and Barker 1964, Blakley 1965, Reichard 1993, Booker et al. 1994), methionine biosynthesis and the anaerobic catabolism of recalcitrant polymers (Frings et al. 1992, Frings and Schink 1994, Frings et al. 1994).

                                  Origins of B12

     Many have postulated that B12 was present prebiotically and may have played an important catalytic role in the "RNA world" (Benner et al. 1989). The porphyrin molecule, a precursor to the central corrin ring of B12, has been synthesized in "primitive earth" experiments (Hodgson and Ponnamperuma 1968), and Eschenmoser (1988) has commented on how the B12 structure could have been formed using energetically favorable reactions. Roth and colleagues (1996) proposed that the original function of B12 was to balance redox reactions by providing an internal electron sink. Later it evolved to allow the use of inorganic electron acceptors. Finally with the development of an oxygen atmosphere on Earth, B12 assumed roles in aerobic metabolism in the form of heme and chlorophyll.

                                Distribution of B12

      Among the kingdoms of life, B12 is unevenly distributed. Although only

microorganisms are capable of synthesizing B12 de novo, both mammals and protists require this cofactor for their metabolism. Plants and fungi, with few exceptions, appear to neither synthesize nor require this cofactor for their metabolism (Duda et al. 1967) and instead may use S-adenosyl methionine as a source of the 5'-deoxyadenosyl radical required in carbon skeleton rearrangements (Ollagnier et al. 1998).

                                 Structure of B12

     With a molecular weight of 1350 Da, vitamin B12 (CN-B12) or cyano-cobalamin is the largest cofactor known to man. It is composed of four parts: a central ring, an