ENZYMATIC MECHANISM FOR THE HYDROLYSIS OF 5,10-METHENYLTETRAHYDROPTEROYLGLUTAMATE TO 5-FORMYLTETRAHYDROPTEROYLGLUTAMATE BY SERINE HYDROXYMETHYLTRANSFERASE

Serine hydroxymethyltransferase in the presence of glycine catalyzes the hydrolysis of (6R)-5,10-methenyltetrahydropteroylpolyglutamate to (6S)-5-formyltetrahydropteroylpolyglutamate. The enzyme also catalyzes the formation of (6S)-5-formyltetrahydropteroylpolyglutamate from a compound in equilibrium with (6R)-5,10-methenyltetrahydropteroylpolyglutamate believed to be (6R,11R)-5,10-hydroxymethylenetetrahydropteroylpolyglutamate, a putative intermediate in the nonenzymatic hydrolysis of 5,10-methenyltetrahydropteroylglutamate to 5-formyltetrahydropteroylglutamate [Stover, P., & Schirch, V. (1992) Biochemistry (preceding paper in this issue)]. The enzymatic mechanism for the formation of (6S)-5-formyltetrahydropteroylpolyglutamate from these substrates and the role of glycine in the reaction was addressed. Evidence suggests that(6R,11R)-5,10-hydroxymethylenetetrahydropteroyltetraglutamate is a catalytically competent intermediate in the enzyme-catalyzed hydrolysis of (6R)-5,10-methenyltetrahydropteroyltetraglutamate. The enzyme displays a high K(m) of 40-mu-M for (6R)-5,10-methenyltetrahydropteroyltetraglutamate, while the K(m) for (6R,11R)-5,10-hydroxymethylenetetrahydropteroyltetraglutamate is below 0.5-mu-M. The k(cat) values for both reactions are identical and equal to the rate of formation of an enzyme ternary complex absorbing at 502 nm which is formed from glycine and (6S)-5-formyltetrahydropteroylpolyglutamate. The hydrolysis reaction proceeds with exchange of the C-11 formyl proton of (6R)-5,10-methenyltetrahydropteroyltetraglutamate, suggesting that the enzyme-catalyzed reaction occurs by the same C-11 carbanion inversion mechanism as the nonenzymatic reaction. Isotope exchange experiments using [2-H-3]glycine and differential scanning calorimetry data suggest both a catalytic and a conformational role for glycine in the enzymatic reaction. The results are discussed in terms of the similarity in mechanisms of the SHMT-catalyzed retroaldol cleavage of serine and hydrolysis of (6R)-5,10-methenyltetrahydropteroylpolyglutamates.