Structural basis of the broad specificity of a general odorant-binding protein from honeybee.

General odorant-binding proteins (GOBPs) are believed to transport a wide range of volatile hydrophobic molecules across the aqueous sensillum lymph toward olfactory receptors in insects. GOBPs are involved in the first step of odorant recognition, which has a great impact in agriculture and in insect-mediated human disease control. We report here the first structural study of a GOBP, the honeybee ASP2, in complex with a small hydrophilic ligand. The overall fold of the NMR structure of ASP2 consists of the packing of six alpha-helices creating an internal cavity and closely resembles that of the related pheromone-binding proteins (PBPs). The predominantly hydrophobic internal cavity of ASP2 provides additional possible interactions (pi-stacking, electrostatic contact) for ligand binding. We also show that the internal cavity of ASP2 has the ability to bind ligands of different structures and properties, including a hydrophobic component of the floral scent [2-isobutyl-3-methoxypyrazine (IBMP)] and a small hydrophilic ligand. We further demonstrate that IBMP binds ASP2 with two stable alternative conformations inside the ASP2 binding pocket. The (15)N NMR relaxation study suggests that significant backbone mobility occurs at the ligand entry site at the millisecond rate, which likely plays a role in the recognition and the uptake-release mechanism of ligands by ASP2. We propose that the broad ligand specificity of GOBPs compared with PBPs is conferred by the cumulative effects of weak nonspecific protein-ligand interactions and of enhanced protein internal dynamics at the ligand entry site.