A Comprehensive Calorimetric Investigation of an Entropically Driven T Cell Receptor-Peptide/Major Histocompatibility Complex Interaction

ABSTRACT The ab T cell receptor (TCR) is responsible for recognizing peptides bound and ‘‘presented’ ’ by major histocompatibility complex (MHC) molecules. We recently reported that at 25C the A6 TCR, which recognizes the Tax peptide presented by the class I MHC human leukocyte antigen-A*0201 (HLA-A2), binds with a weak DH, a favorable DS, and a moderately negative DCp. These observations were of interest given the unfavorable binding entropies and large heat capacity changes measured for many other TCR-ligand interactions, suggested to result from TCR conformational changes occurring upon binding. Here, we further investigated the A6-Tax/HLA-A2 interaction using titration calorimetry. We found that binding results in a pKa shift, complicating interpretation of measured binding thermodynamics. To better characterize the interaction, we measured binding as a function of pH, temperature, and buffer ionization enthalpy. A global analysis of the resulting data allowed determination of both the intrinsic binding thermodynamics separated from the influence of protonation as well as the thermodynamics associated with the pKa shift. Our results indicate that intrinsically, A6 binds Tax/HLA-A2 with a very weak DH, an even more favorable DS than previously thought, and a relatively large negative DCp. Comparison of these energetics with the makeup of the protein-protein interface suggests that conformational adjustments are required for binding, but these are more likely to be structural shifts, rather than disorder-to-order transitions. The thermodynamics of the pKa shift suggest protonation may be linked to an additional process such as ion binding