Organometallic chemistry of b(12) coenzymes.

When coenzyme B(12) was identified as organometallic derivative of vitamin B(12), metal-carbon bonds were revealed to be relevant in life processes. Vitamin B(12), the "antipernicious anaemia factor" required for human health, was isolated earlier as a crystallizable cyano-Co(III)-complex. B(12) cofactors and other cobalt corrinoids play important roles not only in humans, but in the metabolism of archaea and other microorganisms, in particular. Indeed, the microorganisms are the only natural sources of the B(12) derivatives. For other B(12)-requiring organisms the corrinoids are thus "vitamins". However, vitamin B(12) also needs to be converted into organometallic B(12)-forms, which are the typical coenzymes in metabolically important enzymes. One of these, methionine synthase, catalyzes the transfer of a methyl group and its corrinoid cofactor is methylcobalamin. Another one, methylmalonyl-CoA mutase uses a reversible radical process, and coenzyme B(12) (adenosylcobalamin) as its cofactor, to transform methylmalonyl-CoA into succinyl-CoA. In such enzymes, the bound B(12) derivatives engage (or are formed) in exceptional organometallic enzymatic reactions, which depend upon the organometallic reactivity of the B(12) cofactors. Clearly, organometallic B(12) derivatives hold an important position in life and have thus attracted particular interest from the medical sciences, biology, and chemistry. This chapter outlines the unique structures of B(12) derivatives and recapitulates their redox properties and their organometallic chemistry, relevant in the context of the metabolic transformation of B(12) derivatives into the relevant coenzyme forms and for their use in B(12)-dependent enzymes.