Analysis of free energy versus temperature curves in protein folding and macromolecular interactions.

Plots of free energy versus temperature are commonly called stability curves or Gibbs-Helmholtz curves, and they have proven to be extremely useful in protein folding and ligand-binding studies. Curvature in a Gibbs-Helmholtz or stability plot is indicative of a heat capacity change, and some of their primary uses in biochemistry over the past few decades have included determining ΔCp values and comparing ΔCp values between two related processes. This chapter describes basic approaches for analyzing curved Gibbs-Helmholtz plots, along with two specific extensions of standard Gibbs-Helmholtz plot analysis: (1) translating ΔG of folding versus temperature into ΔH and ΔS versus temperature for comparing mesophilic-thermophilic protein pairs, and (2) fitting Gibbs-Helmholtz plots to determine if ΔCp changes with temperature or not. Neither of these extensions is new, but they are infrequently used, and their use is particularly germane to certain molecular interpretations of thermodynamic information from ΔG versus temperature curves. It is shown that translating ΔG of folding into ΔH and ΔS of folding versus temperature for a mesophilic-thermophilic protein pair can immediately influence possible structural hypotheses for thermal stabilization of thermophilic proteins. It is also shown that very small temperature-dependent heat capacity changes (ΔΔCp values) can be obtained from extended fits to ΔG versus temperature plots, and that these very small ΔΔCp values can have serious consequences for any attempt to correlate ΔCp with ΔASA for some reactions. Copyright Â