Heat capacity and conformation of proteins in the denatured state

Abstract

Heat capacity, intrinsic viscosity and ellipticity of a number of globular proteins (pancreatic ribonuclease A, staphylococcal nuclease, hen egg-white lysozyme, myoglobin and cytochrome c) and a fibrillar protein (collagen) in various states (native, denatured, with and without disulfide crosslinks or a heme) have been studied experimentally over a broad range of temperatures. It is shown that the partial heat capacity of denatured protein significantly exceeds the heat capacity of native protein, especially in the case of globular proteins, and is close to the value calculated for an extended polypeptide chain from the known heat capacities of individual amino acid residues. The significant residual structure that appears at room temperature in the denatured states of some globular proteins (e.g. myoglobin and lysozyme) at neutral pH results in a slight decrease of the heat capacity, probably due to partial screening of the protein non-polar groups from water. The heat capacity of the unfolded state increases asymptotically, approaching a constant value at about 100 °C. The temperature dependence of the heat capacity of the native state, which can be determined over a much shorter range of temperature than that of the denatured state and, correspondingly, is less certain, appears to be linear up to 80 °C. Therefore, the denaturational heat capacity increment seems to be temperature-dependent and is likely to decrease to zero at about 140 °C.