Physical chemistry study of protein -protein interactions in a specific model of TEM-1 beta-lactamase and its inhibitors

A study of protein-protein interactions in the binding of TEM-1 beta-lactamase and beta-lactamase inhibitory protein (BLIP) is presented. First, we determined the thermodynamics of binding interactions between TEM-1 and a set of alanine substituted contact residue mutants of BLIP. Based on our measurements, a contact residue of BLIP has been categorized as an anti-canonical residue since its alanine substitution exhibits a change of thermodynamic property, which does not match the change of hydrophobicity due to the mutation. The interactions between TEM-1 and BLIP canonical contact residues contribute directly to binding free energy while the anti-canonical behavior of certain residues is a result of mutation-induced modifications. To understand the structural basis of the binding thermodynamics, we determined the high-resolution structures of several thermodynamically distinctive complexes of BLIP mutants with TEM-1 using protein X-ray crystallography. Detailed structural analyses of a tight complex of BLIP Y51A/TEM-1, which has the most negative binding heat capacity change, and of a weak complex of BLIP W150A/TEM1, which has the least negative binding heat capacity change, have identified a strong long distance coupling which is important for tight binding. The analyses also reveal that an increased number of interface-trapped water molecules is correlated with poor interface packing and inversely correlated with binding heat capacity changes of the BLIP mutants with TEM-1. Furthermore, kinetic measurements showed that an increased number of interface-trapped water molecules enhanced the dissociation rate without significantly changing the association rate.