Vitamin B12: Two Coenzyme Forms

Abstract

The eighth and last of the B vitamins is B12, an apparent evolutionary holdover from the days of anaerobic bacterial metabolism. Among its distinctions B12 has a macrocyclic, contracted tetrapyrrolic corrin ring system, with a cobalt ion in the central equatorial plane. The cobalt can cycle between Co(3+), Co(2+), and Co(1+). The top axial ligand can be water (aquocobalamin) or 5-deoxyadenosyl (adenosyl-B12) or a methyl group (methyl B12). B12 engages in only two steps in human metabolism but both are essential for health. The adenosyl-B12 mediates radical chemistry for converting methylmalonyl-CoA to succinyl-CoA, a central metabolite. Methyl B12 is an intermediate C1 carrier from N5–CH3–tetrahydrofolate to homocysteine to produce the essential amino acid methionine.