title: Factors affecting the stereospecificity and catalytic efficiency of the tryptophan synthase-catalysed exchange of the pro-2R and pro-2S protons of glycine.
abstract: 13C-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 protons of [2-13C]glycine. The first- and second-order rate constants for exchange when the alpha 2 beta 2 enzyme complex is or is not saturated with glycine have been determined at pH 7.0 and 7.8. At pH 7.8 the effects of binding the allosteric effector, DL-alpha-glycerol 3-phosphate, and of removing the alpha-subunits have been examined. The beta-subunits preferentially catalyse the exchange of the pro-2R proton of glycine, but adding alpha-subunits decreases the stereospecificity of the exchange reactions. Likewise, binding of DL-alpha-glycerol 3-phosphate 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-fold larger than that of the alpha 2 beta 2 enzyme complex in the presence of DL-alpha-glycerol 3-phosphate, while there is only a 5-fold decrease in the stereospecificity of the first-order exchange reaction under the same conditions. We discuss how these results relate to current theories which attempt to explain how the alpha-subunits and DL-alpha-glycerol 3-phosphate modify the catalytic properties of tryptophan synthase.
mesh_terms: D002384:Catalysis; D003903:Deuterium / Q000378:metabolism; D005994:Glycerophosphates / Q000494:pharmacology; D005998:Glycine / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D046911:Macromolecular Substances; D009682:Magnetic Resonance Spectroscopy; D012486:Salmonella typhimurium / Q000201:enzymology; D013237:Stereoisomerism; D014367:Tryptophan Synthase / Q000378:metabolism*
chemical_list: D005994:Glycerophosphates; D046911:Macromolecular Substances; D006859:Hydrogen; C029620:alpha-glycerophosphoric acid; D003903:Deuterium; D014367:Tryptophan Synthase; D005998:Glycine