Figure 1.

The structure of tetrahydrofolate. In natural folates, the pterin ring exists in tetrahydro form (as shown) or in 7,8-dihydro form (as in DHF). The ring is fully oxidized in folic acid, which is not a natural folate. Folates usually have a γ-linked polyglutamyl tail of up to about eight residues attached to the first glutamate. One-carbon units (formyl, methyl, etc.) can be coupled to the N5 and/or N10 positions.