translocation of B12 across the outer membrane (Bradbeer and Woodrow 1976, Bradbeer 1993). Once inside the periplasm, it is thought that the BtuF protein binds B12 (White et al. 1973) which then transports it to the BtuCD complex on the inner membrane where translocation of B12 into the cytosol is driven by the hydrolysis of ATP (Kadner and Liggins 1973, Bassford Jr. and Kadner 1977, DeVeaux and Kadner 1985, DeVeaux et al. 1986).

                            B12-Dependent Reactions

     The vitamin B12 coenzymes, Ado-B12 and CH3-B12, are cofactors for at least 15

different enzymes. Ado-B12 is used as a cofactor for enzymes involved in intramolecular rearrangement reactions. In the enteric bacteria, examples of such reactions can be seen in the fermentation of small molecules such as 1,2-propanediol (PD), ethanolamine, and glycerol. In other procaryotes, Ado-B12 is used in the fermentation of amino acids such as glutamate, lysine, leucine, and ornithine (Stroinski 1987). Among higher organisms, Ado-B12 is used in the metabolism of propionyl-CoA. The key step in each of these reactions is the formation of an adenosyl radical on the Ado-B12 molecule, which results from the homolytic cleavage of the carbon-cobalt bond (Stroinski 1987). Ado-B12 then abstracts a proton from the substrate forming an unstable radical that rearranges forming the product. Methylcobalamin (CH3-B12), on the other hand is the cofactor for methyltransferases (Schneider and Stroinski 1987b), which are involved in methionine synthesis in both higher organisms and bacteria, in methanogenesis in the methanogens, and in acetyl-CoA synthesis in anaerobic bacteria. Reactions utilizing this cofactor do not involve homolytic cleavage of the carbon-cobalt bond and instead proceed using ionic interactions (Banerjee and Matthews 1990, Drennan et al. 1994, Ludwig et al. 1996, Ludwig and Matthews 1997).