is an irreversible inhibitor of thymidylate synthase. This substance, like dUMP, binds to the enzyme (an F atom is not much larger than an H atom) and undergoes the first two steps of the normal enzymatic reaction (fig. 23-15).
In step 3, however, the enzyme cannot abstract the F atom as F* (F is the most electronegative element) so that the enzyme is frozen in an enzyme-FdUMP-THF ternary covalent complex.