Fig. 1.

Vitamin B12 Biosynthetic Pathways.

De Novo biosynthesis occurs with divergent enzymatic pathways based on the availability of oxygen. Gut microbes with necessary biochemical machinery convert glutamate and glycine to 5-aminolevulinate, which is then converted to uroporphyrinogen III (UroIII). The cobA gene product converts UroIII to precorrin-2, the last common intermediate between the anaerobic and aerobic pathways which converge with the production of Cob(II)yrinic acid, a,c-diamide, which, through an additional series of reactions, is then converted to adenosylcobalamin (VB12).