Thermodynamic parameters for the glutamate dehydrogenase catalyzed alpha-imino acid-alpha-amino acid interconversion

Delta 1-Piperidine 2-carboxylic acid, an alpha-imino acid, is reduced by 1,4-dihydropyridines to pipecolic acid, an alpha-amino acid, and the corresponding pyridinium ions. This nonenzymatic reaction occurs only in the direction of pipecolic acid production. Glutamate dehydrogenase catalyzes this reaction when the reductant is NADPH and gives as products L-pipecolic acid and NADP+. The reaction velocity for the enzyme-catalyzed reaction is measurable in either direction. The pH-independent equilibrium constant, Keq, for the reduction of the imino acid by NADPH to give pipecolic acid anion and NADP+ was determined from the equilibrium conditions and the pKa values of pipecolic acid (10.72) and of the cyclic imino acid (8.10). The value of Keq was found to be 175 +/- 30; the values of delta G0, delta H0 and delta S0 are -3.1 +/- 0.1 kcal/mol, 5 +/- 1 kcal/mol and 27 +/- 4 e.u., respectively. The data indicate that the reactants are far more solvated than the products and that there must be a large degree of solvent reorganization during the course of the reaction. If these thermodynamic parameters apply to the redox step of the enzyme-catalyzed glutamate reaction, then the burst phase which results upon mixing the enzyme, L-glutamate and NADP+ in stoichiometric amounts must contain a hidden nonredox step of large delta H0 value to account for the curved Arrhenius plot observed for this phase (A.H. Colen, R.T. Medary and H.F. Fisher, Biopolymers 20 (1981) 879).