THE EFFECT OF HISTIDINE-228 ON THE CATALYTIC EFFICIENCY AND STEREOSPECIFICITY OF THE SERINE HYDROXYMETHYLTRANSFERASE CATALYZED EXCHANGE OF THE ALPHA-PROTONS OF AMINO-ACIDS

C-13-NMR has been used to determine how replacing the histidine-228 re sidue of serine hydroxymethyltransferase (EC 2.1.2.1) by an asparagine residue effects the catalysis of the hydrogen-deuterium exchange of t he alpha-protons of [2-C-13]glycine at pH 7.8. The H228N mutation did not lead to a large change in the stereospecificity of the first order exchange rates of the alpha-protons of glycine both in the presence a nd in the absence of tetrahydrofolate. However, the mutation did lead to large decreases in the stereospecificity of the second order exchan ge rate in both the presence and the absence of tetrahydrofolate. In t he absence of tetrahydrofolate this decrease in stereospecificity was largely due to the decrease in the second order exchange rate of the p ro-2S proton, while in the presence of tetrahydrofolate the large incr ease in the second order exchange rate of the pro-2R proton of glycine made a major contribution. We conclude that the H228N mutation has si gnificant effects on the catalytic efficiency and stereospecificity of the second order exchange reactions, but only a small effect on the c orresponding first order exchange reactions. (C) 1998 Elsevier Science B.V, All rights reserved.