Fig. 1: Reaction schemes of typical MTHFR and S6MTHFR.

a In E. coli, MTHFR catalyses the reduction of CH2-THF, which is generated by the Gly cleavage system or serine hydroxymethyltransferase (SHMT). Electrons used in the reduction of CH2-THF were derived from NADH. b Predicted reaction scheme of S6MTHFR. S6MTHFR has been suggested to catalyse the oxidation of CH3-THF, which is generated by the methyl-transfer reaction from vanillate and syringate to THF by LigM and DesA, respectively. The reduced form of FAD (FADH2) must be oxidised by an electron acceptor (X).