Computational Probing of Temperature-Dependent Unfolding of a Small Globular Protein: From Cold to Heat Denaturation.

Fully atomistic replica exchange molecular dynamics simulations are performed to compute the stability curve of a small globular protein as accurately as possible. To investigate the individual roles of the protein and water parts, we compute the conformational entropy change of this protein directly from the simulation ensembles. This entropy calculation enables complete separations of the unfolding changes of enthalpy and the entropy into their own protein and hydration components. From this decomposition, we are able to determine the main thermodynamic factors governing the cold and heat unfolding events: the cold and heat unfolding events are largely driven by the hydration enthalpy gain and the protein conformational entropy gain, respectively. This computational study discloses several temperature-dependent unfolding thermodynamic behaviors of the protein and water compartments and establishes their unique relationship. Upon unfolding, the changes of enthalpy and entropy in the protein part are all positive convex functions of temperature, whereas the equivalent changes in the water part are all negative concave functions of temperature. Hence, these two mutually opposing effects from the protein and water parts dictate the thermodynamics of unfolding. Furthermore, consistent with the temperature-dependent behaviors of the protein part, the changes of the solvent-accessible surface area and the radius of gyration of the protein upon unfolding are also convex functions of temperature. Hence, all these new temperature dependences, combined together, pave the way to unveiling the thermodynamic and structural features of protein denaturation events at various temperature conditions.