Heat capacity effects of water molecules and ions at a protein-DNA interface

The interaction of the TATA-box binding protein from the thermophilic and halophilic archaea Pyrococcus woesei (PwTBP) with an oligonucleotide containing a specific binding site is stable over a very broad range of temperatures and ionic strengths, and is consequently an outstanding system for characterising general features of protein-DNA thermodynamics. In common with other specific protein-DNA recognition events, the PwTBP-TATA box interaction is accompanied by a large negative change in heat capacity (ΔC_p) arising from the total change in solvation that occurs upon binding, which in this case involves a net uptake of cations. Contrary to previous hypotheses, we find no overall effect of ionic strength on this heat capacity change. We investigate the local contributions of site-specific ion and water binding to the overall change in heat capacity by means of a series of site-directed mutations of PwTBP. We find that although changes in the local ion binding capacity affect the enthalpic and entropic contributions to the free energy of the interaction, they do not affect the change in heat capacity. In contrast, we find remarkably large heat capacity effects arising from two particular symmetry-related mutations. The great magnitude of these effects is not explicable in terms of current semi-empirical models of heat capacity change. Previously reported X-ray crystal structures show that these mutated residues are at the centre of an evolutionarily conserved network of water-mediated hydrogen bonds between the protein and the DNA backbone. Consequently, we conclude that, in addition to water molecules buried in the protein-DNA interface that have been previously shown to influence heat capacity, bridging water molecules in a highly polar surface environment can also contribute substantially to negative heat capacity change on formation of a protein-DNA complex.