Interrelationships between folic acid and cobalamin in the synthesis of methionine by extracts of Escherichia coli

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Abstract
Aqueous extracts of acetone-dried E. coli PA 15 (a serine or glycine auxotroph) synthesize methionine from homocysteine and serine. Tetrahydropteroylglutamic acid appears to inhibit the function of the natural folic acid coenzyme present in the extracts. Tetrahydropteroylglutamic acid will, however, itself serve as cofactor, but only provided that the organisms have been grown on a medium containing cobalamin. Addition of cobalamin to the enzymic extract has no effect. A nondiffusible, heat-labile material which promotes methionine synthesis in the presence of tetrahydropteroylglutamic acid has been obtained from extracts of organisms grown with cobalamin and partly purified. The purified fraction still contains a significant amount of cobalamin. The reversible transformation of serine into glycine is catalysed by tetrahydropteroylglutamic acid whether or not the organisms have been grown with cobalamin. With L-serine as the C1 donor the other requirements for methionine synthesis are homocysteine, hexose diphosphate, adenosine triphosphate, diphosphopyridine nucleotide, pyridoxal phosphate and Mg2+ ions. With the formaldehyde derivative of tetrahydropteroylglutamic acid as donor pyridoxal phosphate is not required. Serine is not on the pathway of synthesis of the methyl group of methionine from formaldehyde. Glycine has one-third of the activity of serine as C1 donor when tested under the same conditions.