Energetics of Ligand Recognition and Self-Association of Bovine β-Lactoglobulin: Differences between Variants A and B

An understanding of the interplay between structure and energetics is crucial for the optimization of modern protein engineering techniques. In this context, the study of natural isoforms is a subject of major interest, as it provides the scenario for analyzing mutations that have endured during biological evolution. In this study, we performed a comparative analysis of the ligand-recognition and homodimerization energetics of bovine beta-lactoglobulin variants A (beta lgA) and B (beta lgB). These variants differ by only two amino-acid substitutions: 64th (Asp(A) -> Gly(B)), which is fully exposed to the solvent, and 118th (Val(A) -> Ala(B)), immersed in the hydrophobic core of the protein. Calorimetric measurements revealed significant enthalpic and entropic differences between the isoforms in both binding processes. A structural comparison suggests that a variation in the conformation of the loop C-D, induced by mutation Asp/Gly, could be responsible for the differences in ligand-binding energetics. While recognition of lauric acid was entropically driven, recognition of sodium dodecyl sulfate was both entropically and enthalpically driven, confirming the key role of the ligand polar moiety. Because of a more favorable enthalpy, the dimerization equilibrium constant of beta lgB was larger than that of beta lgA at room temperature, while the two dimers became similarly stable at 35 degrees C. The isoforms exchanged the same number of structural water molecules and protons and shared similar stereochemistry at the dimer interface. MD simulations revealed that the subunits of both variants become more flexible upon dimer formation. It is hypothesized that a larger increase of beta lgA mobility could account for the dimerization energetic differences observed.