The evolution of enzyme kinetic power.

Evolution of the kinetic potential of enzyme reactions is discussed. Quantitative assessment of the evolution of enzyme action has usually focused on optimization of the parametric ratio kcat./Km, which is the apparent second-order rate constant for the reaction of free substrate with free enzyme to give product. We propose that the general form kcat.[E]T/Km (where [E]T is total enzyme concentration), which is designated the 'kinetic power', is the real measure of kinetic/catalytic potential in situ. The standard paradigm of 'perfection' dictates the evolutionary maximum of 'kinetic power' to be k+s[E]T/2, where k+s is the diffusion-controlled rate constant for formation of the ES complex (and, hence, for the overall enzyme reaction). We discuss the role of protein conformational mobility in determining this state of 'perfection', via gating of substrate binding and determination of the catalytic configuration. Going beyond the level of the individual enzyme, we indicate the manner by which the organizational features of enzyme action in vivo may enhance the 'kinetic power'. Through evolutionary 'perfection' of the microenvironment, one finds that the 'kinetic power' of enzymes can be affected by alteration of [E]T as well as the unitary rate constants. At this level of complexity, we begin to realize that the 'kinetic' description of cell metabolism must be supplemented with thermodynamic concepts.