A Thermodynamic Molecular Switch in Biological Systems: Ribonuclease S′ Fragment Complementation Reactions

Abstract

It is well known that essentially all biological systems function over a very narrow temperature range. Most typical macromolecular interactions show ΔH°( T) positive (unfavorable) and a positive ΔS°( T) (favorable) at low temperature, because of a positive ( ΔCp°/T) . Because ΔG°( T) for biological systems shows a complicated behavior, wherein ΔG°( 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 ΔCp°( 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 ΔH°( T) , ΔS°( T) , ΔW°( T) , and ΔG°( T) give rise to the classically observed behavior patterns in biological reactivity, as may be seen in ribonuclease S′ fragment complementation reactions.[[page end]]