MetaCyc Pathway: sulfite oxidation IV
Traceable author statement to experimental supportInferred from experiment

Enzyme View:

Pathway diagram: sulfite oxidation IV

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/AssimilationInorganic Nutrients MetabolismSulfur Compounds MetabolismSulfite Oxidation

Some taxa known to possess this pathway include : Arabidopsis thaliana col, Aves, Bos taurus, Gallus gallus, Homo sapiens, Mammalia, Metazoa, Mus musculus, Rattus norvegicus

Expected Taxonomic Range: Metazoa, Viridiplantae

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 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].

In plants

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

Created 12-Sep-2006 by Caspi R, SRI International


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

Hansch05: Hansch R, Mendel RR (2005). "Sulfite oxidation in plant peroxisomes." Photosynth Res 86(3);337-43. PMID: 16307306

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

Heber98: Heber U, Hueve K (1998). "Action of SO2 on plants and metabolic detoxification of SO2." Int Rev Cytol., 177, 255-286.

Johnson76a: Johnson JL, Rajagopalan KV (1976). "Purification and properties of sulfite oxidase from human liver." J Clin Invest 58(3);543-50. PMID: 956383

Johnson77: Johnson JL, Rajagopalan KV (1977). "Tryptic cleavage of rat liver sulfite oxidase. Isolation and characterization of molybdenum and heme domains." J Biol Chem 252(6);2017-25. PMID: 14956

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

Kisker97: Kisker C, Schindelin H, Rees DC (1997). "Molybdenum-cofactor-containing enzymes: structure and mechanism." Annu Rev Biochem 66;233-67. PMID: 9242907

Lester95: Lester MR (1995). "Sulfite sensitivity: significance in human health." J Am Coll Nutr 14(3);229-32. PMID: 8586770

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

Other References Related to Enzymes, Genes, Subpathways, and Substrates of this Pathway

Garrett95: Garrett RM, Bellissimo DB, Rajagopalan KV (1995). "Molecular cloning of human liver sulfite oxidase." Biochim Biophys Acta 1262(2-3);147-9. PMID: 7599189

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

Latendresse13: Latendresse M. (2013). "Computing Gibbs Free Energy of Compounds and Reactions in MetaCyc."

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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
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