This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Sulfur Compounds Metabolism → Sulfite Oxidation|
Sulfite is generated endogenously by the metabolism of sulfur-containing amino acids such as methionine and cysteine. In addition, sulfite is formed from sulfur dioxide, an environmental pollutant, and is provided exogenously, either as a constituent of certain fruits, or as a preservative in dried fruits, vegetables and wine [Lester95].
Sulfite oxidase (EC 22.214.171.124) is a metallohemoprotein with molybdenum and protoheme as prosthetic groups mainly found in eukaryotes. The enzyme is located in the mitochondrial intermembrane space, and is active mostly in the liver, where it catalyzes the oxidation of sulfite to sulfate. This oxidation is the terminal reaction in the oxidative degradation of the sulfur-containing amino acids cysteine and methionine (see superpathway of L-methionine salvage and degradation).
The enzyme has been isolated from different sources and exists as a homodimer of molecular weight in the range 101-110 kDa. Each subunit can be proteolytically cleaved into two domains [Johnson77], the smaller of which contains a b5 cytochrome, while the larger harbors a molybdenum cofactor [Kisker97].
Sulfite oxidase deficiency is a lethal genetic disease that results from defects either in the genes encoding proteins involved in molybdenum cofactor biosynthesis or in the sulfite oxidase gene itself [Karakas05].
Sulfite in excess will cause damage to the organism, by breaking disulfide bridges (sulfitolysis), forming metabolic inhibitors such as hydroxysulfonates and interfere with the availability of essential cations required for nutrition [Heber98].
Sulfite oxidation is different in plants and animals. In vertebrates the role of sulfite oxidation lays in the detoxification of sulfite derived from the decomposition of sulfur-containing amino acids. Plants assimilate sulfur by reducing sulfate via sulfite to sulfide to form cysteine. One major point is the different location of sulfite oxidase (SO). The enzyme is found in vertebrates in the mitochondria whereas the plant SO has been localized to the peroxisome [Nowak04]. Sulfite oxidation constitutes a converse pathway to sulfate assimilation which has been found in chloroplasts. Therefore, peroxisomal sulfite detoxification solves the problem of having two counteracting pathways in the same cell compartment [Hansch05].
In contrast to its animal counterpart SO lacks the heme domain, hence being the only known Mo enzyme that possesses no other redox-active centers [Eilers01]. It has been demonstrated that the reaction catalyzed by sulfite oxidase involves the conversion of molecular oxygen into hydrogen peroxide [Hansch06].
While sulfite produced during the process of sulfate assimilation is fast converted into sulfide, SO has been discussed to serve as 'safety valve' to detoxify increased levels of sulfite [Hansch05] which would be harmful to plants. Under such conditions (high sulfite concentration) the catalase in the peroxisomes is inhibited and excess hydrogen peroxide adds sulfite oxidizing capacities in a non-enzymatically way, hence very likely both enzymatic and non-enzymatic steps are involved in the sulfite oxidation in plants [Hansch06].
Superpathways: superpathway of L-methionine salvage and degradation
Variants: sulfite oxidation I (sulfite oxidoreductase), sulfite oxidation II, sulfite oxidation III, sulfite oxidation V (SoeABC), superpathway of sulfur metabolism (Desulfocapsa sulfoexigens), superpathway of thiosulfate metabolism (Desulfovibrio sulfodismutans)
Unification Links: AraCyc:PWY-5326
Eilers01: Eilers T, Schwarz G, Brinkmann H, Witt C, Richter T, Nieder J, Koch B, Hille R, Hansch R, Mendel RR (2001). "Identification and biochemical characterization of Arabidopsis thaliana sulfite oxidase. A new player in plant sulfur metabolism." J Biol Chem 276(50);46989-94. PMID: 11598126
Hansch06: Hansch R, Lang C, Riebeseel E, Lindigkeit R, Gessler A, Rennenberg H, Mendel RR (2006). "Plant sulfite oxidase as novel producer of H2O2: combination of enzyme catalysis with a subsequent non-enzymatic reaction step." J Biol Chem 281(10);6884-8. PMID: 16407262
Karakas05: Karakas E, Wilson HL, Graf TN, Xiang S, Jaramillo-Busquets S, Rajagopalan KV, Kisker C (2005). "Structural insights into sulfite oxidase deficiency." J Biol Chem 280(39);33506-15. PMID: 16048997
Nowak04: Nowak K, Luniak N, Witt C, Wustefeld Y, Wachter A, Mendel RR, Hansch R (2004). "Peroxisomal localization of sulfite oxidase separates it from chloroplast-based sulfur assimilation." Plant Cell Physiol 45(12);1889-94. PMID: 15653809
Kisker97a: Kisker C, Schindelin H, Pacheco A, Wehbi WA, Garrett RM, Rajagopalan KV, Enemark JH, Rees DC (1997). "Molecular basis of sulfite oxidase deficiency from the structure of sulfite oxidase." Cell 91(7);973-83. PMID: 9428520
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