Sulfite oxidase

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sulfite oxidase
Sulfite oxidase catalyses the oxidation-reduction reaction of sulfite and water, yielding sulfate.
EC number1.8.3.1
CAS number9029-38-3
IntEnzIntEnz view
ExPASyNiceZyme view
MetaCycmetabolic pathway
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
PDB 1mj4 EBI.jpg
Available structures
PDBOrtholog search: PDBe RCSB
AliasesSUOX, entrez:6821, sulfite oxidase
External IDsOMIM: 606887 MGI: 2446117 HomoloGene: 394 GeneCards: SUOX
Gene location (Human)
Chromosome 12 (human)
Chr.Chromosome 12 (human)[1]
Chromosome 12 (human)
Genomic location for SUOX
Genomic location for SUOX
Band12q13.2Start55,997,180 bp[1]
End56,006,641 bp[1]
RefSeq (mRNA)



RefSeq (protein)



Location (UCSC)Chr 12: 56 – 56.01 MbChr 10: 128.67 – 128.67 Mb
PubMed search[3][4]
View/Edit HumanView/Edit Mouse

Sulfite oxidase (EC is an enzyme in the mitochondria of all eukaryotes, with exception of the yeasts.[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.[5][6][7] 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 (in a case of animals). 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.


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[edit]

A proposed mechanism of the oxidation of sulfite to sulfate by sulfite oxidase.

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).


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.[8] A G473D mutation impairs dimerization and catalysis in human sulfite oxidase.[9][10]

See also[edit]


  1. ^ a b c GRCh38: Ensembl release 89: ENSG00000139531 - Ensembl, May 2017
  2. ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000049858 - Ensembl, May 2017
  3. ^ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. ^ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. ^ D'Errico G, Di Salle A, La Cara F, Rossi M, Cannio R (January 2006). "Identification and characterization of a novel bacterial sulfite oxidase with no heme binding domain from Deinococcus radiodurans". J. Bacteriol. 188 (2): 694–701. doi:10.1128/JB.188.2.694-701.2006. PMC 1347283. PMID 16385059.
  6. ^ Tan WH, Eichler FS, Hoda S, Lee MS, Baris H, Hanley CA, Grant PE, Krishnamoorthy KS, Shih VE (September 2005). "Isolated sulfite oxidase deficiency: a case report with a novel mutation and review of the literature". Pediatrics. 116 (3): 757–66. doi:10.1542/peds.2004-1897. PMID 16140720. S2CID 6506338.
  7. ^ Cohen HJ, Betcher-Lange S, Kessler DL, Rajagopalan KV (December 1972). "Hepatic sulfite oxidase. Congruency in mitochondria of prosthetic groups and activity". J. Biol. Chem. 247 (23): 7759–66. PMID 4344230.
  8. ^ Karakas E, Kisker C (November 2005). "Structural analysis of missense mutations causing isolated sulfite oxidase deficiency". Dalton Transactions (21): 3459–63. doi:10.1039/b505789m. PMID 16234925.
  9. ^ Wilson HL, Wilkinson SR, Rajagopalan KV (February 2006). "The G473D mutation impairs dimerization and catalysis in human sulfite oxidase". Biochemistry. 45 (7): 2149–60. doi:10.1021/bi051609l. PMID 16475804.
  10. ^ Feng C, Tollin G, Enemark JH (May 2007). "Sulfite oxidizing enzymes". Biochim. Biophys. Acta. 1774 (5): 527–39. doi:10.1016/j.bbapap.2007.03.006. PMC 1993547. PMID 17459792.

Further reading[edit]

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