The investigation of enzyme catalysis has traditionally been based on the rapid equilibrium conditions defined by Henri, Michaelis and Menten. The general rate equations derived by Briggs and Haldane provided incremental progress by assuming near steady-state levels for the enzyme-substrate complex. In situ, however, enzymes may operate far from equilibrium, so that the idealizing assumptions of traditional enzyme kinetics do not hold up and the enzyme becomes an active participant in determining the outcome of a reaction. We used computer modeling of an enzyme-catalyzed reaction with substrate activation to assess the impact of the rate constants on the product production per unit time. Mapping of the parameter space in bifurcation plots displayed ranges of instability that were preceded and succeeded by distinct stable states. To test whether this phenomenon may occur in enzymes without substrate activation, we also analyzed the ordinary differential equations that describe general enzyme catalysis. We found stable responses and limit cycles to rhythmic substrate input, but no chaos. We confirmed, on the basis of theoretical calculations and experiment, that in bifurcating reactions (which are common in situ) the presence or absence of an enzyme in one arm shifts the equilibrium. These results contest the paradigm that enzymes accelerate equilibrium formation in chemical reactions without affecting the reaction outcome.
Milanowski P, Carter TJ, Weber GF (2013) Enzyme Catalysis and the Outcome of Biochemical Reactions. J Proteomics Bioinform 6:132-141.