The Role of Folate in Methionine Metabolism

In addition to its role in the synthesis of proteins and the polyamines spermidine tmd spermine, the main metabolic role of methionine is as a methyl donor in a wide variety of biosynthetic reactions. 

There are two separate homocysteine methyltransferases in most tissues. One uses methyl-tetrahydrofolate as the methyl donor and has vitamin B12 (cohalamin Section 10.8.1) as its prosthetic group. This enzyme is also known as methionine synthetase it is the only homocysteine methylti cmsferase in the central nervous system. The other enzyme utilizes betaine (an intermediate in the catabolism of choline Section 14.2.1) tis the methyl donor and does not require vitamin B12. 

Unlike most enzymes utilizing or metabolizing tetrahydrofolate, methionine synthetase has equal activity towtud methyl-tetreihydrofolate mono- emd polyglutamates. As discussed in Section 10.2.2, demethylation of methyl-tetrahydrofolate is essentitd for the polygluttunylation emd intracellular accumulation of folate. 

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