Vitamin B-12: Insights into biosynthesis's mount improbable

The biosynthesis of the corrin ring component of cobalamin (vitamin B12) is reviewed with regard to how two separate though broadly similar pathways m ay have evolved. The more ancient "anaerobic" pathway is characterized by t he early chelation of cobalt and the release of acetaldehyde whereas the "a erobic" pathway is characterized by an absolute dependency on molecular oxy gen, the late chelation of cobalt and the release of acetic acid. Both path ways require the addition of 8 S-adenosyl-L-methionine-derived methyl group s to the periphery of the tetrapyrrole framework. The sequences of these en zymes reveal that they are clearly related, most likely having evolved from an ancestral methylase gene. The three-dimensional structure of one of the se methyltransferases is highlighted and discussed in light of a common mec hanism for this family of enzymes. Moreover, the aerobic and anaerobic chel atases are described and parallels with the chelatases found in heme and ch lorophyll synthesis are drawn, (C) 1999 Academic Press.