Conformational changes of glucose/galactose‐binding protein illuminated by open, unliganded, and ultra‐high‐resolution ligand‐bound structures

Abstract

D‐Glucose/D‐Galactose‐binding protein (GGBP) mediates chemotaxis toward and active transport of glucose and galactose in a number of bacterial species. GGBP, like other periplasmic binding proteins, can exist in open (ligand‐free) and closed (ligand‐bound) states. We report a 0.92 Å resolution structure of GGBP from Escherichia coli in the glucose‐bound state and the first structure of an open, unbound form of GGBP (at 1.55 Å resolution). These structures vary in the angle between the two structural domains; the observed difference of 31° arises from torsion angle changes in a three‐segment hinge. A comparison with the closely related periplasmic receptors, ribose‐ and allose‐binding proteins, shows that the GGBP hinge residue positions that undergo the largest conformational changes are different. Furthermore, the high‐quality data collected for the atomic resolution glucose‐bound structure allow for the refinement of specific hydrogen atom positions, the assignment of alternate side chain conformations, the first description of CO 2 trapped after radiation‐induced decarboxylation, and insight into the role of the exo‐anomeric effect in sugar binding. Together, these structures provide insight into how the hinge‐bending movement of GGBP facilitates ligand binding, transport, and signaling.