On the status of the Michaelis-Menten equation and its implications for enzymology

Abstract

AbstractThe Michaelis-Menten equation (MME) is considered to be the fundamental equation describing the rates of enzyme-catalysed reactions, and thus the 'physicochemical key' to understanding all life processes. It is the basis of the current view of enzymes as generally proteinaceous macromolecules that bind the substrate reversibly at the active site, and convert it to the product in a relatively slow overall sequence of bonding changes ('turnover'). The manifested 'saturation kinetics', by which the rate of the enzymic reaction (essentially) increases linearly with the substrate concentration ([S]) at low [S] but reaches a plateau at high [S], is apparently modelled by the MME. However, it is argued herein that the apparent success of the MME is misleading, and that it is fundamentally flawed by its equilibrium-based derivation (as can be shown mathematically). Thus, the MME cannot be classed as a formal kinetic equation vis-a-vis the law of mass action, as it does not involve the 'incipient concentrations' of enzyme and substrate; indeed, it is inapplicable to the reversible interconversion of substrate and product, not leading to the expected thermodynamic equilibrium constant. Furthermore, the principles of chemical reactivity do not necessarily lead from the above two-step model of enzyme catalysis to the observed 'saturation kinetics': other assumptions are needed, plausibly the inhibition of product release by the substrate itself. (Ironically, thus, the dramatic graphical representation of the MME encrypts its own fundamental flaw!) Perhaps the simplest indictment of the MME, however, lies in its formulation that the rate of the enzymic reaction tends towards a maximum of k~cat~[E~o~] in the 'saturation regime'. This implies - implausibly - that the turnover rate constant k~cat~ can be known from the overall rate, but independently of the dissociation constant (K~M~) of the binding step. (Many of these arguments have been presented previously in preliminary form.)