Molecular Basis of Sulfite Oxidase Deficiency from the Structure of Sulfite Oxidase

Abstract

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.

Structural Studies of the Molybdenum Center of the Pathogenic R160Q Mutant of Human Sulfite Oxidase by Pulsed EPR Spectroscopy and 17O and 33S Labeling

“…No crystal structure is available for hSO, but the structure of the highly homologous chicken SO shows that R138 (equivalent to R160 in hSO) is in the active-site pocket near the Mo center which exhibits approximately square-pyramidal five-coordinate geometry in the fully oxidized Mo(VI) state, with axial and equatorial oxo ligands and three equatorial sulfur ligands (two from the pyranopterindithiolate cofactor and one from a cysteinyl side chain). 5 In the first step of the proposed catalytic cycle (Scheme 1, A→B) the equatorial oxo ligand reacts with sulfite to form an enzyme-product complex in which sulfate is coordinated to Mo (IV). Replacement of the product (sulfate) by water or hydroxide (B→C) and subsequent oneelectron oxidations (Mo(IV/V/VI)) and concomitant deprotonations (C→D→A) return the Mo center to the fully oxidized resting state.…”

Isolated sulfite oxidase deficiency: identification of 12 novel SUOX mutations in 10 patients

“…Y343X was present on both alleles from one patient and Q364X on both alleles from each of two sibs. These residues occur at the Cterminus of the molybdenum domain (343) and near the start of the third domain (364) of the sulfite oxidase protein (Kisker et al, 1997). Nonsense mutations at these positions would be expected to cause a severe form of sulfite oxidase deficiency, as was observed in the three patients, since the third domain is critical for dimer formation and stabilization of the protein for proper enzymatic function.…”

“…The third patient showed a 623C>A predicting an A208D substitution on one allele and a 1019G>A and 1109C>A on the second predicting R340Q and S370Y substitutions, respectively (Edwards et al, 1999). Of the latter two mutations, the S370Y was considered causative on the basis of the conserved nature of S370 in other species and the position of this residue when mapped to the crystal structure of the homologous protein from chicken liver (Kisker et al, 1997). A fourth patient with isolated sulfite oxidase was recently reported (Lam et al, 2002) with the 479G>A mutation as identified earlier.…”

“…The mature protein lacks this presequence. The native enzyme is a dimer of identical subunits, each of which contains three domains, the Nterminal heme domain, the central molybdenum domain and a C-terminal domain with key residues at the dimer interface (Johnson and Rajagopalan, 1977;Kisker et al, 1997). The enzyme catalyzes the oxidation of sulfite to sulfate, the terminal reaction in the degradation of sulfur-containing amino acids, and transfers the electrons derived from substrate oxidation to cytochrome c on the mitochondrial inner membrane.…”

“…In addition, we identified one missense mutation that would likely result in sulfite oxidase deficiency, but which, in this case, was superseded by a frameshift mutation upstream of the base substitution. The location of each altered amino acid arising from a missense mutation is examined using coordinates derived from the crystal structure of chicken liver sulfite oxidase (Kisker et al, 1997).…”