CLINICAL CASES
385
is due to decreased thymidylate synthetase conversion (C1
transfer) of
dUMP to dTMP as a result of insufficient cobalamin or folate pool
impairment.
One-carbon pool: Folate derivatives that carry a single carbon in various
oxidation states (formyl, methenyl, methylene, and methyl) for transfer
to acceptor molecules; that is, transfer to deoxyuridine monophosphate
to form deoxythymidine monophosphate; transfer to homocysteine to
form methionine.
D ISC U SSIO N
The structure of cobalamin (vitamin B12) is shown in Figure 42-1. Cobalamin
is somewhat analogous to heme in its structure having as its base a tetrapyr-
role ring. Instead of iron as a metal cofactor for heme, cobalamin has cobalt in
a coordination state of six with a benzimidazole group nitrogen coordinated to
one axial position, the four equatorial positions coordinated by the nitrogens
of the four pyrrole groups and the sixth position occupied by either a
Figure 42-1. The structure of cobalamin, vitamin B12. X = deoxyadenosine in
deoxyadenyosylcobalamin; X = CH3 in methylcobalamin; X = CN- in
cyanocobalamin, the commercial form found in vitamin tablets.
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