Protein and Nucleic Acid Enzymes

Abstract

As is true of any catalyst, the defining quality of every enzyme is the ability to enhance chemical reactivity without altering the thermodynamic equilibrium of the reaction catalyzed. An enzyme's catalytic proficiency is measured by its rate enhancement factor, a parameter that equals v enz/v non, where v enz and v non are the reaction velocities in the presence and absence of an enzyme respectively. Although the rates of uncatalyzed reactions are often too slow to be determined under physiologic conditions, enhancement factors of enzymes are thought to range typically from 109 to 1017, with a few approaching 1022 and others falling well below 100. Each catalytic cycle shares common features: (1) the enzyme must combine with its substrate, which is then converted into the transition state; (2) and this least stable reactant configuration then undergoes equipartition (i.e. half of the time, reforming EnzymeSubstrate complex, and just as often proceeding to form EnzymeProduct complex, the latter followed by product release). After each catalytic reaction cycle, an enzyme returns to its original form, such that an enzyme's action is without any effect on the reaction's equilibrium constant. Because product release is so often the slowest step in catalysis, discrete bond-making and bond-breaking steps, and even the formation of covalent intermediates, rarely hinder catalysis. The rates of highly perfected enzymes are limited only by the diffusion-limited availability of substrate; other enzymes are so sluggish that each complete catalytic cycle operates on the seconds to minutes timescale. In terms of natural selection, however, an organism accrues no competitive advantage should one of its enzymes evolve catalytic proficiency beyond that needed for efficient metabolism.