Mechanism of the Reaction between Cyanocobalamin and Reduced Flavin Mononucleotide

A study on the kinetics of the reaction between cyanocobalamin (vitamin B-12, CNCbl) and the reduced form of flavin mononucleotide (FMNH2) in weakly acidic, neutral, and slightly alkaline media is performed. It is shown that FMNH2 can reduce CNCbl to cobalamin(II) (Cbl(II)). It is established that protonated, mono-deprotonated, and di-deprotonated forms of FMNH2 can participate in the reaction. The reaction mechanism includes the slow substitution of the 5,6-dimethylbenzimidazole nucleotide with FMNH2 molecule, rapid electron transfer from FMNH2 to Co(III) ions, and the subsequent dissociation of cyanide. The reaction is reversible, due to the ability of the oxidized form of FMNH2 (FMN) to react with the Cbl(II) complex.