A thermodynamic molecular switch in biological systems:: Ribonuclease S′ fragment complementation reactions

It is well known that essentially all biological systems function over a very narrow temperature range. Most typical macromolecular interactions show Delta H degrees(T) positive (unfavorable) and a positive Delta S degrees(T) (favorable) at low temperature, because of a positive (Delta Cp degrees/T). Because Delta G degrees(T) for biological systems shows a complicated behavior, wherein Delta G degrees(T) changes from positive to negative, then reaches a negative value of maximum magnitude (favorable), and finally becomes positive as temperature increases, it is clear that a deeper-lying thermodynamic explanation is required. This communication demonstrates that the critical factor is a temperature-dependent Delta Cp degrees(T) (heat capacity change) of reaction that is positive at low temperature but switches to a negative value at a temperature well below the ambient range. Thus the thermodynamic molecular switch determines the behavior patterns of the Gibbs free energy change and hence a change in the equilibrium constant, K-eq, and/or spontaneity. The subsequent, mathematically predictable changes in Delta H degrees(T), Delta S degrees(T), Delta W degrees(T), and Delta G degrees(T) give rise to the classically observed behavior patterns in biological reactivity, as may be seen in ribonuclease S' fragment complementation reactions.