Transfer of the methyl group from N5-methyltetrahydrofolates to homocysteine in Escherichia coli

Abstract

The 2 reactions by which the methyl group of N5-methyltetrahydrofolates is transferred to homocysteine by E. coli was studied with purified preparations of the relevant enzymes. Enzyme-B, (a N5-methyltetrahydropteroyltriglutamate-homocysteine methyltrans-ferase) requires only Mg2+ ions for full activity; Mn2+ ions are also active, but do not support optimum methionine formation. Enzyme-B, at several stages of purification, completed the enzymic constitution of a methionine mutant with a single metabolic lesion, indicating that it is a single protein. The cobamide-containing enzyme, which can use both N5-methyltetrahydropteroylmonoglutamate and -triglutamate as substrate, requires the presence of another protein fraction (which may be complex) as well as adenosine triphosphate (ATP), Mg2+ ions, flavin adenine dinucleotide (FAD) and a reduced nicotinamide (NADH2)-generating system. S-Adenosylhomocysteine replaces homocysteine as methyl acceptor to only a limited extent, and S-adenosylmethionine does not yield methionine even with unpurified extracts of E. coli. The FADH2, whose formation from FAD is catalyzed by the accessory protein fraction, does not replace that fraction.