Enthalpically-driven ligand recognition and cavity solvation of bovine odorant binding protein.

Lipocalins are a widely distributed family of extracellular proteins typically involved in the transport of small hydrophobic molecules. To gain new insights into the molecular basis that governs ligand recognition by this ancient protein family, the binding properties of the domain-swapped dimer bovine odorant binding protein (bOBP) and its monomeric mutant bOBP121G+ were characterized using calorimetric techniques and molecular dynamics simulations. Thermal unfolding profiles revealed that the isolated bOBP subunits behave as a cooperative folding unit. In addition, bOBP and bOBP121G+ exhibited similar ligand binding properties, characterized by a non-classical hydrophobic effect signature. The energetic differences in the binding of bOBP to 1-hexen-3-ol and the physiological ligand 1-octen-3-ol were strikingly larger than those observed for the interaction of other lipocalins with congeneric ligands. MD simulations revealed that the recurrent opening of transient pores in the submicrosecond timescale allows a profuse exchange of water molecules between the protein interior and the surrounding solvent. This picture contrasts with other lipocalins whose ligand-free binding cavities are devoid of solvent molecules. Furthermore, the simulations indicated that internal water molecules solvate the protein cavity suboptimally, forming fewer hydrogen bonds and having lower density and higher potential energy than bulk water molecules. Upon ligand occupation, water molecules were displaced from the binding cavity in an amount that depended on the ligand size. Taken together, calorimetric and MD-simulation results are consistent with a significant contribution of cavity desolvation to the enthalpically-driven interaction of bOBP with its hydrophobic ligands.

Polyphenolic Compounds and Digestive Enzymes: In Vitro Non-Covalent Interactions.

The digestive enzymes-polyphenolic compounds (PCs) interactions behind the inhibition of these enzymes have not been completely studied. The existing studies have mainly analyzed polyphenolic extracts and reported inhibition percentages of catalytic activities determined by UV-Vis spectroscopy techniques. Recently, pure PCs and new methods such as isothermal titration calorimetry and circular dichroism have been applied to describe these interactions. The present review focuses on PCs structural characteristics behind the inhibition of digestive enzymes, and progress of the used methods. Some characteristics such as molecular weight, number and position of substitution, and glycosylation of flavonoids seem to be related to the inhibitory effect of PCs; also, this effect seems to be different for carbohydrate-hydrolyzing enzymes and proteases. The digestive enzyme-PCs molecular interactions have shown that non-covalent binding, mostly by van der Waals forces, hydrogen binding, hydrophobic binding, and other electrostatic forces regulate them. These interactions were mainly associated to non-competitive type inhibitions of the enzymatic activities. The present review emphasizes on the digestive enzymes such as α-glycosidase (AG), α-amylase (PA), lipase (PL), pepsin (PE), trypsin (TP), and chymotrypsin (CT). Existing studies conducted in vitro allow one to elucidate the characteristics of the structure-function relationships, where differences between the structures of PCs might be the reason for different in vivo effects.