Tetrahydrofolate coenzymes are derived from the vitamin folic acid (see structure here) and they participate in the generation and utilization of single-carbon functional groups-methyl, methylene, and formyl. Once inside of a cell, folate is converted to active forms (dihydrofolate and tetrahydrofolate) by two successive reductions catalyzed by the NADPH-specific enzyme, dihydrofolate reductase (DHFR) (see here). Activity of DHFR is blocked by folate analogs, such as aminopterin (see here) and amethopterin (methotrexate). The coenzymatic function of tetrahydrofolate is the mobilization and utilization of single-carbon functional groups. These reactions are involved in the metabolism of serine (see below), glycine (see below), and methionine (see here), among the amino acids. Single carbon units derived from tetrahydrofolate coenzymes are also used in the synthesis of purine nucleotides (Figure 22.4 and Figure 22.5) and thymine nucleotides (Figure 22.18).

Figure 20.17 shows the metabolic reactions involving single-carbon adducts of tetrahydrofolate. Tetrahydrofolate can acquire single-carbon units from diverse sources. Degradation of histidine, in both bacterial and animal cells, yields 5-formiminotetrahydrofolate, as does the bacterial fermentation of purines. However, most organisms derive their activated single-carbon units from the -carbon of serine and the subsequent oxidation of glycine, as follows:

Serine + Tetrahydrofolate <=> Glycine + 5,10-Methylenetetrahydrofolate

The reaction can be reversed to provide a way to synthesize serine, as well. In mitochondria, the glycine cleavage system can catalyze the following reaction:

Glycine + Tetrahydrofolate + NAD⁺ -> 5,10-Methylenetetrahydrofolate + CO2 + NH3 + NADH + H⁺

This reaction is the chief catabolic fate of glycine in most cells.

5,10-Methenyltetrahydrofolate acts as a chromophore in enzymes called photolyases that absorb light energy and use it to power the breaking of pyrimidine-pyrimidine bonds in damaged DNA.