Enzymatic synthesis of the methyl group of methionine. I. Identification of the enzymes and cofactors involved in the system isolated from Escherichia coli.

The mechanism of formation of the methyl group of methionine is of particular interest at the present time since the reaction may be studied in cell-free systems and is dependent on the direct participation of vitamin Bit. The reported systems for study of methionine biosynthesis in vitro have been derived from mammalian, avian, and bacterial sources (&lo). Serine and formaldehyde have been the most commonly studied precursors of the l-carbon fragment that appears in the methyl group of methionine. Homocysteine has been the required acceptor in all the systems. The studies in both animal and bacterial systems have established cofactor requirements for pyridoxal-P (when serine is the l-carbon unit donor), tetrahydrofolic acid, adenosine triphosphate, Mg++, and a pyridine dinucleotide. Vitamin Biz has been shown to be a required cofactor only in the system derived from a mutant strain of Escherichia coli which requires vitamin Blz or methionine for growth (11, 12). These studies have not identified the necessary enzymes or the reaction steps in the pathway for methionine synthesis. This paper reports a system of three partially purified enzyme fractions from E. coli which are essential for methionine biosynthesis from serine. One of the required enzymes is serine hydroxymethylase (13-15), and another has been found to have vitamin Bi2 as a prosthetic group. The cofactor requirements of the enzymatic system have been extended by demonstration of a requirement for a flavin coenzyme.