The molybdenum-containing enzyme sulfite oxidase catalyzes the conversion of sulfite to sulfate, the terminal step in the oxidative degradation of cysteine and methionine. Deficiency of this enzyme in humans usually leads to major neurological abnormalities and early death. The crystal structure of chicken liver sulfite oxidase at 1.9 A resolution reveals that each monomer of the dimeric enzyme consists of three domains. At the active site, the Mo is penta-coordinated by three sulfur ligands, one oxo group, and one water/hydroxo. A sulfate molecule adjacent to the Mo identifies the substrate binding pocket. Four variants associated with sulfite oxidase deficiency have been identified: two mutations are near the sulfate binding site, while the other mutations occur within the domain mediating dimerization.

X-ray absorption spectroscopy at the molybdenum and sulfur K-edges has been used to probe the active site of wild-type and cysteine 207 → serine mutant human sulfite oxidases. We compare the active site structures in the Mo(VI) oxidation states:  the wild-type enzyme possesses two MoO ligands at 1.71 Å and three Mo−S ligands at 2.41 Å. The mutant molybdenum site is a novel trioxo site with MoO bond lengths of 1.74 Å, with two Mo−S ligands at 2.47 Å. We conclude that cysteine 207 is a ligand of molybdenum in wild-type human sulfite oxidase, and that, in the mutant, the Mo is ligated to an extra oxo group rather than the hydroxyl of the substituent serine 207.

Photooxidation is a potentially significant process in the degradation of crude oil spilled at sea. Moreover, a fundamental understanding of the effect of photochemical degradation on crude oil is a prerequisite for providing an accurate description of the recent history and potential fate of oil spilled in a marine environment. In this report we examine the effect of ultraviolet illumination on crude oil using a variety of techniques including gas chromatography/mass spectroscopy and X-ray absorption spectroscopy. The saturated compounds are resistant, but the aromatic compounds are particularly sensitive to photooxidation. Greater size and increasing alkyl substitution increase the sensitivity of aromatic compounds to photochemical oxidation. The photooxidized products appear in the resin and polar fractions as determined by thin-layer chroma tography. Thus, the effect of photooxidation is distinctly different from that of biodegradation, where larger and more substituted compounds are more resistant to degradation. Perhaps surprisingly, X-ray absorption spectroscopy indicates that the aliphatic sulfur compounds are more readily oxidized than the thiophenic compounds with the sulfur being oxidized to sulfoxides, sulfones, sulfonates, and sulfates in approximately equal amounts.

Sulfite oxidase catalyzes the terminal reaction in the degradation of sulfur amino acids. Genetic deficiency of sulfite oxidase results in neurological abnormalities and often leads to death at an early age. The mutation in the sulfite oxidase gene responsible for sulfite oxidase deficiency in a 5-year-old girl was identified by sequence analysis of cDNA obtained from fibroblast mRNA to be a guanine to adenine transition at nucleotide 479 resulting in the amino acid substitution of Arg-160 to Gln. Recombinant protein containing the R160Q mutation was expressed in Escherichia coli, purified, and characterized. The mutant protein contained its full complement of molybdenum and heme, but exhibited 2% of native activity under standard assay conditions. Absorption spectroscopy of the isolated molybdenum domains of native sulfite oxidase and of the R160Q mutant showed significant differences in the 480-and 350-nm absorption bands, suggestive of altered geometry at the molybdenum center. Kinetic analysis of the R160Q protein showed an increase in K m for sulfite combined with a decrease in k cat resulting in a decrease of nearly 1,000-fold in the apparent second-order rate constant k cat ͞K m . Kinetic parameters for the in vitro generated R160K mutant were found to be intermediate in value between those of the native protein and the R160Q mutant. Native sulfite oxidase was rapidly inactivated by phenylglyoxal, yielding a modified protein with kinetic parameters mimicking those of the R160Q mutant. It is proposed that Arg-160 attracts the anionic substrate sulfite to the binding site near the molybdenum.

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Each of the four cysteines in rat sulfite oxidase was altered by site-directed mutagenesis to serine, and the mutant proteins were expressed in Escherichia coli. Three of the replacements proved to be silent mutations, while a single cysteine, Cys-207, was found to be essential for enzyme activity. The C207S mutation was also generated in cloned human sulfite oxidase. The mutant human enzyme also displayed severely attenuated activity but was expressed at higher levels allowing purification and spectroscopic analysis. The absorption spectrum of the isolated molybdenum domain of the human C207S mutant displayed marked attenuation of the peak at 350 nm and a lesser decrease in absorbance from 450 -600 nm as compared with the native human molybdenum domain. The molybdenum and molybdopterin contents of the two samples were comparable. These data suggest that the major features in the absorption spectrum of the native molybdenum domain arise from the binding of Cys-207 to the molybdenum and indicate that this residue functions as a ligand of the metal.Sulfite oxidase, located in the intermembrane space of animal mitochondria, catalyzes the oxidation of sulfite to sulfate, the terminal reaction in the oxidative degradation of the sulfurcontaining amino acids, cysteine and methionine. The enzyme is a dimer of identical subunits of mass 52 kDa. The N-terminal domain of mass 10 kDa forms a b 5 -type cytochrome, and the C-terminal domain of mass 42 kDa anchors the molybdenum cofactor. The molybdenum cofactor in sulfite oxidase consists of molybdopterin (MPT), 1 a 6-alkyl-dihydropterin containing a unique cis-dithiolene moiety coordinated to molybdenum (1). EXAFS studies of rat liver sulfite oxidase have provided evidence for the presence of 2 MoϭO, 2 to 3 Mo-S, and 1 Mo-O(N) bonds at the molybdenum center (2, 3).The complete amino acid sequences of sulfite oxidase from chicken (4), rat (5), and human (6) sources have been reported. In addition, the amino acid sequences of a related enzyme nitrate reductase have been reported from a variety of fungal and plant sources (9 -17). 2, 3 Nitrate reductase catalyzes the reduction of nitrate to nitrite, a critical reaction in the nitrogen assimilation pathway in fungi and higher plants. The enzyme contains three prosthetic groups: the molybdenum cofactor, a b 557 cytochrome, and FAD in binding domains encoded by distinct segments of the primary sequence. Unlike in sulfite oxidase, the molybdenum domain of nitrate reductase is at the N terminus, followed by the central heme domain and the C-terminal flavin domain. The amino acid sequences of the molybdenum domains of sulfite oxidase and nitrate reductase are approximately 37% identical, and a single cysteine residue, corresponding to Cys-207 of sulfite oxidase, is invariant in all of the sulfite oxidases and nitrate reductases sequenced to date. It has been postulated that this cysteine functions as a ligand to molybdenum (4, 6). Recently it was shown that mutation of the corresponding cysteine residue in nitrate reductase leads t...

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We report twelve novel mutations in patients with isolated sulfite oxidase deficiency. The mutations are in SUOX, the gene that encodes the molybdohemoprotein sulfite oxidase. These include two frameshift mutations, a four-basepair deletion (562del4) and a singlebasepair insertion (113insC), both resulting in premature termination. Nonsense mutations predicting Y343X and Q364X substitutions were identified in a homozygous state in three patients, the latter in two sibs. The remaining eight are missense mutations generating single amino acid substitutions. From the position of the substituted residues, seven of these mutations are considered to be causative of the enzyme deficiency: I201L, R211Q, G305S, R309H, K322R, Q339R, and W393R. The eighth, a C>T transition, predicts an R319C substitution, which could affect the binding of the molybdenum cofactor and thus severely reduce sulfite oxidase activity. This mutation, however, is downstream of a frameshift mutation and is therefore not the causative mutation in this individual.