Enzyme Kinetics: Steady State

Abstract

Abstract The kinetics of enzyme‐catalysed reactions can be analysed in terms of steady state models if the substrate concentrations are more than an order of magnitude higher than the enzyme level. Such analysis yields Michaelis constants, maximum velocities and useful information on reaction mechanisms. Thus when there are two or more substrates, the number of terms in the denominator of the rate equation tells the type of mechanism. A sequential mechanism has a constant term plus ones in the concentration of each substrate and one including both concentrations. A ping pong mechanism has no constant term in the denominator. An ordered sequential mechanism where the binding of the first substrate is at equilibrium lacks a denominator term in the concentration of the second substrate. Other types of steady state studies include the use of inhibitors, determination of pH profiles and the effect of isotopic substitutions on the rates. Key Concepts: As long as substrate concentrations are more than an order of magnitude greater than the enzyme level, the reaction will be in a steady state. Steady state rate equations are simple ratios of a numerator that is a function of substrate concentrations and a maximum velocity divided by a denominator that represents the distribution of the enzyme among all possible forms that are present in the steady state. Steady state kinetic studies enable one to distinguish kinetic mechanisms and can also give information on rate limiting steps and in favorable cases, chemical mechanism.