Fig. 7. Interactions with aromatic amino acids are important for xenon and glycine binding. Stereo views showing (A ) Xenon atoms (red spheres ) are predicted to bind in the N -methyl-D-aspartate receptor glycine site and stabilize the open conformation of the ligand binding domain. Interactions with phenylalanine 758 are important for xenon binding. Mutating F758 to a tryptophan, leucine, or alanine eliminates xenon binding. (B ) With glycine bound the ligand binding domain adopts a closed conformation. The W731 moves as the cleft closes and maintains close contact with the glycine molecule. Mutation of W731 to alanine or leucine greatly reduces glycine binding. The F758 residue is also involved in glycine binding; mutating it to an aromatic tryptophan has no effect on glycine binding but eliminates xenon binding. Crystallographic structures (1PBQ and 1PB7)51were obtained from the Protein Data Bank. The positions of xenon atoms shown were predicted using Grand Canonical Monte Carlo modeling simulations.11Images were created using the PyMOL Molecular Graphics System (see http://www.pymol.org; accessed May 21, 2012). Phe 484 = phenylalanine 484; Phe 758 = phenylalanine 758; Trp 731 = tryptophan 731.