The structure of glutamine-binding protein complexed with glutamine at 1.94 A resolution: comparisons with other...

The crystal structure of the glutamine-binding protein (GlnBP) complexed with its ligand (Gln) was determined and refined to 1.94 A resolution. This ellipsoidal protein has two globular domains and is approximately 52 Ax40 Ax35 A in size. The glutamine ligand is located in the cleft between the two domains and stablized by hydrogen bondings and ionic interactions with Asp10, Gly68, Thr70, Ala67, Asp157, Arg75, Lys115, Gly119 and His156. The aliphatic portion of the glutamine ligand is sandwiched in a hydrophobic pocket formed between Phe13 and Phe50 and has 21 van der Waals contacts with GlnBP. Lys115 and His156, that are unique to GlnBP among amino acid binding proteins, apparently contribute to the ligand binding specificity of GlnBP. Asp10 is within 3 A of Lys115. These two residues are over 10 A apart in the ligand-free form of the GlnBP. In addition, GlnBP-Gln exhibits a large-scale movement of the two hinges connecting the two globular domains upon ligand binding. The most significant 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 upon ligand binding. A hydrogen bond network connects the large domain to the second hinge and a second hydrogen bond network coalesces the small 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 from the second hinge may be involved in the cascade of the conformational change that is induced by ligand binding.