheim an der RuhrD-45470, Germany
| | - Michael Reus
- Max Planck Institute for Chemical Energy Conversion, Mülheim an der RuhrD-45470, Germany
| | - Eiri Heyno
- Max Planck Institute for Chemical Energy Conversion, Mülheim an der RuhrD-45470, Germany
| | - Alexander Schnegg
- Max Planck Institute for Chemical Energy Conversion, Mülheim an der RuhrD-45470, Germany
| | - Marc M. Nowaczyk
- Department of Plant Biochemistry, Ruhr-Universität Bochum, BochumD-44780, Germany
| | - Wolfgang Lubitz
- Max Planck Institute for Chemical Energy Conversion, Mülheim an der RuhrD-45470, Germany
| | - Johannes Messinger
- Department of Chemistry- Ångström Laboratorium, Uppsala University, UppsalaS-75120, Sweden
- Department of Chemistry, Chemical Biological Centre, Umeå University, UmeåS-90187, Sweden
| | - Nicholas Cox
- Max Planck Institute for Chemical Energy Conversion, Mülheim an der RuhrD-45470, Germany
- Research School of Chemistry, Australian National University, Acton ACT2601, Australia
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Enemark JH. Mechanistic complexities of sulfite oxidase: An enzyme with multiple domains, subunits, and cofactors. J Inorg Biochem 2023; 247:112312. [PMID: 37441922 DOI: 10.1016/j.jinorgbio.2023.112312] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2023] [Revised: 05/13/2023] [Accepted: 07/01/2023] [Indexed: 07/15/2023]
Abstract
Sulfite oxidase (SO) deficiency, an inherited disease that causes severe neonatal neurological problems and early death, arises from defects in the biosynthesis of the molybdenum cofactor (Moco) (general sulfite oxidase deficiency) or from inborn errors in the SUOX gene for SO (isolated sulfite oxidase deficiency, ISOD). The X-ray structure of the highly homologous homonuclear dimeric chicken sulfite oxidase (cSO) provides a template for locating ISOD mutation sites in human sulfite oxidase (hSO). Catalysis occurs within an individual subunit of hSO, but mutations that disrupt the hSO dimer are pathological. The catalytic cycle of SO involves five metal oxidation states (MoVI, MoV, MoIV, FeIII, FeII), two intramolecular electron transfer (IET) steps, and couples a two-electron oxygen atom transfer reaction at the Mo center with two one-electron transfers from the integral b-type heme to exogenous cytochrome c, the physiological oxidant. Several ISOD examples are analyzed using steady-state, stopped-flow, and laser flash photolysis kinetics and physical measurements of recombinant variants of hSO and native cSO. In the structure of cSO, Mo…Fe = 32 Å, much too long for efficient IET through the protein. Interdomain motion that brings the Mo and heme centers closer together to facilitate IET is supported indirectly by decreasing the length of the interdomain tether, by changes in the charges of surface residues of the Mo and heme domains, as well as by preliminary molecular dynamics calculations. However, direct dynamic measurements of interdomain motion are in their infancy.
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Affiliation(s)
- John H Enemark
- Department of Chemistry and Biochemistry, The University of Arizona, 1306 East University Blvd, Tucson, AZ 85721-0041, United States of America.
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Colaneri MJ, Vitali J. Probing Axial Water Bound to Copper in Tutton Salt Using Single Crystal 17O-ESEEM Spectroscopy. J Phys Chem A 2018; 122:6214-6224. [PMID: 29989412 DOI: 10.1021/acs.jpca.8b04075] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
Electron spin-echo envelope modulation (ESEEM) signals attributed to axial water bound to Cu2+ have been detected and analyzed in Cu(II)-doped 17O-water-enriched potassium zinc sulfate hexahydrate (Tutton salt) crystals. The magnetic field orientation dependences of low frequency modulations were measured to fit hyperfine and quadrupole coupling tensors of a 17O ( I = 5/2) nucleus. The hyperfine tensor ( A xx, A yy, A zz: 0.13, 0.23, -3.81 MHz) exhibits almost axial symmetry with the largest value directed normal to the metal equatorial plane in the host structure. Comparisons with quantum chemical calculations position this nucleus about 2.3 Å from the copper. The isotropic coupling (-1.15 MHz) is small and reflects the weak axial water interaction with a dx2-y2 unshared orbital of copper. The 17O-water quadrupole interaction parameters ( e2 qQ/ h = 6.4 MHz and η = 0.93) are close to the average of those found in a variety of solid hydrates. In addition, the coupling tensor directions correlate very closely with the O8 water geometry, with the maximum quadrupole direction 3° from the water plane normal, and its minimum coupling about 2° from the H-H direction. In almost all previous magnetic resonance 17O-water studies, the quadrupole tensor orientation was based on theoretical considerations. This work represents one of the few experimental confirmations of its principal axis frame. When Cu2+ dopes into the Tutton salt, a Jahn-Teller distortion interchanges the relative long and intermediate metal O7 and O8 bond lengths of the zinc host. Therefore, only those unit cells containing the impurity conform to the pure copper Tutton structure. This study provides further support for this model. Moreover, coupling interactions from distant H217O such as in the present case have important implications in studies of copper enzymes and proteins where substrates have been proposed to displace weakly bound water in the active site.
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Affiliation(s)
- Michael J Colaneri
- Department of Chemistry and Physics , State University of New York at Old Westbury , Old Westbury , New York 11568 , United States
| | - Jacqueline Vitali
- Department of Physics and Department of Biological, Geological and Environmental Sciences , Cleveland State University , Cleveland , Ohio 44115 , United States
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4
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Ramirez Cohen M, Mendelman N, Radoul M, Wilson TD, Savelieff MG, Zimmermann H, Kaminker I, Feintuch A, Lu Y, Goldfarb D. Thiolate Spin Population of Type I Copper in Azurin Derived from 33S Hyperfine Coupling. Inorg Chem 2017; 56:6163-6174. [DOI: 10.1021/acs.inorgchem.7b00167] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Affiliation(s)
- Marie Ramirez Cohen
- Department of Chemical
Physics, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Netanel Mendelman
- Department of Chemical
Physics, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Marina Radoul
- Department of Chemical
Physics, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Tiffany D. Wilson
- Department of Chemistry, University of Illinois, Urbana, Illinois 61801, United States
| | - Masha G. Savelieff
- Department of Chemistry, University of Illinois, Urbana, Illinois 61801, United States
| | - Herbert Zimmermann
- Abteilung Biophysik, Max Planck-Institut für Medizinische Forschung, Heidelberg 69120, Germany
| | - Ilia Kaminker
- Department of Chemical
Physics, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Akiva Feintuch
- Department of Chemical
Physics, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Yi Lu
- Department of Chemistry, University of Illinois, Urbana, Illinois 61801, United States
| | - Daniella Goldfarb
- Department of Chemical
Physics, Weizmann Institute of Science, Rehovot 76100, Israel
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Rendon J, Biaso F, Ceccaldi P, Toci R, Seduk F, Magalon A, Guigliarelli B, Grimaldi S. Elucidating the Structures of the Low- and High-pH Mo(V) Species in Respiratory Nitrate Reductase: A Combined EPR, 14,15N HYSCORE, and DFT Study. Inorg Chem 2017; 56:4423-4435. [DOI: 10.1021/acs.inorgchem.6b03129] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Julia Rendon
- Aix Marseille Univ, CNRS, BIP, Marseille, France
| | | | - Pierre Ceccaldi
- Aix Marseille Univ, CNRS, BIP, Marseille, France
- Aix Marseille Univ, CNRS, LCB, Marseille, France
| | - René Toci
- Aix Marseille Univ, CNRS, LCB, Marseille, France
| | - Farida Seduk
- Aix Marseille Univ, CNRS, LCB, Marseille, France
| | - Axel Magalon
- Aix Marseille Univ, CNRS, LCB, Marseille, France
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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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7
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Heinze K. Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms. Coord Chem Rev 2015. [DOI: 10.1016/j.ccr.2015.04.010] [Citation(s) in RCA: 40] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
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Sparacino-Watkins C, Stolz JF, Basu P. Nitrate and periplasmic nitrate reductases. Chem Soc Rev 2014; 43:676-706. [PMID: 24141308 DOI: 10.1039/c3cs60249d] [Citation(s) in RCA: 189] [Impact Index Per Article: 18.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
The nitrate anion is a simple, abundant and relatively stable species, yet plays a significant role in global cycling of nitrogen, global climate change, and human health. Although it has been known for quite some time that nitrate is an important species environmentally, recent studies have identified potential medical applications. In this respect the nitrate anion remains an enigmatic species that promises to offer exciting science in years to come. Many bacteria readily reduce nitrate to nitrite via nitrate reductases. Classified into three distinct types--periplasmic nitrate reductase (Nap), respiratory nitrate reductase (Nar) and assimilatory nitrate reductase (Nas), they are defined by their cellular location, operon organization and active site structure. Of these, Nap proteins are the focus of this review. Despite similarities in the catalytic and spectroscopic properties Nap from different Proteobacteria are phylogenetically distinct. This review has two major sections: in the first section, nitrate in the nitrogen cycle and human health, taxonomy of nitrate reductases, assimilatory and dissimilatory nitrate reduction, cellular locations of nitrate reductases, structural and redox chemistry are discussed. The second section focuses on the features of periplasmic nitrate reductase where the catalytic subunit of the Nap and its kinetic properties, auxiliary Nap proteins, operon structure and phylogenetic relationships are discussed.
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9
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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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10
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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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11
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Rapatskiy L, Cox N, Savitsky A, Ames WM, Sander J, Nowaczyk MM, Rögner M, Boussac A, Neese F, Messinger J, Lubitz W. Detection of the Water-Binding Sites of the Oxygen-Evolving Complex of Photosystem II Using W-Band 17O Electron–Electron Double Resonance-Detected NMR Spectroscopy. J Am Chem Soc 2012; 134:16619-34. [DOI: 10.1021/ja3053267] [Citation(s) in RCA: 224] [Impact Index Per Article: 18.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Affiliation(s)
- Leonid Rapatskiy
- Max-Planck-Institut für
Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim
an der Ruhr, Germany
| | - Nicholas Cox
- Max-Planck-Institut für
Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim
an der Ruhr, Germany
| | - Anton Savitsky
- Max-Planck-Institut für
Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim
an der Ruhr, Germany
| | - William M. Ames
- Max-Planck-Institut für
Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim
an der Ruhr, Germany
| | - Julia Sander
- Plant
Biochemistry, Ruhr University Bochum, Universitätsstrasse
150, D-44780 Bochum, Germany
| | - Marc. M. Nowaczyk
- Plant
Biochemistry, Ruhr University Bochum, Universitätsstrasse
150, D-44780 Bochum, Germany
| | - Matthias Rögner
- Plant
Biochemistry, Ruhr University Bochum, Universitätsstrasse
150, D-44780 Bochum, Germany
| | - Alain Boussac
- iBiTec-S, URA UMR 8221, CEA Saclay,
91191 Gif-sur-Yvette, France
| | - Frank Neese
- Max-Planck-Institut für
Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim
an der Ruhr, Germany
| | - Johannes Messinger
- Department of Chemistry, Chemical
Biological Centre (KBC), Umeå University, S-90187 Umeå, Sweden
| | - Wolfgang Lubitz
- Max-Planck-Institut für
Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim
an der Ruhr, Germany
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12
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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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13
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Rajapakshe A, Astashkin AV, Klein EL, Reichmann D, Mendel RR, Bittner F, Enemark JH. Structural studies of the molybdenum center of mitochondrial amidoxime reducing component (mARC) by pulsed EPR spectroscopy and 17O-labeling. Biochemistry 2011; 50:8813-22. [PMID: 21916412 DOI: 10.1021/bi2005762] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
Mitochondrial amidoxime reducing components (mARC-1 and mARC-2) represent a novel group of Mo-containing enzymes in eukaryotes. These proteins form the catalytic part of a three-component enzyme complex known to be responsible for the reductive activation of several N-hydroxylated prodrugs. No X-ray crystal structures are available for these enzymes as yet. A previous biochemical investigation [Wahl, B., et al. (2010) J. Biol. Chem., 285, 37847-37859 ] has revealed that two of the Mo coordination positions are occupied by sulfur atoms from a pyranopterindithiolate (molybdopterin, MPT) cofactor. In this work, we have used continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy and density functional theoretical (DFT) calculations to determine the nature of remaining ligands in the Mo(V) state of the active site of mARC-2. Experiments with samples in D(2)O have identified the exchangeable equatorial ligand as a hydroxyl group. Experiments on samples in H(2)(17)O-enriched buffer have shown the presence of a slowly exchangeable axial oxo ligand. Comparison of the experimental (1)H and (17)O hyperfine interactions with those calculated using DFT has shown that the remaining nonexchangeable equatorial ligand is, most likely, protein-derived and that the possibility of an equatorial oxo ligand can be excluded.
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Affiliation(s)
- Asha Rajapakshe
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, USA
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14
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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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15
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Rajapakshe A, Johnson-Winters K, Nordstrom AR, Meyers KT, Emesh S, Astashkin AV, Enemark JH. Characterization of chloride-depleted human sulfite oxidase by electron paramagnetic resonance spectroscopy: experimental evidence for the role of anions in product release. Biochemistry 2010; 49:5154-9. [PMID: 20491442 DOI: 10.1021/bi902172n] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The Mo(V) state of the molybdoenzyme sulfite oxidase (SO) is paramagnetic and can be studied by electron paramagnetic resonance (EPR) spectroscopy. Vertebrate SO at pH <7 and >9 exhibits characteristic EPR spectra that correspond to two structurally different forms of the Mo(V) active center termed the low-pH (lpH) and high-pH (hpH) forms, respectively. Both EPR forms have an exchangeable equatorial OH ligand, but its orientation in the two forms is different. It has been hypothesized that the formation of the lpH species is dependent on the presence of chloride. In this work, we have prepared and purified samples of the wild type and various mutants of human SO that are depleted of chloride. These samples do not exhibit the typical lpH EPR spectrum at low pH but rather exhibit spectra that are characteristic of the blocked species that contains an exchangeable equatorial sulfate ligand. Addition of chloride to these samples results in the disappearance of the blocked species and the formation of the lpH species. Similarly, if chloride is added before sulfite, the lpH species is formed instead of the blocked one. Qualitatively similar results were observed for samples of sulfite-oxidizing enzymes from other organisms that were previously reported to form a blocked species at low pH. However, the depletion of chloride has no effect upon the formation of the hpH species.
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Affiliation(s)
- Asha Rajapakshe
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, USA
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