Role of conserved tyrosine 343 in intramolecular electron transfer in human sulfite oxidase

Mechanistic complexities of sulfite oxidase: An enzyme with multiple domains, subunits, and cofactors

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.

Functional mononuclear molybdenum enzymes: challenges and triumphs in molecular cloning, expression, and isolation

Mononuclear molybdenum enzymes catalyze a variety of reactions that are essential in the cycling of nitrogen, carbon, arsenic, and sulfur. For decades, the structure and function of these crucial enzymes have been investigated to develop a fundamental knowledge for this vast family of enzymes and the chemistries they carry out. Therefore, obtaining abundant quantities of active enzyme is necessary for exploring this family's biochemical capability. This mini-review summarizes the methods for overexpressing mononuclear molybdenum enzymes in the context of the challenges encountered in the process. Effective methods for molybdenum cofactor synthesis and incorporation, optimization of expression conditions, improving isolation of active vs. inactive enzyme, incorporation of additional prosthetic groups, and inclusion of redox enzyme maturation protein chaperones are discussed in relation to the current molybdenum enzyme literature. This article summarizes the heterologous and homologous expression studies providing underlying patterns and potential future directions.

Role of a Conserved Tyrosine Residue in the FMN-Heme Interdomain Electron Transfer in Inducible Nitric Oxide Synthase

The interdomain electron transfer (IET) between the flavin mononucleotide (FMN) and heme domains is essential in the biosynthesis of nitric oxide (NO) by the NO synthase (NOS) enzymes. A conserved tyrosine residue in the FMN domain (Y631 in human inducible NOS) was proposed to be a key part of the electron transfer pathway in the FMN/heme docked complex model. In the present study, the FMN-heme IET kinetics in the Y631F mutant and wild type of a bidomain oxygenase/FMN construct of human inducible NOS were determined by laser flash photolysis. The rate constant of the Y631F mutant is significantly decreased by ∼75% (compared to the wild type), showing that the tyrosine residue indeed facilitates the FMN-heme IET through the protein medium. The IET rate constant of the wild type protein decreases from 345 to 242 s^-1 on going from H₂O to 95% D₂O, giving a solvent kinetic isotope effect of 1.4. In contrast, no deuterium isotope effect was observed for the Tyr-to-Phe mutant. Moreover, an appreciable change in the wild type iNOS IET rate constant value was observed upon changing pH. These results indicate that the FMN-heme IET is proton coupled, in which the conserved tyrosine residue may play an important role.

Oxygen reactivity of mammalian sulfite oxidase provides a concept for the treatment of sulfite oxidase deficiency

Mammalian sulfite oxidase (SO) is a dimeric enzyme consisting of a molybdenum cofactor- (Moco) and haem-containing domain and catalyses the oxidation of toxic sulfite to sulfate. Following sulfite oxidation, electrons are passed from Moco via the haem cofactor to cytochrome c, the terminal electron acceptor. In contrast, plant SO (PSO) lacks the haem domain and electrons shuttle from Moco to molecular oxygen. Given the high similarity between plant and mammalian SO Moco domains, factors that determine the reactivity of PSO towards oxygen, remained unknown. In the present study, we generated mammalian haem-deficient and truncated SO variants and demonstrated their oxygen reactivity by hydrogen peroxide formation and oxygen-consumption studies. We found that intramolecular electron transfer between Moco and haem showed an inverse correlation to SO oxygen reactivity. Haem-deficient SO variants exhibited oxygen-dependent sulfite oxidation similar to PSO, which was confirmed further using haem-deficient human SO in a cell-based assay. This finding suggests the possibility to use oxygen-reactive SO variants in sulfite detoxification, as the loss of SO activity is causing severe neurodegeneration. Therefore we evaluated the potential use of PEG attachment (PEGylation) as a modification method for future enzyme substitution therapies using oxygen-reactive SO variants, which might use blood-dissolved oxygen as the electron acceptor. PEGylation has been shown to increase the half-life of other therapeutic proteins. PEGylation resulted in the modification of up to eight surface-exposed lysine residues of SO, an increased conformational stability and similar kinetic properties compared with wild-type SO.

Kinetic results for mutations of conserved residues H304 and R309 of human sulfite oxidase point to mechanistic complexities

Several point mutations in the gene of human sulfite oxidase (hSO) result in isolated sulfite oxidase deficiency, an inherited metabolic disorder. Three conserved residues (H304, R309, K322) are hydrogen bonded to the phosphate group of the molybdenum cofactor, and the R309H and K322R mutations are responsible for isolated sulfite oxidase deficiency. The kinetic effects of the K322R mutation have been previously reported (Rajapakshe et al., Chem. Biodiversity, 2012, 9, 1621-1634); here we investigate several mutants of H304 and R309 by steady-state kinetics, laser flash photolysis studies of intramolecular electron transfer (IET), and spectroelectrochemistry. An unexpected result is that all of the mutants show decreased rates of IET but increased steady-state rates of catalysis. However, in all cases the rate of IET is greater than the overall turnover rate, showing that IET is not the rate determining step for any of the mutations.

Shifting the metallocentric molybdoenzyme paradigm: the importance of pyranopterin coordination

In this review, we test the hypothesis that pyranopterin coordination plays a critical role in defining substrate reactivities in the four families of mononuclear molybdenum and tungsten enzymes (Mo/W-enzymes). Enzyme families containing a single pyranopterin dithiolene chelate have been demonstrated to have reactivity towards two (sulfite oxidase, SUOX-fold) and five (xanthine dehydrogenase, XDH-fold) types of substrate, whereas the major family of enzymes containing a bis-pyranopterin dithiolene chelate (dimethylsulfoxide reductase, DMSOR-fold) is reactive towards eight types of substrate. A second bis-pyranopterin enzyme (aldehyde oxidoreductase, AOR-fold) family catalyzes a single type of reaction. The diversity of reactions catalyzed by each family correlates with active site variability, and also with the number of pyranopterins and their coordination by the protein. In the case of the AOR-fold enzymes, inflexibility of pyranopterin coordination correlates with their limited substrate specificity (oxidation of aldehydes). In examples of the SUOX-fold and DMSOR-fold enzymes, we observe three types of histidine-containing charge-transfer relays that can: (1) connect the piperazine ring of the pyranopterin to the substrate-binding site (SUOX-fold enzymes); (2) provide inter-pyranopterin communication (DMSOR-fold enzymes); and (3) connect a pyran ring oxygen to deeply buried water molecules (the DMSOR-fold NarGHI-type nitrate reductases). Finally, sequence data mining reveals a number of bacterial species whose predicted proteomes contain large numbers (up to 64) of Mo/W-enzymes, with the DMSOR-fold enzymes being dominant. These analyses also reveal an inverse correlation between Mo/W-enzyme content and pathogenicity.

Structure-based alteration of substrate specificity and catalytic activity of sulfite oxidase from sulfite oxidation to nitrate reduction

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.

Molybdenum enzymes in higher organisms

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.

Effects of large-scale amino acid substitution in the polypeptide tether connecting the heme and molybdenum domains on catalysis in human sulfite oxidase

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.

Active-site dynamics and large-scale domain motions of sulfite oxidase: a molecular dynamics study

The physiologically vital enzyme sulfite oxidase employs rapid intramolecular electron transfer between a molybdenum ion in the C-terminal domain (the site of sulfite oxidation) and a heme moeity in the N-terminal domain to complete its catalytic cycle. Crystal structures of the enzyme show C- and N-terminal domain orientations that are not consistent with rapid intramolecular electron transfer. Domain motion has been postulated to explain this discrepancy. In the present work we employ molecular dynamics simulations to understand the large-scale domain motions of the enzyme. We observe motion of the N-terminal domain into an orientation similar to that postulated for rapid electron transfer. Our simulations also probe the dynamics of the active site and surrounding residues, adding a further level of structural and thermodynamic detail in understanding sulfite oxidase function.

The structures of the C185S and C185A mutants of sulfite oxidase reveal rearrangement of the active site

Sulfite oxidase (SO) catalyzes the physiologically critical conversion of sulfite to sulfate. Enzymatic activity is dependent on the presence of the metal molybdenum complexed with a pyranopterin-dithiolene cofactor termed molybdopterin. Comparison of the amino acid sequences of SOs from a variety of sources has identified a single conserved Cys residue essential for catalytic activity. The crystal structure of chicken liver sulfite oxidase indicated that this residue, Cys185 in chicken SO, coordinates the Mo atom in the active site. To improve our understanding of the role of this residue in the catalytic mechanism of sulfite oxidase, serine and alanine variants at position 185 of recombinant chicken SO were generated. Spectroscopic and kinetic studies indicate that neither variant is capable of sulfite oxidation. The crystal structure of the C185S variant was determined to 1.9 A resolution and to 2.4 A resolution in the presence of sulfite, and the C185A variant to 2.8 A resolution. The structures of the C185S and C185A variants revealed that neither the Ser or Ala side chains appeared to closely interact with the Mo atom and that a third oxo group replaced the usual cysteine sulfur ligand at the Mo center, confirming earlier extended X-ray absorption fine structure spectroscopy (EXAFS) work on the human C207S mutant. An unexpected result was that in the C185S variant, in the absence of sulfite, the active site residue Tyr322 became disordered as did the loop region flanking it. In the C185S variant crystallized in the presence of sulfite, the Tyr322 residue relocalized to the active site. The C185A variant structure also indicated the presence of a third oxygen ligand; however, Tyr322 remained in the active site. EXAFS studies of the Mo coordination environment indicate the Mo atom is in the oxidized Mo(VI) state in both the C185S and C185A variants of chicken SO and show the expected trioxodithiolene active site. Density functional theory calculations of the trioxo form of the cofactor reasonably reproducd the Mo horizontal lineO distances of the complex; however, the calculated Mo-S distances were slightly longer than either crystallographic or EXAFS measurements. Taken together, these results indicate that the active sites of the C185S and C185A variants are essentially catalytically inactive, the crystal structures of C185S and C185A variants contain a fully oxidized, trioxo form of the cofactor, and Tyr322 can undergo a conformational change that is relevant to the reaction mechanism. Additional DFT calculations demonstrated that such methods can reasonably reproduce the geometry and bond lengths of the active site.

Effects of interdomain tether length and flexibility on the kinetics of intramolecular electron transfer in human sulfite oxidase

Sulfite oxidase (SO) is a vitally important molybdenum enzyme that catalyzes the oxidation of toxic sulfite to sulfate. The proposed catalytic mechanism of vertebrate SO involves two intramolecular one-electron transfer (IET) steps from the molybdenum cofactor to the iron of the integral b-type heme and two intermolecular one-electron steps to exogenous cytochrome c. In the crystal structure of chicken SO [Kisker, C., et al. (1997) Cell 91, 973-983], which is highly homologous to human SO (HSO), the heme iron and molybdenum centers are separated by 32 A and the domains containing these centers are linked by a flexible polypeptide tether. Conformational changes that bring these two centers into greater proximity have been proposed [Feng, C., et al. (2003) Biochemistry 42, 5816-5821] to explain the relatively rapid IET kinetics, which are much faster than those theoretically predicted from the crystal structure. To explore the proposed role(s) of the tether in facilitating this conformational change, we altered both its length and flexibility in HSO by site-specific mutagenesis, and the reactivities of the resulting variants have been studied using laser flash photolysis and steady-state kinetics assays. Increasing the flexibility of the tether by mutating several conserved proline residues to alanines did not produce a discernible systematic trend in the kinetic parameters, although mutation of one residue (P105) to alanine produced a 3-fold decrease in the IET rate constant. Deletions of nonconserved amino acids in the 14-residue tether, thereby shortening its length, resulted in more drastically reduced IET rate constants. Thus, the deletion of five amino acid residues decreased IET by 70-fold, so that it was rate-limiting in the overall reaction. The steady-state kinetic parameters were also significantly affected by these mutations, with the P111A mutation causing a 5-fold increase in the sulfite K(m) value, perhaps reflecting a decrease in the ability to bind sulfite. The electron paramagnetic resonance spectra of these proline to alanine and deletion mutants are identical to those of wild-type HSO, indicating no significant change in the Mo active site geometry.

Structural studies of the molybdenum center of the pathogenic R160Q mutant of human sulfite oxidase by pulsed EPR spectroscopy and 17O and 33S labeling

Electron paramagnetic resonance (EPR) investigation of the Mo(V) center of the pathogenic R160Q mutant of human sulfite oxidase (hSO) confirms the presence of three distinct species whose relative abundances depend upon pH. Species 1 is exclusively present at pH < or = 6, and remains in significant amounts even at pH 8. Variable-frequency electron spin echo envelope modulation (ESEEM) studies of this species prepared with (33)S-labeled sulfite clearly show the presence of coordinated sulfate, as has previously been found for the "blocked" form of Arabidopsis thaliana at low pH (Astashkin, A. V.; Johnson-Winters, K.; Klein, E. L.; Byrne, R. S.; Hille, R.; Raitsimring, A. M.; Enemark, J. H. J. Am. Chem. Soc. 2007, 129, 14800). The ESEEM spectra of Species 1 prepared in (17)O-enriched water show both strongly and weakly magnetically coupled (17)O atoms that can be assigned to an equatorial sulfate ligand and the axial oxo ligand, respectively. The nuclear quadrupole interaction (nqi) of the axial oxo ligand is substantially stronger than those found for other oxo-Mo(V) centers studied previously. Additionally, pulsed electron-nuclear double resonance (ENDOR) measurements reveal a nearby weakly coupled exchangeable proton. The structure for Species 1 proposed from the pulsed EPR results using isotopic labeling is a six-coordinate Mo(V) center with an equatorial sulfate ligand that is hydrogen bonded to an exchangeable proton. Six-coordination is supported by the (17)O nqi parameters for the axial oxo group of the model compound, (dttd)Mo(17)O((17)Otms), where H2dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane; tms = trimethylsilyl. Reduction of R160Q to Mo(V) with Ti(III) gives primarily Species 2, another low pH form, whereas reduction with sulfite at higher pH values gives a mixture of Species 1 and 2, as well as the "primary" high pH form of wild-type SO. The occurrence of significant amounts of the "sulfate-blocked" form of R160Q (Species 1) at physiological pH suggests that this species may be a contributing factor to the lethality of this mutation.

Studies of the Mo(V) Center of the Y343F Mutant of Human Sulfite Oxidase by Variable Frequency Pulsed EPR Spectroscopy

The Mo(V) forms of the Tyr343Phe (Y343F) mutant of human sulfite oxidase (SO) have been investigated by continuous wave (CW) and variable frequency pulsed EPR spectroscopies as a function of pH. The CW EPR spectrum recorded at low pH (∼6.9) has g-values similar to those known for the low-pH form of the native vertebrate SO (original lpH form); however, unlike the spectrum of original lpH SO, it does not show any hyperfine splittings from a nearby exchangeable proton. The detailed electron spin echo (ESE) envelope modulation (ESEEM) and pulsed electron-nuclear double resonance (ENDOR) experiments also did not reveal any nearby protons that could belong to an exchangeable ligand at the molybdenum center. These results suggest that under low-pH conditions the active site of Y343F SO is in the "blocked" form, with the Mo(V) center coordinated by sulfate. With increasing pH the EPR signal from the "blocked" form decreases, while a signal similar to that of the original lpH form appears and becomes the dominant signal at pH>9. In addition, both the CW EPR and ESE-detected field sweep spectra reveal a considerable contribution from a signal similar to that usually detected for the high-pH form of native vertebrate SO (original hpH form). The nearby exchangeable protons in both of the component forms observed at high pH were studied by the ESEEM spectroscopy. These results indicate that the Y343F mutation increases the apparent pK(a) of the transition from the lpH to hpH forms by ∼2 pH units.

Sulfite oxidizing enzymes

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.

Structural analysis of missense mutations causing isolated sulfite oxidase deficiency

The physiologically essential oxidation of sulfite to sulfate is catalyzed by the molybdoheme enzyme, sulfite oxidase. Deficiencies of this enzyme in human patients lead to severe neurological symptoms, which usually result in death in early childhood. Up to date eleven missense mutations in the gene encoding sulfite oxidase have been identified from sulfite oxidase deficient patients. The structural characterization of these mutants is now possible after the chicken sulfite oxidase gene has been synthesized chemically and due to the high homology to the human enzyme it provides a good model of human sulfite oxidase. This review focuses on the possible effects of the sulfite oxidase deficiency causing mutations based on our new structures of recombinant chicken sulfite oxidase.

Total truncation of the molybdopterin/dimerization domains of SUOX protein in an Arab family with isolated sulfite oxidase deficiency

We ascertained a patient with the full-blown phenotype of isolated sulfite oxidase deficiency in a consanguineous Arab family. The proband's phenotype included the presence of intractable seizures in the neonatal period, some dysmorphic features, neuroradiologic findings reminiscent of hypoxic ischemic encephalopathy and rapidly progressive brain destruction leading to severe neurodevelopmental impairment. Biochemically, the patient excreted a large amount of S-sulfocysteine with normal amounts of xanthene and hypoxanthine and had normal plasma uric acid, which was consistent with isolated sulfite oxidase deficiency. We report the identification of the first Arab mutation in SUOX, the gene for sulfite oxidase enzyme, in the ascertained family. The newly identified Arab mutation in the SUOX gene (a single nucleotide deletion, del G1244) is predicted to cause a frame shift at amino acid 117 of the translated protein with the generation of a stop codon and total truncation of the molybdo-pterin- and the dimerizing-domain(s) of SUOX protein expressed from the mutant allele. The identification of this new Arab SUOX mutation should facilitate pre-implantation genetic diagnosis and selection of unaffected embryos for future pregnancy in the ascertained family with the mutation and related families with the same mutation.

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

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.

The crystal structure of plant sulfite oxidase provides insights into sulfite oxidation in plants and animals

The molybdenum cofactor (Moco) containing sulfite oxidase (SO) from Arabidopsis thaliana has recently been identified and biochemically characterized. The enzyme is found in peroxisomes and believed to detoxify excess sulfite that is produced during sulfur assimilation, or due to air pollution. Plant SO (PSO) is homodimeric and homologous to animal SO, but contains only a single Moco domain without an additional redox center. Here, we present the first crystal structure of a plant Moco enzyme, the apo-state of Arabidopsis SO at 2.6 A resolution. The overall fold and coordination of the Moco are similar to chicken SO (CSO). Comparisons of conserved surface residues and the charge distribution in PSO and CSO reveal major differences near the entrance to both active sites reflecting different electron acceptors. Arg374 has been identified as an important substrate binding residue due to its conformational change when compared to the sulfate bound structure of CSO.

The Role of Tyrosine 343 in Substrate Binding and Catalysis by Human Sulfite Oxidase

In the crystal structure of chicken sulfite oxidase, the residue Tyr(322) (Tyr(343) in human sulfite oxidase) was found to directly interact with a bound sulfate molecule and was proposed to have an important role in mediating the substrate specificity and catalytic activity of this molybdoprotein. In order to understand the role of this residue in the catalytic mechanism of sulfite oxidase, steady-state and stopped-flow analyses were performed on wild-type and Y343F human sulfite oxidase over the pH range 6-10. In steady-state assays of Y343F sulfite oxidase using cytochrome c as the electron acceptor, k(cat) was somewhat impaired ( approximately 34% wild-type activity at pH 8.5), whereas the K(m)(sulfite) showed a 5-fold increase over wild type. In rapid kinetic assays of the reductive half-reaction of wild-type human sulfite oxidase, k(red)(heme) changed very little over the entire pH range, with a significant increase in K(d)(sulfite) at high pH. The k(red)(heme) of the Y343F variant was significantly impaired across the entire pH range, and unlike the wild-type protein, both k(red)(heme) and K(d)(sulfite) were dependent on pH, with a significant increase in both kinetic parameters at high pH. Additionally, reduction of the molybdenum center by sulfite was directly measured for the first time in rapid reaction assays using sulfite oxidase lacking the N-terminal heme-containing domain. Reduction of the molybdenum center was quite fast (k(red)(Mo) = 972 s(-1) at pH 8.65 for wild-type protein), indicating that this is not the rate-limiting step in the catalytic cycle. Reduction of the molybdenum center of the Y343F variant by sulfite was more significantly impaired at high pH than at low pH. These results demonstrate that the Tyr(343) residue is important for both substrate binding and oxidation of sulfite by sulfite oxidase.

Intramolecular electron transfer in a bacterial sulfite dehydrogenase

Sulfite dehydrogenase (SDH) from Starkeya novella, a sulfite-oxidizing molybdenum-containing enzyme, has a novel tightly bound alphabeta-heterodimeric structure in which the Mo cofactor and the c-type heme are located on different subunits. Flash photolysis studies of intramolecular electron transfer (IET) in SDH show that the process is first-order, independent of solution viscosity, and not inhibited by sulfate, which strongly indicates that IET in SDH proceeds directly through the protein medium and does not involve substantial movement of the two subunits relative to each other. The IET results for SDH contrast with those for chicken and human sulfite oxidase (SO) in which the molybdenum domain is linked to a b-type heme domain through a flexible loop, and IET shows a remarkable dependence on sulfate concentration and viscosity that has been ascribed to interdomain docking. The results for SDH provide additional support for the interdomain docking hypothesis in animal SO and clearly demonstrate that dependence of IET on viscosity and sulfate is not an inherent property of all sulfite-oxidizing molybdenum enzymes.

Re-evaluation of intramolecular long-range electron transfer between tyrosine and tryptophan in lysozymes. Evidence for the participation of other residues

One-electron oxidation of six different c-type lysozymes from hen egg white, turkey egg white, human milk, horse milk, camel stomach and tortoise was studied by gamma- and pulse-radiolysis. In the first step, one tryptophan side chain is oxidized to indolyl free radical, which is produced quantitatively. As shown already, the indolyl radical subsequently oxidizes a tyrosine side chain to the phenoxy radical in an intramolecular reaction. However this reaction is not total and its stoichiometry depends on the protein. Rate constants also vary between proteins, from 120 x s(-1) to 1000 x s(-1) at pH 7.0 and room temperature [extremes are hen and turkey egg white (120 x s(-1)) and human milk (1000 x s(-1))]. In hen and turkey egg white lysozymes we show that another reactive site is the Asn103-Gly104 peptidic bond, which gets broken radiolytically. Tryptic digestion followed by HPLC separation and identification of the peptides was performed for nonirradiated and irradiated hen lysozyme. Fluorescence spectra of the peptides indicate that Trp108 and/or 111 remain oxidized and that Tyr20 and 53 give bityrosine. Tyr23 appears not to be involved in the process. Thus new features of long-range intramolecular electron transfer in proteins appear: it is only partial and other groups are involved which are silent in pulse radiolysis.