Kinetics and thermodynamics of metabolite transfer between enzymes

Based on experimental evidence put forward by Bernhard and others, we explore the kinetics and thermodynamics of the proposed direct and diffusional transfer mechanisms of enzymatic catalysis. Data for transient transfer of NADH between two cognate dehydrogenases (E1 and E2) are combined with steady-state catalytic data to quantify the kinetics of the two transfer mechanisms. In order to rationalize these data we find: (1) that the rate constants for direct transfer of NADH from E1-NADH to E2 must be much larger when a reactive metabolite, M, is bound to E2, (2) that a significant amount of noncatalytic complex E1-E2-M must be formed, and (3) that dissociation constants of the order of 1 microM are required for the ternary complexes involved in direct transfer (E1-NADH-E2). Using values of rate parameters similar to those assumed in these calculations, we proceed to explore the kinetics and thermodynamics of a hypothetical two-enzyme segment of a metabolic pathway that involves direct and diffusional metabolite transfer operating in parallel. Under steady-state conditions, we conclude from our calculations: (1) that the flux through the direct transfer branch would be comparable to or greater than that through the diffusional transfer branch under physiological conditions, (2) that activity effects resulting from physiological concentrations of inert protein enhance the predominance of the direct transfer flux, (3) that significant concentrations of ternary transfer complexes are formed, (4) that changes in the catalytic mechanism involving the ternary complex have little effect, (5) that direct transfer significantly moderates the reduction in metabolic flux caused by protein complexation, and (6) that direct transfer greatly alters the thermodynamics and kinetics of our hypothetical pathways. We conclude, therefore, that direct transfer--when it exists--would have important kinetic, thermodynamic, and physiological consequences in vivo.