In addition, it serves as a carrier molecule for single-carbon moieties, that is, groups containing one carbon atom e.g. methyl, methylene, methenyl, formyl, or formimino.

When combined with one such single-carbon moiety as in 10-formyltetrahydrofolate, it acts as a donor of a group with one carbon atom.

Tetrahydrofolate gets this extra carbon atom by sequestering formaldehyde produced in other processes.

These single-carbon moieties are important in the formation of precursors for DNA synthesis.

[2][3][4] Methotrexate acts on dihydrofolate reductase, like pyrimethamine or trimethoprim, as an inhibitor and thus reduces the amount of tetrahydrofolate made.