Comparison of the energetics of the uncatalyzed and glutamate dehydrogenase catalyzed .alpha.-imino acid-.alpha.-amino acid interconversion

Abstract

The thermodynamic and activation parameters for the reduction of delta 1-pyrroline-2-carboxylic acid (an alpha-imino acid) by reduced nicotinamide adenine dinucleotide phosphate (NADPH) are compared with those for the reduction of the same imino acid by the glutamate dehydrogenase-NADPH complex. The enthalpies of activation and standard free energy changes for these two reactions are found to be virtually the same. The catalysis by the enzyme, expressed as the ratio of the reactivity of the enzyme--NADPH complex to that of NADPH itself in reducing the iminium ion, is entirely accounted for by a more favorable entropy of activation with enzyme--NADPH as the reductant. This entropic driving force is large enough to overcome the exergonic formation of the binary complex and still lead to considerable catalysis by glutamate dehydrogenase. Comparison of delta S not equal to and delta So values for the reduction of the iminium ion by NADPH suggests that the solvation of the transition state resembles that of the reactants, even though the substituent effects on rate have shown that the hydride transfer from the reduced coenzyme is complete at the transition state [Srinivasan, R., Medary, R. T., Fisher, H. F., Norris, D. J., & Stewart, R. (1982) J. Am. Chem. Soc. 104, 807]. The delta Go and delta S not equal to/delta So values for the reduction by the enzyme--NADPH complex indicate that this reaction has a fairly symmetric transition state, the solvation properties of which are intermediate between those of the reactants and those of the products.(ABSTRACT TRUNCATED AT 250 WORDS)

A View at the Millennium: the Efficiency of Enzymatic Catalysis

“…For enzymatic reactions involving more than one substrate, it is expected that the value of TΔ S ‡ would increase. This is so for a glutamate dehydrogenase reaction, the only two-substrate reaction for which activation parameters have been published . It is well-known (by NMR 19 ) that protein structures are in constant motion.…”

“…This is so for a glutamate dehydrogenase reaction, the only two-substrate reaction for which activation parameters have been published. 18 It is well-known (by NMR 19 ) that protein structures are in constant motion. One may observe the motions of the substrate and transition state in enzyme-catalyzed reactions by molecular dynamic (MD) simulations.…”

The Depth of Chemical Time and the Power of Enzymes as Catalysts

“…Indeed, a recent comparison of the enzymatic with the spontaneous phosphorylation of glucose by ATP, in the presence and absence of hexokinase, suggests that enthalpic and entropic contributions to transition-state affinity are of comparable magnitude, 32 and the enhancement of the rate of reduction of an imino acid by glutamate dehydrogenase appears to be entirely entropic in origin. 33 …”

“…That generalization seems less likely to apply to two-substrate reactions (including reactions involving coenzymes or other cofactors) which often require that the two substrates be bound in a configuration conducive to reaction. Indeed, a recent comparison of the enzymatic with the spontaneous phosphorylation of glucose by ATP, in the presence and absence of hexokinase, suggests that enthalpic and entropic contributions to transition-state affinity are of comparable magnitude, and the enhancement of the rate of reduction of an imino acid by glutamate dehydrogenase appears to be entirely entropic in origin …”