The thermodynamics of a negatively interacting allosteric effector system. The glutamate dehydrogenase . NADPH . ADP complexes.

The reaction catalyzed by bovine L-glutamate dehydrogenase is subjected to allosteric activation by ADP. We have measured the thermodynamic parameters (delta G0, delta H0, delta S0, and delta Cp) of the formation of the various possible binary and ternary complexes formed between the enzyme, NADPH, and either ADP or its analogs, adenosine, AMP, and ATP. delta H0 and delta Cp have been measured by flow calorimetry; delta G0 values obtained by calorimetry itself, differences spectroscopy, or gel filtration have been selected on the basis of accuracy under the conditions required for the formation of each complex. The data are interpreted in terms of "interaction parameters," the differences between the thermodynamic parameters of the formation of a ternary complex and the sum of those of the two related binary complexes. Both adnosine and ATP appear to loosen the binding of NADPH by simply preventing a subsite interaction of NADPH. AMP appears to have only minor and probably secondary effects. The negative effect of the binding of ADP on that of NADPH, however, involves the formation of a new interaction, which is exothermic, entropy compensated, has a moderately large negative delta Cp, and does not occur in either binary complex.