Serine transhydroxymethylase in methionine biosynthesis in Saccharomyces cerevisiae.

Serine transhydroxymethylase appears to be the first enzyme in the synthesis of the methyl group of methionine. Properties of serine transhydroxymethylase activity as assayed by the production of formaldehyde were correlated with properties of cell-free extracts for the methylation of homocysteine deriving the methyl group from the beta-carbon of serine. The reaction required pyridoxal phosphate and tetrahydrofolic acid, and was characterized in cell-free extracts with respect to Michaelis constant, pH optimum, incubation time, and optimal enzyme concentration. The activity was sensitive to inhibition by methionine, and to a much greater extent by S-adenosylmethionine. Serine transhydroxymethylase and the methylation of homocysteine reactions were not repressed by methionine and were stimulated by glycine. The activities of cell-free extracts for these reactions were significantly higher in cells in exponential than in stationary growth. When cells were grown in 10 mm glycine, the activities remained high throughout the culture cycle. The data indicated that glycine rather than methionine is involved in the control of the formation of the enzyme.