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This study led to the conclusion that naturally occurring osmolytes which are known to protect proteins against denaturing stresses, do not perturb the Gibbs energy of stabilization of proteins at 25°C (ΔG_D°) which has been shown to control the in vivo rate of degradative protein turnover (Pace et al., Acta Biol. Med. Germ 40 (1981) 1385–1392). This conclusion has been reached from our studies of heat-induced denaturation of lysozyme, ribonuclease A, cytochrome c and myoglobin in the presence of different concentrations of osmolytes, namely, glycine, proline, sarcosine and glycine-betaine. At a fixed concentration of osmolyte a heat-induced denaturation curve measured by following changes in the molar absorption coefficient of the protein, was analyzed for T_m, the midpoint of the denaturation and ΔH_m, the enthalpy change of denaturation at T_m. Values of ΔG_D° were determined with Gibbs–Helmoltz equation …