EC - Heme a synthase

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IntEnz Enzyme Nomenclature


Accepted name:
heme a synthase
Systematic name:
ferroheme o:acceptor C-81-oxidoreductase (heme a-forming)



Contains a heme b cofactor. The enzyme catalyses the conversion of heme o to heme a by two successive hydroxylations of the methyl group at C-8, using water as the oxygen source. The first hydroxylation forms heme i, the second hydroxylation results in an unstable dihydroxymethyl group, which spontaneously dehydrates, resulting in the formyl group of heme a [2,4]. The electrons produced by the reaction are transferred to a heme b cofactor [6]. However, the electron acceptor that is used to restore the heme b cofactor to its oxidized state is still not known (both a thioredoxin-like protein and menaquinol have been proposed).

Links to other databases

Enzymes and pathways: NC-IUBMB , BRENDA , ExplorEnz , ENZYME@ExPASy , KEGG , MetaCyc , UniPathway
Structural data: CSA , EC2PDB


  1. Barros, M. H., Carlson, C. G., Glerum, D. M., Tzagoloff, A.
    Involvement of mitochondrial ferredoxin and Cox15p in hydroxylation of heme O.
    FEBS Lett. 492 : 133-138 (2001). [PMID: 11248251]
  2. Brown, K. R., Allan, B. M., Do, P., Hegg, E. L.
    Identification of novel hemes generated by heme A synthase: evidence for two successive monooxygenase reactions.
    Biochemistry 41 : 10906-10913 (2002). [PMID: 12206660]
  3. Brown, K. R., Brown, B. M., Hoagland, E., Mayne, C. L., Hegg, E. L.
    Heme A synthase does not incorporate molecular oxygen into the formyl group of heme A.
    Biochemistry 43 : 8616-8624 (2004). [PMID: 15236569]
  4. Hederstedt, L., Lewin, A., Throne-Holst, M.
    Heme A synthase enzyme functions dissected by mutagenesis of Bacillus subtilis CtaA.
    J. Bacteriol. 187 : 8361-8369 (2005). [PMID: 16321940]
  5. Hederstedt, L.
    Heme A biosynthesis.
    Biochim. Biophys. Acta 1817 : 920-927 (2012). [PMID: 22484221]
  6. Niwa, S., Takeda, K., Kosugi, M., Tsutsumi, E., Mogi, T., Miki, K.
    Crystal structure of heme A synthase from Bacillus subtilis.
    Proc Natl Acad Sci U S A 115 : 11953-11957 (2018). [PMID: 30397130]

[EC created 2020]