The role of backbone conformational heat capacity in protein stability: Temperature dependent dynamics of the B1 domain of Streptococcal protein G

The contributions of backbone NH group dynamics to the conformational heat capacity of the B1 domain of Streptococcal protein G have been estimated from the temperature dependence of ¹⁵N NMR-derived order parameters. Longitudinal (R₁) and transverse (R₂) relaxation rates, transverse cross- relaxation rates (η(xy)), and steady state {¹H}-¹⁵N nuclear Overhauser effects were measured at temperatures of 0, 10, 20, 30, 40, and 50 °C for 89-100% of the backbone secondary amide nitrogen nuclei in the B1 domain. The ratio R₂/η(xy) was used to identify nuclei for which conformational exchange makes a significant contribution to R₂. Relaxation data were fit to the extended model-free dynamics formalism, incorporating an axially symmetric molecular rotational diffusion tensor. The temperature dependence of the order parameter (S²) was used to calculate the contribution of each NH group to conformational heat capacity (C(p)) and a characteristic temperature (T*), representing the density of conformational energy states accessible to each NH group. The heat capacities of the secondary structure regions of the B1 domain are significantly higher than those of comparable regions of other proteins, whereas the heat capacities of less structured regions are similar to those in other proteins. The higher local heat capacities are estimated to contribute up to ~0.8 kJ/mol K to the total heat capacity of the B1 domain, without which the denaturation temperature would be ~9°C lower (78°C rather than 87°C). Thus, variation of backbone conformational heat capacity of native proteins may be a novel mechanism that contributes to high temperature stabilization of proteins.