 The Michaelis – Menten kinetic model of a single-substrate reaction is shown on the right. There is an initial bimolecular reaction between the enzyme E and substrate S to form the enzyme – substrate complex ES but the rate of enzymatic reaction increases with the increase of the substrate concentration up to a certain level but then an increase in substrate concentration does not cause any increase in reaction rate as there no more E available for reacting with S and the rate of reaction becomes dependent on ES and the reaction becomes unimolecular reaction. Although the enzymatic mechanism for the unimolecular reaction <formula> can be quite complex, there is typically one rate-determining enzymatic step that allows this reaction to be modelled as a single catalytic step with an apparent unimolecular rate constant kcat. If the reaction path proceeds over one or several intermediates, kcat will be a function of several elementary rate constants, whereas in the simplest case of a single elementary reaction (e.g. no intermediates) it will be identical to the elementary unimolecular rate constant k2. The apparent unimolecular rate constant kcat is also called turnover number and denotes the maximum number of enzymatic reactions catalysed per second.
