FACTORS AFFECTING THE STEREOSPECIFICITY AND CATALYTIC EFFICIENCY OF THE TRYPTOPHAN SYNTHASE-CATALYZED EXCHANGE OF THE PRO-2R AND PRO-2S PROTONS OF GLYCINE

C-13-NMR has been used to follow the tryptophan synthase (EC 4.2.1.20) catalysed hydrogen-deuterium exchange of the pro-2R and pro-2S proton s of [2-C-13]glycine. The first- and second-order rate constants for e xchange when the alpha(2) beta(2) enzyme complex is or is not saturate d with glycine have been determined at pH 7.0 and 7.8. At pH 7.8 the e ffects of binding the allosteric effector, DL-alpha-glycerol 3-phospha te, and of removing the alpha-subunits have been examined. The beta-su bunits preferentially catalyse the exchange of the pro-2R proton of gl ycine, but adding alpha-subunits decreases the stereospecificity of th e exchange reactions. Likewise, binding of DL-alpha-glycerol 3-phospha te to the alpha(2) beta(2) enzyme complex causes a further decrease in the stereospecificity of this reaction. The stereospecificity of the second-order exchange reaction catalysed by the beta-subunits is 136-f old larger than that of the alpha(2) beta(2) enzyme complex in the pre sence of DL-a-glycerol 3-phosphate, while there is only a 5-fold decre ase in the stereospecificity of the first-order exchange reaction unde r the same conditions. We discuss how these results relate to current theories which attempt to explain how the a-subunits and DL-alpha-glyc erol 3-phosphate modify the catalytic properties of tryptophan synthas e.