Entropic stabilization of myoglobin by subdenaturing concentrations of guanidine hydrochloride

To find out the changes in the internal dynamics and function of proteins as a consequence of their binding interactions with guanidine hydrochloride (GdnHCl), laser flash photolysis and optical absorption methods have been used to study the dynamic events in the horse myoglobin–CO complex (MbCO) in the presence of subdenaturing concentrations of GdnHCl at pH 7, 22 °C. The rate coefficients for geminate rebinding and bimolecular rebinding (kon) were measured by laser photolysis of CO in MbCO, and the CO dissociation rate (koff) was determined by the CO replacement method using hexacyanoferrate ion or NO. Starting from the native-state condition, the values of kon and koff decrease by approximately 1.4 (±0.1)-fold in the presence of 0.1–0.3 M GdnHCl, and then increase at higher concentrations of the denaturant. This has been taken as evidence for internal motional constraints and increased stability of the protein in the subdenaturing region giving rise to gated entry of the photolyzed CO from the solvent. The rate for geminate rebinding does not show any decrease in the rate versus GdnHCl concentration plots. The values for the activation enthalpy for the CO dissociation reaction and the entropy loss relative to the native-state entropy, both measured as a function of GdnHCl concentration, indicate that the protein is indeed stabilized under subdenaturing conditions. Analyses of thermal unfolding transitions of myoglobin in the presence of different concentrations of GdnHCl indicate that the stability of this protein extracted from the linear free energy model is approximately 3–4 kcal mol−1 less than the true stability. The results indicate the appropriateness of the denaturant binding model for the analysis of GdnHCl-induced unfolding data, and provide a value of 7.9 kcal mol−1 as the true stability of the protein.