A THERMODYNAMIC INVESTIGATION OF REACTIONS CATALYZED BY TRYPTOPHAN SYNTHASE

Microcalorimetry and high-performance liquid chromatography have been used to conduct a thermodynamic investigation of the following reactio ns catalyzed by the tryptophan synthase alpha(2) beta(2) complex (EC 4 .2.1.20) and its subunits: indole(aq) + L-serine(aq) = L-tryptophan(aq ) + H2O(1); L-serine(aq) = pyruvate(aq) + ammonia(aq); indole(aq) + D- glyceraldehyde 3-phosphate(aq) = 1-(indol-3-yl)glycerol 3-phosphate(aq ); L-serine(aq) + 1-(indol-3-yl)glycerol 3-phosphate(aq) = L-tryptopha n(aq) + D-glyceraldehyde 3-phosphate(aq) + H2O(1). The calorimetric me asurements led to standard molar enthalpy changes for all four of thes e reactions. Direct measurements yielded an apparent equilibrium const ant for the third reaction; equilibrium constants for the remaining th ree reactions were obtained by using thermochemical cycle calculations . The results of the calorimetric and equilibrium measurements were an alyzed in terms of a chemical equilibrium model that accounted for the multiplicity of the ionic states of the reactants and products. Therm odynamic quantities for chemical reference reactions involving specifi c ionic forms have been obtained. These quantities permit the calculat ion of the position of equilibrium of the above four reactions as a fu nction of temperature, pH, and ionic strength. Values of the apparent equilibrium constants and standard transformed Gibbs free energy chang es Delta(r)G'(o)(m) under approximately physiological conditions are g iven. Le Chatelier's principle provides an explanation as to why, in t he metabolic pathway leading to the synthesis of L-tryptophan, the thi rd reaction proceeds in the direction of formation of indole and D-gly ceraldehyde 3-phosphate even though the apparent equilibrium constant greatly favors the formation of 1-(indol-3-yl) glycerol 3-phosphate. ( C) 1998 Elsevier Science B.V. All rights reserved.