Homology model of ATD of NR2B subunit. A. Ribbon representation of the amino-terminal domain of NR2B was constructed as described under Materials and Methods using mGluR1 crystal structure (1 EWK) as template . Model resembles the clam-shell configuration as seen in the bacterial periplasmic leucine-isoleucine-valine binding protein (LIVBP) (2 LIV). Critical residues (Asp101, Ile150, Phe176) that reduced ifenprodil sensitivity 60-fold and more, when mutated, as compared to wild-type NMDA receptors  are shown as red sticks using Rasmol. A mutation (D113A) outside the putative ifenprodil binding pocket is identified by blue sticks. B. Putative ifenprodil binding pocket highlighting the three critical amino acid residues that play critical role in interacting with ligand and the inhibition sensitivity of NR2B-containing NMDA receptors. C. Ifenprodil-docked into ATD2B putative binding pocket model. The relative orientations of side chains of Asp101 (top red residue in red sticks), Ile150 and Phe176 towards the putative centre cleft suggest plausible ligand-protein interactions with various functional groups on ifenprodil (see Discussion).