Fig. 4. Enzymatic strategies to methylate l-homocysteine back to l-methionine using betaine or S-methylmethionine (SMM) as methyl donor. The orange marked methyl group is transferred to l-homocysteine. BHMT, betaine-l-homocysteine S-methyltransferase; HSMT; l-homocysteine S-methyltransferase.