Thymidylate synthesis. Thymidylate synthase catalyzes the addition of a methyl group (derived from N5, N10-methylenetetrahydrofolate) to dUMP to form TMP. The addition of a thiolate from the enzyme activates dUMP. Opening the five-membered ring of the THF derivative prepares the methylene group for nucleophilic attack by the activated dUMP. The reaction is completed by the transfer of a hydride ion to form dihydrofolate.