On the Meaning of Km and V/K in Enzyme Kinetics

Most biochemistry textbooks describe V / K, or k cat / K m, as one of the fundamental kinetic constants for catalysis in enzymatic reactions and associate it with some measure of the rate of the chemical transformation of substrate into product. However, in the reactions of all enzymes except isomerases and mutases, V / K fails to encompass a complete turnover. Instead, it can be shown that V / K actually provides a measure of the rate of capture of substrate by free enzyme into a productive complex or complexes destined to form products and complete a turnover at some later time. Similarly, V or k cat provides a measure of the rate of release of product from the productive enzyme complexes that constitute capture. It is here suggested that the symbols V / K and k cat be replaced by k cap and k rel, respectively, at least in the teaching of enzyme kinetics. Capture and release are equally necessary to generate a complete catalytic turnover, but they are determined by different things, and the proposed symbolism is less abstract than older alternatives. Used together, they provide a more accurate definition of the Michaelis constant, as K m = k rel / k cap, which is the kinetic equivalent of the thermodynamic dissociation constant, K d = k off / k on .