Crystallographic analysis of the epimeric and anomeric specificity of the periplasmic transport/chemosensory protein receptor for D-glucose and D-galactose.

The D-glucose/D-galactose-binding protein (M(r) = 33,000) found in the periplasm of bacterial cells serves as the primary high-affinity receptor of active transport for and chemotaxis toward both sugar epimers. This protein from Escherichia coli binds D-glucose with a Kd of 2 x 10(-7) M, which is about 2 times tighter than D-galactose. The 2.0-A resolution crystal structure of the binding protein complexed with D-galactose has been refined to a crystallographic R-factor of 0.167. This structure, combined with that previously refined for the complex with D-glucose [Vyas, N.K., Vyas., M. N., & Quiocho, F. A. (1988) Science 242, 1290-1295], provides understanding, in atomic detail, of recognition of sugar epimers and anomers. In the two complex structures, the sugar ring is positioned identically in the binding site, and each hydroxyl group common to both is involved in very similar cooperative hydrogen-bonding interactions with protein residues and ordered water molecules. Only the beta-anomer of both monosaccharides is bound, with Asp154 OD1 primarily responsible for accepting a hydrogen bond from the anomeric hydroxyl. Recognition of both sugar epimers is accomplished principally by hydrogen bonding of Asp14 OD1 with the equatorial OH4 of D-glucose and OD2 with the axial OH4 of D-galactose. These results are reconciled with equilibrium and fast kinetics data, which indicate binding of both anomers of the two sugars, and further compared with sugar recognition by other periplasmic sugar-binding proteins with specificities for arabinose/galactose/fucose, maltooligosaccharides, and ribose.