Figure 7 Cyanocobalamin, one of the forms of vitamin B12. Shown are the corrin ring and the attachment of the dimethylbenzimidazole to the cobalt above the ring. The cyano group is shown attached to the cobalt below the plane of the ring.

group, or a 5' deoxyadenosyl group depending on the reaction or enzyme. As noted, 5' deoxyade-nosylcobalamin is the most common form of the coenzyme. The 5' deoxyadenosylcobalamine arises by an attack on the 5' carbon of ATP by Co+, which displaces the triphosphate group of ATP, a rare action in biochemistry. Known enzymes that require B12 fit one of two functional categories: those that transfer methyl groups from the coenzyme to the substrate, and those that take part in positional rearrangements of neighboring groups on the substrate, or group transfer reactions.

As noted above, methylation reactions in mammalian systems that involve B12 are limited to the transfer of a methyl group to homocysteine to form methionine. Recall, N5-methyl-THF is the methyl group donor in the reaction and B12 mediates the transfer. Restoring the methyl group on methio-nine primes the system to further methylation since methionine itself, acting through its active form, S-adenosylmethionine (see below), is a primary donor of methyl groups to other substrates.

Of late, there has been considerable interest in vitamin B12 reactions that have free radicals as intermediates. This may be one of the principal advantages of the coenzyme, i.e., the ability to form and retain a stable free radical in its structure. The stability of the free radical is due to the unusual chemistry of the cobalt ion.

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