Reaction: 2 cob(II)alamin-[cyanocobalamin reductase] + 2 hydrogen cyanide + NADP+ = 2 cyanocob(III)alamin + 2 [cyanocobalamin reductase] + NADPH + H+
Other name(s): MMACHC (gene name); CblC; cyanocobalamin reductase (NADPH, cyanide-eliminating); cyanocobalamin reductase (NADPH, CN-eliminating); NADPH:cyanocob(III)alamin oxidoreductase (cyanide-eliminating); cob(I)alamin, cyanide:NADP+ oxidoreductase; cyanocobalamin reductase (cyanide-eliminating)
Systematic name: cob(II)alamin, hydrogen cyanide:NADP+ oxidoreductase
Comments: The mammalian enzyme, which is cytosolic, can bind internalized cyanocobalamin and process it to cob(II)alamin by removing the upper axial ligand. The product remains bound to the protein, which, together with its interacting partner MMADHC, transfers it directly to downstream enzymes involved in adenosylcobalamin and methylcobalamin biosynthesis. In addition to its decyanase function, the mammalian enzyme also catalyses an entirely different chemical reaction with alkylcobalamins, using the thiolate of glutathione for nucleophilic displacement, generating cob(I)alamin and the corresponding glutathione thioether (cf. EC 2.5.1.151, alkylcobalamin dealkylase).
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 131145-00-1
References:
1. Watanabe, F., Oki, Y., Nakano, Y. and Kitaoka, S. Occurrence and characterization of cyanocobalamin reductase (NADPH; CN-eliminating) involved in decyanation of cyanocobalamin in Euglena gracilis. J. Nutr. Sci. Vitaminol. 34 (1988) 1-10. [PMID: 3134526]
2. Kim, J., Gherasim, C. and Banerjee, R. Decyanation of vitamin B12 by a trafficking chaperone. Proc. Natl. Acad. Sci. USA 105 (2008) 14551-14554. [PMID: 18779575]
3. Koutmos, M., Gherasim, C., Smith, J.L. and Banerjee, R. Structural basis of multifunctionality in a vitamin B12-processing enzyme. J. Biol. Chem. 286 (2011) 29780-29787. [PMID: 21697092]
4. Mah, W., Deme, J.C., Watkins, D., Fung, S., Janer, A., Shoubridge, E.A., Rosenblatt, D.S. and Coulton, J.W. Subcellular location of MMACHC and MMADHC, two human proteins central to intracellular vitamin B12 metabolism. Mol Genet Metab 108 (2013) 112-118. [PMID: 23270877]