Massive thermal acceleration of the emergence of primordial chemistry, the incidence of spontaneous mutation, and the evolution of enzymes

Abstract

Kelvin considered it unlikely that sufficient time had elapsed on the earth for life to have reached its present level of complexity. In the warm surroundings in which life first appeared, however, elevated temperatures would have reduced the kinetic barriers to reaction. Recent experiments disclose the profound extent to which very slow reactions are accelerated by elevated temperatures, collapsing the time that would have been required for early events in primordial chemistry before the advent of enzymes. If a primitive enzyme, like model catalysts and most modern enzymes, accelerated a reaction by lowering its enthalpy of activation, then the rate enhancement that it produced would have increased automatically as the environment cooled, quite apart from any improvements in catalytic activity that arose from mutation and natural selection. The chemical events responsible for spontaneous mutation are also highly sensitive to temperature, furnishing an independent mechanism for accelerating evolution.

Keywords: Energy of Activation; Enzyme; Enzyme Catalysis; Enzyme Inhibitor; Enzyme Mechanism; Heat of Activation; Spontaneous Mutation; Tempo of Mutation; Thermodynamics.

FIGURE 1.

Free energy changes during enzyme catalysis. Estimation of K_tx, the dissociation constant of the enzyme from the altered substrate in the transition state, requires knowledge of k_non, the rate constant of the uncatalyzed reaction in water.

FIGURE 2.

Entropies of activation (TΔS^‡) plotted as a function of enthalpies of activation (ΔH^‡) of some uncatalyzed reactions (11). The yellow point represents an average value for a collection of enzyme reactions (23). OMP, orotidine 5′-phosphate decarboxylase.

FIGURE 3.

Arrhenius plots for the enzymatic and nonenzymatic α-glucosidase reactions. The bold segments of the line show the temperature range over which rate measurements were conducted (11).

FIGURE 4.

Half-lives of bonds in DNA at pH 7, based on work with individual 2′-deoxynucleosides and other model compounds (40).