Compatibility of osmolytes with Gibbs energy of stabilization of proteins

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 degrees C (Delta G(D)degrees) which has been shown to control the in vivo rate of degradative protein turnover (Pace et al., Acta Biol. Med. Germ 30 (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 Delta H-m, the enthalpy change of denaturation at T-m. Values of Delta G(D)degrees were determined with Gibbs-Helmoltz equation using known values of T-m, Delta H-m, and Delta C-p, the constant-pressure heat capacity change. It has been observed that T-m increases with the osmolyte concentration, whereas Delta G(D)degrees remains unaffected in the presence of the osmolyte. This observation on Delta G(D)degrees in the presence of osmolytes has been considered in the physiological context. (C) 2000 Elsevier Science B.V. All rights reserved.