Indeed, only two enzymatic processes in humans are known to utilize B12, although other organisms make wider use of B12 compounds [6,9-12]. One may

Figure 1 Schematic representation of the molecular structure, numbering of atoms, and designations of pyrrole rings of cobalamins. R Me is methyl B12; R Ado is adenosylcobalamin (coenzyme B12); X CN is cyanocobalamin (vitamin B12). Five-membered rings are labeled A-D, and the amide side-chains are labeled a-g.