Enzyme kinetics and the rate of biological processes.

Abstract

It is found empirically that a simple modification of the usual theoretical kinetic formulation (in which a transformation in the temperature scale is made) describes the temperature dependence of a wide variety of biochemical processes with a greater accuracy than hitherto achieved. Used in conjunction with the formulation of the theory of absolute reaction rates this empirical relation facilitates the determination of the thermodynamic functions. The results of applying these relations to biochemical processes support the contention that in the lower temperature range of enzyme activity a thermodynamic equilibrium exists between catalytically active and inactive forms of the enzyme. It is suggested that at low temperatures the formation of intramolecular hydrogen bridges converts reactive enzyme particles to a catalytically inactive condition, in which the active centers either lose their specific configuration or are no longer exposed to the substrate. Upon the basis of this interpretation, values of the entropy changes that are calculated theoretically are found to be in agreement with those calculated from the experimental data. The reactive configuration of the enzyme is apparently possessed in only a relatively narrow temperature band, being lost at both high and low temperatures. The kinetics of biological processes appear to differ only quantitatively from those of in vitro enzyme-catalyzed reactions. In both cases the non-linearity of the Arrhenius plots appears to be due to the fact that in the lower temperature range of enzyme activity a series of reactions are involved in the formation of the activated complex. These include reactions which lead to the formation of the catalytically reactive form of the enzyme followed by that which leads to the formation of the activated complex. The conversion of enzymes to the catalytically inactive form is essentially completed at temperatures of -10-0 degrees C. in living systems, whereas in in vitro experiments with purified enzyme preparations this condition is not attained until temperatures 30-60 degrees C. lower have been reached.