THE EFFECT OF DIFFERENT AMINO-ACID SIDE-CHAINS ON THE STEREOSPECIFICITY AND CATALYTIC EFFICIENCY OF THE TRYPTOPHAN SYNTHASE-CATALYZED EXCHANGE OF THE ALPHA-PROTONS OF AMINO-ACIDS

H-1-NMR has been used to follow the tryptophan synthase (EC 4.2.1.20) catalysed hydrogen-deuterium exchange of the alpha-protons of L- and D -alanine and -tryptophan. The first-order and second-order rate consta nts for exchange have been determined at pH 7.8 in the presence and ab sence of the allosteric effector, DL-alpha-glycerol 3-phosphate. In th e presence of DL-alpha-glycerol 3-phosphate the stereospecificity of t he tryptophan synthase-catalysed first-order exchange rates was in the order tryptophan > alanine > glycine. This increase in stereospecific ity was largely due to the decrease in the magnitude of the first-orde r exchange rate of the slowly exchanged alpha-proton, A similar increa se in the stereospecificity of the second-order exchange rates for ala nine was also largely due to the decrease in the magnitude of the firs t-order exchange rate of the slowly exchanged alpha-proton of Dalanine . Adding DL-alpha-glycerol 3-phosphate produced an increase in the ste reospecificity of the second-order exchange rate observed with alanine but no significant change in the stereospecificity of the first-order exchange rate with tryptophan. The alpha-subunits are shown to increa se the exchange rates of the alpha-protons of L-alanine and L-tryptoph an. We conclude that the contribution of the R-group of an amino acid to the stereospecificity of the exchange reactions of its a-proton can be similar to or larger than that of its alpha-carboxylate group. Pos sible mechanisms that could explain the stereospecificity of these exc hange reactions are discussed.