Role of residual structure in the unfolded state of a thermophilic protein

Ribonucleases H from the thermophilic bacterium Thermus thermophilus and the mesophile Escherichia coli demonstrate a dramatic and surprising difference in their change in heat capacity upon unfolding (DeltaCpdegrees). The lower DeltaCpdegrees of the thermophilic protein directly contributes to its higher thermal denaturation temperature (T-m). We propose that this DeltaCpdegrees difference originates from residual structure in the unfolded state of the thermophilic protein; we verify this hypothesis by using a mutagenic approach. Residual structure in the unfolded state may provide a mechanism for bala icing a high T. with the optimal thermodynamic stability for a protein's function. Structure in the unfolded state is shown to differentially affect the thermodynamic profiles of thermophilic and mesophilic proteins.