CRYSTALLOGRAPHIC ANALYSIS OF THE EPIMERIC AND ANOMERIC SPECIFICITY OFTHE PERIPLASMIC TRANSPORT CHEMOSENSORY PROTEIN-RECEPTOR FOR D-GLUCOSEAND 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 rece ptor of active transport for and chemotaxis toward both sugar epimers. This protein from Escherichia coli binds D-glucose with a K-d Of 2 X 10(-7) M, which is about 2 times tighter than D-galactose. The 2.0-Ang strom resolution crystal structure of the binding protein complexed wi th D-galactose has been refined to a crystallographic R-factor of 0.16 7. This structure, combined with that previously refined for the compl ex with D-glucose Vyas, N.K., Vyas, M.N., and Quiocho, F.A. (1988) Sc ience 242, 1290-1295, provides understanding, in atomic detail, of re cognition of sugar epimers and anomers. In the two complex structures, the sugar ring is positioned identically in the binding site, and eac h hydroxyl group common to both is involved in very similar cooperativ e hydrogen-bonding interactions with protein residues and ordered wate r molecules. Only the P-anomer of both monosaccharides is bound, with Asp154 OD1 primarily responsible for accepting a hydrogen bond from th e 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 spe cificities for arabinose/galactose/fucose, maltooligosaccharides, and ribose.