14. Vitamin B

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Although the nutritional literature still uses the term vitamin B … no species of plants have the enzymes necessary for vitamin B. 12. synthesis. This …

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14. Vitamin B12
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14.1 Role of vitamin B12 in human metabolic processes
Although the nutritional literature still uses the term vitamin B12, a more
specific name for vitamin B12 is cobalamin. Vitamin B12 is the largest of the B
complex vitamins, with a relative molecular mass of over 1000. It consists of
a corrin ring made up of four pyrroles with cobalt at the centre of the ring
(1, 2).
There are several vitamin B12-dependent enzymes in bacteria and algae, but
no species of plants have the enzymes necessary for vitamin B12 synthesis. This
fact has significant implications for the dietary sources and availability of
vitamin B12. In mammalian cells, there are only two vitamin B12-dependent
enzymes (3). One of these enzymes, methionine synthase, uses the chemical
form of the vitamin which has a methyl group attached to the cobalt
and is called methylcobalamin (see Chapter 15, Figure 15.2). The other
enzyme, methylmalonyl coenzyme (CoA) mutase, uses a form of vitamin B12
that has a 5?-adeoxyadenosyl moiety attached to the cobalt and is called 5?-
deoxyadenosylcobalamin, or coenzyme B12. In nature, there are two other
forms of vitamin B12: hydroxycobalamin and aquacobalamin,…