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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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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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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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Qiu JA, Wilson HL, Rajagopalan KV. Structure-based alteration of substrate specificity and catalytic activity of sulfite oxidase from sulfite oxidation to nitrate reduction. Biochemistry 2012; 51:1134-47. [PMID: 22263579 DOI: 10.1021/bi201206v] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Eukaryotic sulfite oxidase is a dimeric protein that contains the molybdenum cofactor and catalyzes the metabolically essential conversion of sulfite to sulfate as the terminal step in the metabolism of cysteine and methionine. Nitrate reductase is an evolutionarily related molybdoprotein in lower organisms that is essential for growth on nitrate. In this study, we describe human and chicken sulfite oxidase variants in which the active site has been modified to alter substrate specificity and activity from sulfite oxidation to nitrate reduction. On the basis of sequence alignments and the known crystal structure of chicken sulfite oxidase, two residues are conserved in nitrate reductases that align with residues in the active site of sulfite oxidase. On the basis of the crystal structure of yeast nitrate reductase, both positions were mutated in human sulfite oxidase and chicken sulfite oxidase. The resulting double-mutant variants demonstrated a marked decrease in sulfite oxidase activity but gained nitrate reductase activity. An additional methionine residue in the active site was proposed to be important in nitrate catalysis, and therefore, the triple variant was also produced. The nitrate reducing ability of the human sulfite oxidase triple mutant was nearly 3-fold greater than that of the double mutant. To obtain detailed structural data for the active site of these variants, we introduced the analogous mutations into chicken sulfite oxidase to perform crystallographic analysis. The crystal structures of the Mo domains of the double and triple mutants were determined to 2.4 and 2.1 Å resolution, respectively.
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Affiliation(s)
- James A Qiu
- Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, United States
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Simultaneous true, gated, and coupled electron-transfer reactions and energetics of protein rearrangement. J Inorg Biochem 2012; 106:143-50. [DOI: 10.1016/j.jinorgbio.2011.09.017] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2011] [Revised: 09/06/2011] [Accepted: 09/09/2011] [Indexed: 11/19/2022]
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6
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Li W, Fan W, Elmore BO, Feng C. Effect of solution viscosity on intraprotein electron transfer between the FMN and heme domains in inducible nitric oxide synthase. FEBS Lett 2011; 585:2622-6. [PMID: 21803041 DOI: 10.1016/j.febslet.2011.07.022] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2011] [Revised: 07/13/2011] [Accepted: 07/14/2011] [Indexed: 11/15/2022]
Abstract
The FMN-heme intraprotein electron transfer (IET) kinetics in a human inducible NOS (iNOS) oxygenase/FMN construct were determined by laser flash photolysis as a function of solution viscosity (1.0-3.0 cP). In the presence of ethylene glycol or sucrose, an appreciable decrease in the IET rate constant value was observed with an increase in the solution viscosity. The IET rate constant is inversely proportional to the viscosity for both viscosogens. This demonstrates that viscosity, and not other properties of the added viscosogens, causes the dependence of IET rates on the solvent concentration. The IET kinetics results indicate that the FMN-heme IET in iNOS is gated by a large conformational change of the FMN domain. The kinetics and NOS flavin fluorescence results together indicate that the docked FMN/heme state is populated transiently.
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Affiliation(s)
- Wenbing Li
- Department of Pharmaceutical Sciences, University of New Mexico, Albuquerque, NM 87131, USA
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Magalon A, Fedor JG, Walburger A, Weiner JH. Molybdenum enzymes in bacteria and their maturation. Coord Chem Rev 2011. [DOI: 10.1016/j.ccr.2010.12.031] [Citation(s) in RCA: 87] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
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8
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Kappler U. Bacterial sulfite-oxidizing enzymes. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2011; 1807:1-10. [DOI: 10.1016/j.bbabio.2010.09.004] [Citation(s) in RCA: 60] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/12/2010] [Revised: 09/05/2010] [Accepted: 09/14/2010] [Indexed: 11/25/2022]
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Rapson TD, Kappler U, Hanson GR, Bernhardt PV. Short circuiting a sulfite oxidising enzyme with direct electrochemistry: Active site substitutions and their effect on catalysis and electron transfer. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2011; 1807:108-18. [DOI: 10.1016/j.bbabio.2010.09.005] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/14/2010] [Revised: 09/06/2010] [Accepted: 09/15/2010] [Indexed: 10/19/2022]
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Kalimuthu P, Tkac J, Kappler U, Davis JJ, Bernhardt PV. Highly Sensitive and Stable Electrochemical Sulfite Biosensor Incorporating a Bacterial Sulfite Dehydrogenase. Anal Chem 2010; 82:7374-9. [DOI: 10.1021/ac101493y] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Palraj Kalimuthu
- Centre for Metals in Biology, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, 4072, Australia, and Department of Chemistry, University of Oxford, South Parks Road, Oxford, U.K
| | - Jan Tkac
- Centre for Metals in Biology, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, 4072, Australia, and Department of Chemistry, University of Oxford, South Parks Road, Oxford, U.K
| | - Ulrike Kappler
- Centre for Metals in Biology, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, 4072, Australia, and Department of Chemistry, University of Oxford, South Parks Road, Oxford, U.K
| | - Jason J. Davis
- Centre for Metals in Biology, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, 4072, Australia, and Department of Chemistry, University of Oxford, South Parks Road, Oxford, U.K
| | - Paul V. Bernhardt
- Centre for Metals in Biology, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, 4072, Australia, and Department of Chemistry, University of Oxford, South Parks Road, Oxford, U.K
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Rapson TD, Astashkin AV, Johnson-Winters K, Bernhardt PV, Kappler U, Raitsimring AM, Enemark JH. Pulsed EPR investigations of the Mo(V) centers of the R55Q and R55M variants of sulfite dehydrogenase from Starkeya novella. J Biol Inorg Chem 2010; 15:505-14. [PMID: 20084533 DOI: 10.1007/s00775-009-0619-0] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2009] [Accepted: 12/13/2009] [Indexed: 10/20/2022]
Abstract
Continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopy have been used to characterize two variants of bacterial sulfite dehydrogenase (SDH) from Starkeya novella in which the conserved active-site arginine residue (R55) is replaced by a neutral amino acid residue. Substitution by the hydrophobic methionine residue (SDH(R55M)) has essentially no effect on the pH dependence of the EPR properties of the Mo(V) center, even though the X-ray structure of this variant shows that the methionine residue is rotated away from the Mo center and a sulfate anion is present in the active-site pocket (Bailey et al. in J Biol Chem 284:2053-2063, 2009). For SDH(R55M) only the high-pH form is observed, and samples prepared in H(2)(17)O-enriched buffer show essentially the same (17)O hyperfine interaction and nuclear quadrupole interaction parameters as SDH(WT) enzyme. However, the pH dependence of the EPR spectra of SDH(R55Q), in which the positively charged arginine is replaced by the neutral hydrophilic glutamine, differs significantly from that of SDH(WT). For SDH(R55Q) the blocked form with bound sulfate is generated at low pH, as verified by (33)S couplings observed upon reduction with (33)S-labeled sulfite. This observation of bound sulfate for SDH(R55Q) supports our previous hypothesis that sulfite-oxidizing enzymes can exhibit multiple pathways for electron transfer and product release (Emesh et al. in Biochemistry 48:2156-2163, 2009). At pH > or = 8 the high-pH form dominates for SDH(R55Q).
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Affiliation(s)
- Trevor D Rapson
- Centre for Metals in Biology, School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, QLD, 4072, Australia
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Emesh S, Rapson TD, Rajapakshe A, Kappler U, Bernhardt PV, Tollin G, Enemark JH. Intramolecular electron transfer in sulfite-oxidizing enzymes: elucidating the role of a conserved active site arginine. Biochemistry 2009; 48:2156-63. [PMID: 19226119 DOI: 10.1021/bi801553q] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
All reported sulfite-oxidizing enzymes have a conserved arginine in their active site which hydrogen bonds to the equatorial oxygen ligand on the Mo atom. Previous studies on the pathogenic R160Q mutant of human sulfite oxidase (HSO) have shown that Mo-heme intramolecular electron transfer (IET) is dramatically slowed when positive charge is lost at this position. To improve our understanding of the function that this conserved positively charged residue plays in IET, we have studied the equivalent uncharged substitutions R55Q and R55M as well as the positively charged substitution R55K in bacterial sulfite dehydrogenase (SDH). The heme and molybdenum cofactor (Moco) subunits are tightly associated in SDH, which makes it an ideal system for improving our understanding of residue function in IET without the added complexity of the interdomain movement that occurs in HSO. Unexpectedly, the uncharged SDH variants (R55Q and R55M) exhibited increased IET rate constants relative to that of the wild type (3-4-fold) when studied by laser flash photolysis. The gain in function observed in SDH(R55Q) and SDH(R55M) suggests that the reduction in the level of IET seen in HSO(R160Q) is not due to a required role of this residue in the IET pathway itself, but to the fact that it plays an important role in heme orientation during the interdomain movement necessary for IET in HSO (as seen in viscosity experiments). The pH profiles of SDH(WT), SDH(R55M), and SDH(R55Q) show that the arginine substitution also alters the behavior of the Mo-heme IET equilibrium (K(eq)) and rate constants (k(et)) of both variants with respect to the SDH(WT) enzyme. SDH(WT) has a k(et) that is independent of pH and a K(eq) that increases as pH decreases; on the other hand, both SDH(R55M) and SDH(R55Q) have a k(et) that increases as pH decreases, and SDH(R55M) has a K(eq) that is pH-independent. IET in the SDH(R55Q) variant is inhibited by sulfate in laser flash photolysis experiments, a behavior that differs from that of SDH(WT), but which also occurs in HSO. IET in SDH(R55K) is slower than in SDH(WT). A new analysis of the possible mechanistic pathways for sulfite-oxidizing enzymes is presented and related to available kinetic and EPR results for these enzymes.
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Affiliation(s)
- Safia Emesh
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, USA
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Bailey S, Rapson T, Johnson-Winters K, Astashkin AV, Enemark JH, Kappler U. Molecular basis for enzymatic sulfite oxidation: how three conserved active site residues shape enzyme activity. J Biol Chem 2009; 284:2053-63. [PMID: 19004819 PMCID: PMC2629113 DOI: 10.1074/jbc.m807718200] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2008] [Revised: 11/12/2008] [Indexed: 11/06/2022] Open
Abstract
Sulfite dehydrogenases (SDHs) catalyze the oxidation and detoxification of sulfite to sulfate, a reaction critical to all forms of life. Sulfite-oxidizing enzymes contain three conserved active site amino acids (Arg-55, His-57, and Tyr-236) that are crucial for catalytic competency. Here we have studied the kinetic and structural effects of two novel and one previously reported substitution (R55M, H57A, Y236F) in these residues on SDH catalysis. Both Arg-55 and His-57 were found to have key roles in substrate binding. An R55M substitution increased Km(sulfite)(app) by 2-3 orders of magnitude, whereas His-57 was required for maintaining a high substrate affinity at low pH when the imidazole ring is fully protonated. This effect may be mediated by interactions of His-57 with Arg-55 that stabilize the position of the Arg-55 side chain or, alternatively, may reflect changes in the protonation state of sulfite. Unlike what is seen for SDHWT and SDHY236F, the catalytic turnover rates of SDH R55M and SDHH57A are relatively insensitive to pH (approximately 60 and 200 s(-1), respectively). On the structural level, striking kinetic effects appeared to correlate with disorder (in SDHH57A and SDHY236F) or absence of Arg-55 (SDHR55M), suggesting that Arg-55 and the hydrogen bonding interactions it engages in are crucial for substrate binding and catalysis. The structure of SDHR55M has sulfate bound at the active site, a fact that coincides with a significant increase in the inhibitory effect of sulfate in SDHR55M. Thus, Arg-55 also appears to be involved in enabling discrimination between the substrate and product in SDH.
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Affiliation(s)
- Susan Bailey
- Molecular Biophysics Group, Science and Technology Facilities Council Daresbury Laboratory, Warrington WA4 4AD, United Kingdom
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Abstract
Phototrophic sulfur bacteria are characterized by oxidizing various inorganic sulfur compounds for use as electron donors in carbon dioxide fixation during anoxygenic photosynthetic growth. These bacteria are divided into the purple sulfur bacteria (PSB) and the green sulfur bacteria (GSB). They utilize various combinations of sulfide, elemental sulfur, and thiosulfate and sometimes also ferrous iron and hydrogen as electron donors. This review focuses on the dissimilatory and assimilatory metabolism of inorganic sulfur compounds in these bacteria and also briefly discusses these metabolisms in other types of anoxygenic phototrophic bacteria. The biochemistry and genetics of sulfur compound oxidation in PSB and GSB are described in detail. A variety of enzymes catalyzing sulfur oxidation reactions have been isolated from GSB and PSB (especially Allochromatium vinosum, a representative of the Chromatiaceae), and many are well characterized also on a molecular genetic level. Complete genome sequence data are currently available for 10 strains of GSB and for one strain of PSB. We present here a genome-based survey of the distribution and phylogenies of genes involved in oxidation of sulfur compounds in these strains. It is evident from biochemical and genetic analyses that the dissimilatory sulfur metabolism of these organisms is very complex and incompletely understood. This metabolism is modular in the sense that individual steps in the metabolism may be performed by different enzymes in different organisms. Despite the distant evolutionary relationship between GSB and PSB, their photosynthetic nature and their dependency on oxidation of sulfur compounds resulted in similar ecological roles in the sulfur cycle as important anaerobic oxidizers of sulfur compounds.
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Dahl C. Inorganic Sulfur Compounds as Electron Donors in Purple Sulfur Bacteria. SULFUR METABOLISM IN PHOTOTROPHIC ORGANISMS 2008. [DOI: 10.1007/978-1-4020-6863-8_15] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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Feng C, Tollin G, Enemark JH. Sulfite oxidizing enzymes. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2007; 1774:527-39. [PMID: 17459792 PMCID: PMC1993547 DOI: 10.1016/j.bbapap.2007.03.006] [Citation(s) in RCA: 143] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2007] [Revised: 03/02/2007] [Indexed: 11/19/2022]
Abstract
Sulfite oxidizing enzymes are essential mononuclear molybdenum (Mo) proteins involved in sulfur metabolism of animals, plants and bacteria. There are three such enzymes presently known: (1) sulfite oxidase (SO) in animals, (2) SO in plants, and (3) sulfite dehydrogenase (SDH) in bacteria. X-ray crystal structures of enzymes from all three sources (chicken SO, Arabidopsis thaliana SO, and Starkeya novella SDH) show nearly identical square pyramidal coordination around the Mo atom, even though the overall structures of the proteins and the presence of additional cofactors vary. This structural information provides a molecular basis for studying the role of specific amino acids in catalysis. Animal SO catalyzes the final step in the degradation of sulfur-containing amino acids and is critical in detoxifying excess sulfite. Human SO deficiency is a fatal genetic disorder that leads to early death, and impaired SO activity is implicated in sulfite neurotoxicity. Animal SO and bacterial SDH contain both Mo and heme domains, whereas plant SO only has the Mo domain. Intraprotein electron transfer (IET) between the Mo and Fe centers in animal SO and bacterial SDH is a key step in the catalysis, which can be studied by laser flash photolysis in the presence of deazariboflavin. IET studies on animal SO and bacterial SDH clearly demonstrate the similarities and differences between these two types of sulfite oxidizing enzymes. Conformational change is involved in the IET of animal SO, in which electrostatic interactions may play a major role in guiding the docking of the heme domain to the Mo domain prior to electron transfer. In contrast, IET measurements for SDH demonstrate that IET occurs directly through the protein medium, which is distinctly different from that in animal SO. Point mutations in human SO can result in significantly impaired IET or no IET, thus rationalizing their fatal effects. The recent developments in our understanding of sulfite oxidizing enzyme mechanisms that are driven by a combination of molecular biology, rapid kinetics, pulsed electron paramagnetic resonance (EPR), and computational techniques are the subject of this review.
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Affiliation(s)
- Changjian Feng
- College of Pharmacy, University of New Mexico, Albuquerque, NM 87131
| | - Gordon Tollin
- Department of Biochemistry and Molecular Biophysics, University of Arizona, Tucson, Arizona 85721, USA
| | - John H. Enemark
- Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA
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Wilson HL, Wilkinson SR, Rajagopalan KV. The G473D mutation impairs dimerization and catalysis in human sulfite oxidase. Biochemistry 2006; 45:2149-60. [PMID: 16475804 DOI: 10.1021/bi051609l] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Among the mutations identified in patients with isolated sulfite oxidase deficiency, the G473D variant is of particular interest since sedimentation analysis reveals that this variant is a monomer, and the importance of the wild-type dimeric state of mammalian sulfite oxidase is not yet well understood. Analysis of recombinant G473D sulfite oxidase indicated that it is severely impaired both in the ability to bind sulfite and in catalysis, with a second-order rate constant 5 orders of magnitude lower than that of the wild type. To elucidate the specific reasons for the severe effects seen in the G473D variant, several other variants were created, including G473A, G473W, and the double mutant R212A/G473D. Despite the inability to form a stable dimer, the G473W variant had 5-fold higher activity than G473D and nearly wild-type activity at pH 7.0 when ferricyanide was the electron acceptor. In contrast, the R212A/G473D variant demonstrated some ability to oligomerize but had undetectable activity. The G473A variant retained the ability to dimerize and had steady-state activity that was comparable to that of the wild type. Furthermore, stopped-flow analysis of the reductive half-reaction of this variant yielded a rate constant nearly 3 times higher than that of the wild type. Examination of the secondary structures of the variants by CD spectroscopy indicated significant random-coil formation in G473D, G473W, and R212A/G473D. These results demonstrate that both the charge and the large size of an Asp residue in this position contribute to the severe effects seen in a patient with the G473D mutation, by causing partial misfolding and monomerization of sulfite oxidase and attenuating both substrate binding and catalytic efficiency during the reaction cycle.
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Affiliation(s)
- Heather L Wilson
- Department of Biochemistry, Box 3711, Duke University Medical Center, Durham, North Carolina 27710, USA
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Feng C, Wilson HL, Tollin G, Astashkin AV, Hazzard JT, Rajagopalan KV, Enemark JH. The pathogenic human sulfite oxidase mutants G473D and A208D are defective in intramolecular electron transfer. Biochemistry 2006; 44:13734-43. [PMID: 16229463 DOI: 10.1021/bi050907f] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Mutations G473D and A208D were identified in patients with isolated sulfite oxidase (SO) deficiency, and the equivalent amino acids (G451 and A186, respectively) have been localized to the vicinity of the molybdopterin active site in the X-ray structure of chicken SO [Kisker, C., Schindelin, H., Pacheco, A., Wehbi, W., Garrett, R. M., Rajagopalan, K. V., Enemark, J. H., and Rees, D. C. (1997) Cell 91, 973-983]. To assess the effects of these mutations in human SO, steady-state kinetic studies of enzyme turnover and laser flash photolysis measurements of intramolecular electron transfer (IET) rate constants between the reduced heme [Fe(II)] and Mo(VI) centers were carried out in the recombinant G473D, G473A, G473W, G473D/R212A, and A208D human SO mutants. In the G473D and A208D mutants, the IET rate constants at pH 6.0 are decreased by 3 orders of magnitude relative to that of the wild type. Steady-state kinetic measurements indicate that the IET process is the rate-limiting step in the catalytic cycle of these two mutants. Thus, the large decreases in the IET rate constants and the kcat values, and the large increases in the Km(sulfite) values, rationalize the fatal impact of these mutations. Far-UV CD spectra of G473D indicate that the protein backbone conformation is remarkably changed, and the sedimentation equilibrium indicates that the protein is monomeric. Furthermore, EPR studies also suggest that the active site structure of the Mo(V) form of A208D is different from that of the wild type. In contrast, similar studies on G473A show that it is dimeric, that its Mo(V) active site structure is similar to that of the wild type, and that its IET rate constant is only 2.6-fold smaller than that of the wild type. IET in G473W is severely impaired, and no IET is observed for G473D/R212A. In chicken SO, the equivalent residues (G451 and A186) are both buried inside the protein. Thus, for human SO, the mutations to charged residues at the equivalent sites most likely cause crucial global or localized structural changes, and expose an alternative docking site that may compete with the Mo domain for docking of the heme, thereby retarding IET and efficient catalytic turnover of the sulfite oxidation reaction.
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Affiliation(s)
- Changjian Feng
- Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA
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Kappler U, Bailey S. Molecular Basis of Intramolecular Electron Transfer in Sulfite-oxidizing Enzymes Is Revealed by High Resolution Structure of a Heterodimeric Complex of the Catalytic Molybdopterin Subunit and a c-Type Cytochrome Subunit. J Biol Chem 2005; 280:24999-5007. [PMID: 15863498 DOI: 10.1074/jbc.m503237200] [Citation(s) in RCA: 109] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Sulfite-oxidizing molybdoenzymes convert the highly reactive and therefore toxic sulfite to sulfate and have been identified in insects, animals, plants, and bacteria. Although the well studied enzymes from higher animals serve to detoxify sulfite that arises from the catabolism of sulfur-containing amino acids, the bacterial enzymes have a central role in converting sulfite formed during dissimilatory oxidation of reduced sulfur compounds. Here we describe the structure of the Starkeya novella sulfite dehydrogenase, a heterodimeric complex of the catalytic molybdopterin subunit and a c-type cytochrome subunit, that reveals the molecular mechanism of intramolecular electron transfer in sulfite-oxidizing enzymes. The close approach of the two redox centers in the protein complex (Mo-Fe distance 16.6 A) allows for rapid electron transfer via tunnelling or aided by the protein environment. The high resolution structure of the complex has allowed the identification of potential through-bond pathways for electron transfer including a direct link via Arg-55A and/or an aromatic-mediated pathway. A potential site of electron transfer to an external acceptor cytochrome c was also identified on the SorB subunit on the opposite side to the interaction with the catalytic SorA subunit.
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Affiliation(s)
- Ulrike Kappler
- Centre for Metals in Biology, University of Queensland, St. Lucia, Queensland 4072, Australia
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