A semi-synthetic form of the vitamin, cyanocobalamin (CNCbl, where a cyanide group is in the upper axial position), is produced from bacterial hydroxocobalamin and used in many pharmaceuticals, supplements and as a food additive. It is presumed to take the same route after ingestion as other forms of cobalamin (Cbl) (Randaccio et al. 2010). At this point in the pathway, CNCbl is reductively decyanated by methylmalonic aciduria and homocystinuria type C protein (MMACHC) to produce cob(II)alamin (B12r, vitaman B12r) and hydrogen cyanide (HCN) (Kim et al. 2008). Decyanation of CNCbl is required for it to be made available for conversion to active cofactors. MMACHC can remove the methyl (Me) and adenosyl (Ado) groups from MeCbl and AdoCbl respectively (not shown in this reaction), as well as CN from CNCbl.
