Figure 4-2.
Structures of the various 1-carbon carriers of tetrahydrofolate
(THF). THF can carry one-carbon units in the oxidation states of methanol
(N5-methyl THF), formaldehyde (N5,N10-methylene THF) or formic acid
(remaining structures).
To restore the levels of methionine (an essential amino acid),
homocysteine must be methylated (Figure 4-3). This reaction is dependent on
N5-methyl THF and vitamin B12.
Methionine levels can be limiting,
the availability of N5-methyl THF for conversion of homocysteine to methion-
ine critical. Further, in the absence of B12, THF can be trapped in the N5-methyl
THF form and thus be removed from the THF pool. This is referred to as
“methyl trap,”
which can impact other areas of 1-carbon metabolism, such as
dTMP production.
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