Novel mode of antagonist binding in NMDA receptors revealed by the crystal structure of the GluN1-GluN2A ligand-binding domain complexed to NVP-AAM077

Competitive antagonists against N-methyl-D-aspartate (NMDA) receptors have played critical roles throughout the history of neuropharmacology and basic neuroscience. There are currently numerous NMDA receptor antagonists containing a variety of chemical groups. Among those compounds, a GluN2-specific antagonist, (R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5 -yl)-methyl]-phosphonic acid (NVP-AAM077), contains a unique combination of a dioxoquinoxalinyl ring, a bromophenyl group, and a phosphono group. Here, we present the crystal structure of the isolated ligand-binding domain (LBD) of the GluN1-GluN2A NMDA receptor in complex with the GluN1 agonist glycine and the GluN2A antagonist NVP-AAM077. The structure shows placement of the dioxoquinoxalinyl ring and the phosphono group of NVP-AAM077 in the glutamate binding pocket in GluN2A and the novel interaction between the bromophenyl group and GluN1-Glu781 at the GluN1-GluN2A subunit interface. Site-directed mutagenesis of GluN1-Glu781 reduced the potency of inhibition by NVP-AAM077, thus confirming the involvement of the GluN1 subunit for binding of NVP-AAM077. The unique antagonist binding pattern shown in this study provides a novel dimension to design and create antagonists with potential therapeutic values