Chicken sulfite oxidase

One-electron reduction of chicken liver sulfite oxidase produces a species in which the molybdenum centre is Mo(V) and the b-type heme is low-spin Fe(III).90... 

Protein sequence homology suggests that sulfite oxidase and assimilatory nitrate reductase are members of the same molybdenum enzyme subfamily [31]. Consistent with this classification, the cofactors of sulfite oxidase and assimilatory nitrate reductase differ significantly from those in dmso reductase, aldehyde oxido-reductase, xanthine oxidase (see Section IV.E.), and even respiratory nitrate reductase (Section IV.D). The EXAFS of both sulfite oxidase [132-136] and assimilatory nitrate reductase [131,137,138] and x-ray studies of sulfite oxidase (chicken liver) [116] confirm that the molybdenum center is coordinated by two sulfur atoms from a single MPT ligand and by the sulfur atom of a cysteine side chain. The Movl state is bis(oxido) coordinated (Figure 14). 

Sulfite Oxidase. This enzyme, isolated from bovine (26, 27) and chicken liver (28), catalyzes the oxidation of sulfite to sulfate. This is possibly a crucial function in animals as S032" (or S02, its gaseous precursor) is toxic while S042" is relatively innocuous. For example, one of the first signs of molybdenum deficiency in rats is a greatly increased susceptibility to S02 poisoning (28). In addition, a human child bom without sulfite oxidase activity did not survive for very long (29). 

The chlorides of the product can be substituted by other ligands and mono(-dithiolene) complexes [MoO(AdS)2(S2C2Me2)], [MoO(SR)2(bdt)l (R = Ad or 2,4,6-/-Pr3CeH2) and [MoOCl(SC6H2-2,4,6- -Pr3)(bdt)] have been synthesized and characterized (90). These complexes represent the first examples of five-coordinate monodithiolene MovO complexes each has a square-pyramidal structure with an apical oxogroup. Representatives of these MovO centers exhibit rhombic EPR spectra that show some similarities to signals exhibited by the low- and high-pH forms of chicken liver sulfite oxidase (91). 

FIGURE 6. The structure of chicken liver sulfite oxidase (from Kisker et al., 1997). The heme domain is shown at left, the molybdenum-binding portion of the protein at right. The redox-active centers are rendered in wireframe representation. 

Brody, M. S., and Hille, R., 1995, The reaction of chicken liver sulfite oxidase with dimethyl-sulfite, Biochim. Biophys. Acta. 1253 133iil35. 

Sulfite Oxidase Family Sulfite oxidase Chicken liver 2, 106 Mo02(SCys)(MPT) 65-heme S03 " — S04 " ISOX... 

Figure 3 Active site of chicken liver sulfite oxidase. The apical group is a terminal 0x0 ligand, the second oxygen is an 0x0 ligand in the oxidized enzyme...

The postulated catalytic cycles for pterin-containing molybdenum enzymes involve a two-electron change at the molybdenum atom (Mo(VI) Mo(IV)). Microcoulometric titrations of nitrate reductase Chlorella vulgaris) (76), milk xanthine oxidase (77), and sulfite oxidase (78) show that their molybdenum centers are reduced by two electrons. The reduction potentials for the molybdenum center of chicken liver sulfite oxidase are strongly dependent upon pH and upon anion concentration (78). 

Kessler DL, Rajagopalan KV (1972) Purification and properties of sulfite oxidase from chicken liver. J Biol Chem 247 6566-6573... 

Sulfite oxidase bovine liver, chicken liver, r ... 

Sulfite oxidase is a molybdoenzyme which catalyzes the conversion of sulfite derived from cysteine, methionine and related compounds to inorganic sulfate. Sulfite oxidase has been isolated from bovine, chicken, rat, and human liver. It is located in the intermembrane space of mitochondria, and its physiological electron acceptor is mitochondrial cytochrome c. The purified enzymes consist of two identical subunits with a molecular weight range of 55-60 kDa, containing each one atom Mo and one cytochrome b5-type heme. 

Amino acid sequence comparisons have shown that sulfite oxidase is related to the heme-containing assimilatory nitrate reductases with a 31 % identity. Therefore, the nature of the crystal structure of chicken liver sulfite oxidase has added relevance. The crystal structure of chicken liver oxidase at... 

The sulfite oxidase enzymes are widespread in Nature, and are found in plants, bacteria (the sulfite dehydrogenases) and in birds and mammals. In addition, this family also includes the assimilatory plant nitrate reductases, which have essentially similar molybdenum coordination and differ structurally in lacking an active site arginine that is present in sulfite oxidase, and in showing somewhat different active site structures on turnover. We will focus here on the animal sulfite oxidase enzymes, of which chicken and human are the best studied. In animals the enzyme is responsible for the physiologically essential oxidation of sulfite to sulfate. It is a dimer of 52 kDa subunits and resides in the mitochondrial inner-membrane space. Each monomer contains Mo associated with one molybdopterin, plus a cytochrome heme. The enzymes catalyze the following reaction, which occurs at the Mo site which is reduced from Mo(vi) to Mo(iv) in the process ... 

The sulfite-oxidizing enzymes can be separated into two elasses, the eukaryotic sulfite oxidases (SO), found in animals and plants, and the sulfite dehydrogenases (SDH), found in bacteria. Plant [Arabidopsis thaliana) SO is the simplest Mo enzyme from this family bearing only a Mo eofaetor. This enzyme is found in peroxisomes and donates electrons directly to dioxygen. Vertebrate SO is a mitochondrial enzyme also containing a heme cofactor, which acts as an electron relay between the Mo active site and its physiological electron acceptor cytochrome c. The two most studied vertebrate sulfite oxidizing enzymes are chicken SO and human SO. ... 

Ferapontova, E. E., Ruzgas, T., and Gorton, L. 2003. Direct electron transfer of heme-and molybdopterin cofactor-containing chicken liver sulfite oxidase on alkanethiol-modified gold electrodes. Anal. Chem. 75 4841—1850. 

The evidence for the essentiality of molybdenum is substantial and conclusive. Molybdenum functions as a cofactor in enzymes that catalyze the hydroxylation of various substrates. Aldehyde oxidase oxidizes and detoxifies various pyrimidines, purines, pteridines, and related compounds. Xanthine oxidase/dehydrogenase catalyzes the transformation of hypoxanthine to xanthine, and xanthine to uric acid. Sulfite oxidase catalyzes the transformation of sulfite to sulfate. Attempts to produce molybdenum deficiency signs in rats, chickens, and humans have resulted in only limited success, and no success in healthy humans. 

Fig. 3. Schematic diagram of the Arabidopsis thaliana nitrate reductase domains and the amino acid homology with nitrate reductases from other species and other proteins. Arabidopsis thaliana amino acid sequence is from Crawford et al. (1988). Other sequence data are as follows Nicotiana tabacum, Nia 2, N. sylvestris type (Vaucheret et al., 1989b) Hordeum vu/ ore(K. M. Schnorr, A. Kleinhofs, and R. L. Warner, unpublished data) Oryza sativa (Choi et al., 1989) Aspergillus nidulans (Kinghom and Campbell, 1989) rat liver sulfite oxidase (Rlso) (Crawford et al., 1988) chicken microsomal cytochrome (CM cyt b5) (Ndbrega and Ozols, 1971) cytochrome bf reductase, bovine (Ozols et al., 1985) cytochrome reductase, human (Yubisui et al., 1984).

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