Complexes of porcine odorant binding protein with odorant molecules belonging to different chemical classes

Porcine odorant binding protein (pOBP) is a monomer of 157 amino acid resid ues, purified in abundance from pig nasal mucosa. In contrast to the observ ation on lipocalins as retinol binding protein (RBP), major urinary protein (MUP) or bovine odorant binding protein (bOBP), no naturally occurring lig and was found in the beta-barrel cavity of pOBP. Porcine OBP was therefore chosen as a simple model for structure/function studies with odorant molecu les. In competition experiments with tritiated pyrazine, the affinity of pO BP towards several odorant molecules belonging to different chemical classe s has been found to be of the micromolar order, with a 1:1 stoichiometry. T he X-ray structures of pOBP complexed to these molecules were determined at resolution between 2.15 and 1.4 Angstrom. As expected, the electron densit y of the odorant molecules was observed into the hydrophobic beta-barrel of the lipocalin. Inside this cavity, very few specific interactions were est ablished between the odorant molecule and the amino acid side-chains, which did not undergo significant conformational change. The high beta-factors o bserved for the odorant molecules as well as the existence of alternative c onformations reveal a non-specific mode of binding of the odorant molecules in the cavity. (C) 2000 Academic Press.