An investigation of the equilibrium of the reaction {L-aspartate(aq)+2-oxoglutarate(aq)=oxaloacetate(aq)+L-glutamate(aq)}

Abstract

Apparent equilibrium constants have been measured for the following biochemical reaction:L-aspartate(aq) + 2-oxoglutarate(aq) = oxaloacetate(aq) +L-glutamate(aq). This reaction, catalysed by aspartate transaminase, was studied over the ranges 283.15≤T/K≤303.15, 6.94≤pH≤7.13, and 0.163≤Im/(mol·kg−1)≤0.167, whereTis temperature andImis ionic strength. The instability of the oxaloacetate in solution required the use of an experimental procedure that was brief. Thus, the procedure used was to measure the change in the chromatographic response ΔRof the oxaloacetate chromatographic peak that accompanied the reaction. Values of ΔRwere measured for several solutions under near equilibrium conditions. The chromatographic response ΔRis expected to be zero for a solution that is at equilibrium with regard to the above reaction and prior to the addition of the enzyme. The results were used to calculate the standard molar Gibbs energy change ΔrGm°=(4.82±0.21) kJ·mol−1, the equilibrium constantK=(0.143±0.012), the standard molar enthalpy change ΔrHm°=(1.9±2.9) kJ·mol−1, and the standard molar entropy change ΔrSm°=− -03(10±10) J·K−1·mol−1for the following chemical reference reaction atT=298.15 K andIm=0:L-aspartate−(aq) + 2-oxoglutarate2−(aq) = oxaloacetate2−(aq) +L-glutamate−(aq). Under near physiological conditions (T= 311.15 K, pH = 7.0,Im= 0.25 mol·kg−1) the apparent equilibrium constantK′ for the overall biochemical reaction is calculated to have the value 0.147; the standard transformed Gibbs energy change ΔrGm′°=4.96 kJ·mol−1under these conditions.