Serine transhydroxymethylase. Studies on the role of tetrahydrofolate.

Studies of the reversible interconversion of glycine and serine catalyzed by rabbit liver serine transhydroxymethylase in the presence and in the absence of tetrahydrofolate are described. The studies include measurements of pHrate profiles, isotope effects, and spectral properties of the enzyme-substrate complexes. Vma, values for the synthesis of serine from glycine and formaldehyde and the exchange of the 2s proton of glycine with protons of the solvent were found to be constant between pH 6.0 and 8.0. The V max for the degradation of serine to glycine and formaldehyde decreased as the pH was increased from 6.0 to 8.4. A dissociable group on the enzyme with a pK of about 7 appears to be involved. An isotope effect of 2 was found for [2S-ZH]glycine in the synthesis of serine both in the presence and in the absence of tetrahydrofolate. The rate of serine synthesis in the presence of tetrahydrofolate is limited at high enzyme concentration by the rate of the nonenzymatic formation of 5, lo-methylenetetrahydrofolate. That formaldehyde can react directly with the enzymeglycine complex in the absence of tetrahydrofolate is suggested by the rapid quenching of the 495-m and 343-nm absorption peaks of this complex by formaldehyde. Spectral evidence (the appearance of absorption maxima at 343 nm and 495 nm above pH 8 for solutions of enzyme and serine) suggests that serine is cleaved to an enzyme-glycine complex in the absence of tetrahydrofolate. The data are interpreted to support a mechanism in which formaldehyde is bound as an imine at the active site. The role of tetrahydrofolate appears to be catalyzing the formation or breakdown of this intermediate through the formation of 5, lo-methylenetetrahydrofolate.