The N-terminal domain and C-terminal domains of cobalamin-independent synthases together define a catalytic cleft in the enzyme. The N-terminal domain is thought to bind the substrate, in particular, the negatively charged polyglutamate chain. The N-terminal domain is also thought to stabilise a loop from the C-terminal domain [1].

1: Crystal structures of cobalamin-independent methionine synthase complexed with zinc, homocysteine, and methyltetrahydrofolate. Ferrer JL, Ravanel S, Robert M, Dumas R; J Biol Chem 2004;279:44235-44238. PMID:15326182