IUBMB Enzyme Nomenclature


Accepted name: 5-hydroxybenzimidazole synthase

Reaction: 5-amino-1-(5-phospho-β-D-ribosyl)imidazole + S-adenosyl-L-methionine + reduced acceptor = 5-hydroxybenzimidazole + 5'-deoxyadenosine + L-methionine + formate + NH3 + phosphate + oxidized acceptor

For diagram of reaction click here

Other name(s): bzaF (gene name); HBI synthase

Systematic name: 5-amino-1-(5-phospho-β-D-ribosyl)imidazole formate-lyase (decarboxylating, 4-amino-2-methyl-5-(phosphooxymethyl)pyrimidine-forming)

Comments: The enzyme, purified from bacteria, is part of the anaerobic pathway for cobalamin biosynthesis. It binds a [4Fe-4S] cluster that is coordinated by 3 cysteines and an exchangeable S-adenosyl-L-methionine molecule. The first stage of catalysis is reduction of the S-adenosyl-L-methionine to produce L-methionine and a 5'-deoxyadenosin-5'-yl radical that is crucial for the conversion of the substrate.

Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number:


1. Mehta, A.P., Abdelwahed, S.H., Fenwick, M.K., Hazra, A.B., Taga, M.E., Zhang, Y., Ealick, S.E. and Begley, T.P. Anaerobic 5-hydroxybenzimidazole formation from aminoimidazole ribotide: an unanticipated intersection of thiamin and vitamin B12 biosynthesis. J. Am. Chem. Soc. 137 (2015) 10444-10447. [PMID: 26237670]

2. Hazra, A.B., Han, A.W., Mehta, A.P., Mok, K.C., Osadchiy, V., Begley, T.P. and Taga, M.E. Anaerobic biosynthesis of the lower ligand of vitamin B12. Proc. Natl Acad. Sci. USA 112 (2015) 10792-10797. [PMID: 26246619]

[EC created 2017]

Return to EC 4.1.99 home page
Return to EC 4.3 home page
Return to EC 4 home page
Return to Enzymes home page
Return to IUBMB Biochemical Nomenclature home page