Sulfite oxidizing enzymes

Johnson-Winters, K., Tollin, G., Enemark, J. H. (2010). Elucidating the catalytic mechanism of sulfite oxidizing enzymes using structural, spectroscopic, and kinetic analyses. Biochemistry, 49, 7242—7254. 

Enzymes that belong to the sulfite oxidase family are comprised of the assim-ilatory eukaryotic nitrate reductases, bacterial YedY and the sulfite oxidizing enzymes. The latter are found in bacteria, plants, animals and humans, and are the primary focus of this section. A similar protein fold of the Mo domain, the so-called SUOX fold, characterizes these enzymes. The nature of the protein fold is one key factor that distinguishes SO family enzymes from the MOSC family proteins, which possess a very similar... 

Figure 2.14 Bond line drawing describing the first coordination sphere of oxidized Mo(vi), Mo(v) and reduced Mo(iv) forms of sulfite oxidizing enzyme active sites.

The active site geometries (Figure 2.14) of the sulfite oxidizing enzymes are quite similar and examples of high-resolution X-ray crystal structures can be found for the S. novella sulfite dehydrogenase (bacterial)/ A. thaliana sulfite oxidase (plant)/ bacterial YedY/ and vertebrate sulfite oxidase. ... 

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. ... 

There is considerable structural diversity amongst the bacterial sulfite oxidizing enzymes. The two most studied bacterial SDH enzymes have been isolated from the soil bacteria Starkeya novella, SorAB, a heterodimer bearing both a Mo and heme-containing subunit, and Sinorhizobium meliloti, SorT, which only has a Mo cofactor but donates electrons to an independent cytochrome (SorU). Neither enzyme shows significant oxidase activity. 

Biochemically, Mo draws attention because it is an essential enzyme cofactor in nearly all organisms, with particular importance for nitrogen fixation, nitrate reduction and sulfite oxidation. Such biochemical ubiquity is surprising in view of the general scarcity of Mo at the Earth s surface. 

Addition of AMP to the sulfite-treated APS reductase resulted in further bleaching between 350 and 500 nm. Peck et al. (S8S, S8S) have shown by EPR spectroscopy near liquid helium temperature that addition of either sulfite or AMP alone does not result in the formation of an iron signal at g = 1.94. However, when AMP and sulfite are added together, a g = 1.94 signal is produced, which is approximately 80 of that obtained when the enzyme is reduced with dithionite. Thus, the authors suggested that APS reductase catalyzes an intramolecular electron transfer during sulfite oxidation as shown in Fig. 42 from Peck et al. iS8S). 

The pH dependenee of both the oxidative and reductive half-reactions of sulfite oxidase has reeently been examined (Brody and Hille, 1999). From a comparison of the pH dependence of kcat with the limiting rate constants for the two half-reactions, kred and kox, respectively, it is evident that k d i s principally rate-limiting above pH 7, but at lower pH kox becomes increasingly important, kred is essentially pH-independent, consistent with a reaction mechanism in which nucleophilic attack by the substrate lone pair on a Mo=0 group initiates the eatalytie sequence. The pH dependence of kred/ d " " indieates an aetive site group having a p a of 9.3 must be depro-tonated for reaetion of oxidized enzyme with free sulfite, possibly Tyr 322 whieh from the erystal strueture of the enzyme constitutes part of the... 

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...

Three different mechanisms have been proposed to explain the reaction to sulfites in asthmatic patients. The first is explained by the inhalation of sulfur dioxide, which produces bronchoconstriction in aU asthmatics through direct stimulation of afferent parasympathetic irritant receptors. Furthermore, inhalation of atropine or the ingestion of doxepin protects sulfite-sensitive patients from reacting to the ingestion of sulfites. The second theory, IgE-mediated reaction, is supported by reported cases of sulfite-sensitive anaphylaxis reaction in patients with positive sulfite skin test. Finally, a reduced concentration of sulfite oxidase enzyme (the enzyme that catalyzes oxidation of sulfites to sulfates) compared with normal individuals has been demonstrated in a group of sulfite-sensitive asthmatics. 

Lyric RM, Suzuki I (1970c) Kinetic studies of sulfite cytochrome c oxidoreductase, thiosulfate-oxidizing enzyme, and adenosine-5 -phosphosulfate reductase from Thiobacillus thioparus. Can J Microbiol 48 594-603... 

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