THE 1.9 ANGSTROM X-RAY STRUCTURE OF A CLOSED UNLIGANDED FORM OF THE PERIPLASMIC GLUCOSE GALACTOSE RECEPTOR FROM SALMONELLA-TYPHIMURIUM/

The three-dimensional structure of a ligand-free closed form of the gl ucose/galactose binding protein from Salmonella typhimurium has been d etermined at a resolution of 1,9 Angstrom The crystallographic R-facto r for the refined structure is 17.9%. The model contains all the atoms of the 309 residues of the protein sequence, a calcium ion, and 174 w ater molecules. The root mean square (r.m.s.) deviations for the whole molecule are: 0.010 A for bond lengths and 2.44 degrees for bond angl es, indicating a good stereochemistry for the model. This structure sh ows that the protein is able to close in the absence of ligand, adopti ng a conformation similar to the liganded form but slightly more open. Water molecules satisfy the hydrogen bonding ability of the hydrophil ic side chains of the binding site in a manner which is reminiscent of the sugars' hydrogen-bonding patterns. Since packing forces are weak, the crystallization event is unlikely to trigger a change from an ope n to a closed conformation. Instead, the latter must be one of the spe cies in equilibrium in solution which is selected by packing in the cr ystal lattice.