L-homocysteine (LHCYS) is derived from L-methionine (L-Met) and can either be remethylated to reform L-Met or take part in cysteine biosynthesis via the trans-sulfuration pathway. LHCYS remethylation can occur by the action of two enzymes; cobalamin-dependent methionine synthase and betaine-homocysteine methyltransferase, using methyltetrahydrofolate and betaine respectively as methyl donors. A third enzyme, S-methylmethionine-homocysteine S-methyltransferase (BHMT2), can use S-methylmethionine (SMM) as the methyl donor to methylate LHCYS and reform L-Met. BHMT2 is a tetrameric, cytosolic enzyme that requires one Zn2+ ion per subunit as cofactor (Szegedi et al. 2008).