Usefulness of kinetic enzyme parameters in biotechnological practice

The kcat /KM ratio, where kcat is the catalytic constant for the conversion of substrate into product, and KM is the Michaelis constant, has been widely used as a measure of enzyme performance, but recent analyses have underscored the inadequacy of this ratio to describe the efficiency of a biocatalyst, particularly when employed as a criterion for selecting between enzyme variants for industrial purposes. The main problem with this kinetic relationship is that it neglects the contribution of important factors operating in actual bioprocess conditions, such as substrate concentrations and product inhibition, leading to unreal expectations on enzyme performance and erroneous selection of the most adequate biocatalyst. Two complementary formalisms, the efficiency function and the catalytic effectiveness, have been introduced to incorporate important features of any bio-industrial system. We review herein the rationales underlying each derivation, and the strengths and fields of application of both strategies. Examples of different situations, including continuous and batch-type reactors, as well as reversible and irreversible processes, are provided, together with recommendations on the use of both approaches.