THE STRUCTURE OF GLUTAMINE-BINDING PROTEIN COMPLEXED WITH GLUTAMINE AT 1.94 ANGSTROM RESOLUTION - COMPARISONS WITH OTHER AMINO-ACID BINDING-PROTEINS

The crystal structure of the glutamine-binding protein (GLnBP) complex ed with its ligand (Gln) was determined and refined to 1.94 Angstrom r esolution. This ellipsoidal protein has two globular domains and is ap proximately 52 Angstrom x 40 Angstrom x 35 Angstrom in size. The gluta mine ligand is located in the cleft between the two domains and stabil ized by hydrogen bondings and ionic interactions with Asp10, Gly68, Th r70, Ala67, Asp157, Arg75, Lys115, Gly119 and His156. The aliphatic po rtion of the glutamine ligand is sandwiched in a hydrophobic pocket fo rmed between Phe13 and Phe50 and has 21 van der Waals contacts with GL nBP. Lys115 and His156, that are unique to GLnBP among amino acid bind ing proteins, apparently contribute to the ligand binding specificity of GLnBP. Asp10 is within 3 Angstrom of Lys115. These two residues are over 10 Angstrom apart in the ligand-free form of the GLnBP. In addit ion, GLnBP-GLn exhibits a large-scale movement of the two hinges conne cting the two globular domains upon ligand binding. The most significa nt changes are 41.1 degrees in the phi angle of Gly89 and 34.3 degrees in the psi angle of Glu181 from the first and the second hinge of the protein, respectively. Besides the original six hydrogen bonds, three extra hydrogen bonds can be observed between the two hinge strands up on ligand binding. A hydrogen bond network connects the large domain t o the second hinge and a second hydrogen bond network coalesces the sm all domain to the same strand, both via interaction with the glutamine ligand. Although the two strands of the hinge connecting the domains do not directly participate in the ligand binding, GLn183 and Tyr185 f rom the second binge may be involved in the cascade of the conformatio nal change that is induced by ligand binding. (C) 1998 Academic Press Limited.