Odorant-binding proteins: structural aspects.

Structural data on odorant-binding proteins (OBPs), both in vertebrates and in insects, are reviewed and discussed. OBPs are soluble proteins interacting with odor molecules and pheromones in the perireceptor areas, the nasal mucus in vertebrates and the sensillar lymph in insects. The physiological function of these proteins is still uncertain, but information on their structure is abundant and accurate. Based on complete amino acid sequences, several subclasses have been identified, suggesting a role in odor discrimination. The OBPs of vertebrates belong to the family of lipocalins that includes proteins involved in the delivery of pheromonal messages. Those of insects do not bear significant similarity to any other class of proteins. The three-dimensional structure of the bovine OBP is a beta-barrel, while for insect OBPs a model has been proposed, mainly containing alpha-helix motifs. In some cases the amino acid residues involved in ligand binding have been identified with the use of photoaffinity label analogues.