A comprehensive calorimetric investigation of an entropically driven T cell receptor-peptide/MHC interaction

¹ University of Notre Dame

^* To whom correspondence should be addressed. E-mail: bbaker2{at}nd.edu.

Submitted on January 17, 2007
Revised on February 19, 2007
Accepted on 13 March 2007

	Abstract

The T cell receptor (TCR) is responsible for recognizing peptides bound and "presented" by major histocompatibility complex (MHC) molecules. We recently reported that at 25 °C the A6 TCR, which recognizes the Tax peptide presented by the class I MHC HLA-A2, binds with a weak H°, a favorable S°, and a moderately negative C_p. 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 pK 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 pK shift. Our results indicate that intrinsically, A6 binds Tax/HLA-A2 with a very weak H°, an even more favorable S° than previously thought, and a relatively large negative C_p. 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 pK shift suggest it may be linked to an additional process such as ion binding.

Key Words: T cell receptor, binding thermodynamics, major histocompatibility complex, proton linkage, titration calorimetry

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