Sulfite oxidase
| sulfite oxidase | |||||||||
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Sulfite oxidase catalyses the oxidation-reduction reaction of sulfite and water, yielding sulfate.
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| Identifiers | |||||||||
| EC number | 1.8.3.1 | ||||||||
| CAS number | Template:CAS | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / EGO | ||||||||
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Sulfite oxidase (EC 1.8.3.1) is an enzyme in the mitochondria of all eukaryotes.[citation needed] It oxidizes sulfite to sulfate and, via cytochrome c, transfers the electrons produced to the electron transport chain, allowing generation of ATP in oxidative phosphorylation.[1][2][3] This is the last step in the metabolism of sulfur-containing compounds and the sulfate is excreted.
Sulfite oxidase is a metallo-enzyme that utilizes a molybdopterin cofactor and a heme group. It is one of the cytochromes b5 and belongs to the enzyme super-family of molybdenum oxotransferases that also includes DMSO reductase, xanthine oxidase, and nitrite reductase.
In mammals, the expression levels of sulfite oxidase is high in the liver, kidney, and heart, and very low in spleen, brain, skeletal muscle, and blood.
Contents
Structure
As a homodimer, sulfite oxidase contains two identical subunits with an N-terminal domain and a C-terminal domain. These two domains are connected by ten amino acids forming a loop. The N-terminal domain has a heme cofactor with three adjacent antiparallel beta sheets and five alpha helices. The C-terminal domain hosts a molybdopterin cofactor that is surrounded by thirteen beta sheets and three alpha helices. The molybdopterin cofactor has a Mo(VI) center, which is bonded to a sulfur from cysteine, an ene-dithiolate from pyranopterin, and two terminal oxygens. It is at this molybdenum center that the catalytic oxidation of sulfite takes place.
Active site and mechanism
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File:Sulfite-oxidase-mechanism.png
A proposed mechanism of the oxidation of sulfite to sulfate by sulfite oxidase.
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The active site of sulfite oxidase contains the molybdopterin cofactor and supports molybdenum in its highest oxidation state, +6 (MoVI). In the enzyme's oxidized state, molybdenum is coordinated by a cysteine thiolate, the dithiolene group of molybdopterin, and two terminal oxygen atoms (oxos). Upon reacting with sulfite, one oxygen atom is transferred to sulfite to produce sulfate, and the molybdenum center is reduced by two electrons to MoIV. Water then displaces sulfate, and the removal of two protons (H+) and two electrons (e−) returns the active site to its original state. A key feature of this oxygen atom transfer enzyme is that the oxygen atom being transferred arises from water, not from dioxygen (O2).
Deficiency
Sulfite oxidase is required to metabolize the sulfur-containing amino acids cysteine and methionine in foods. Lack of functional sulfite oxidase causes a disease known as sulfite oxidase deficiency. This rare but fatal disease causes neurological disorders, mental retardation, physical deformities, the degradation of the brain, and death. Reasons for the lack of functional sulfite oxidase include a genetic defect that leads to the absence of a molybdopterin cofactor and point mutations in the enzyme.[4] A G473D mutation impairs dimerization and catalysis in human sulfite oxidase.[5][6]
See also
References
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Further reading
- Kisker, C. “Sulfite oxidase”, Messerschimdt, A.; Huber, R.; Poulos, T.; Wieghardt, K.; eds. Handbook of Metalloproteins, vol 2; John Wiley and Sons, Ltd: New York, 2002
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External links
- Sulfite oxidase at the US National Library of Medicine Medical Subject Headings (MeSH)
- Research Activity of Sarkar Group
