Folic acid (Vitamin B9)

Folic acid (or, folate), which plays a key role in one-carbon metabolism, is essential for the biosynthesis of several compounds. Folic acid deficiency is probably the most common vitamin deficiency in the United States, particularly among pregnant women and individuals with alcoholism. [Note: Leafy, darkgreen vegetables are a good source of folic acid.]
 Function
Tetrahydrofolate (THF), the reduced, coenzyme form of folate, receives one-carbon fragments from donors such as serine, glycine, and histidine and transfers them to intermediates in the synthesis of amino acids, purine nucleotides, and thymidine monophosphate (TMP), a pyrimidine nucleotide incorporated into DNA (Fig. 1).

Figure 1: Production and use of tetrahydrofolate. NADP(H) = nicotinamide adenine dinucleotide phosphate.