A molecular model of agonist and nonpeptide antagonist binding to the human V-1 vascular vasopressin receptor

The affinity of the nonpeptide antagonist OPC-21268 is greater for the rat V-1 arginine vasopressin (AVP) receptor (V1R) than for the human V1R. Site- specific mutagenesis was carried out to identify the residues that determin e interspecies selectivity for nonpeptide antagonist binding. The introduct ion of rat amino acids in position 224, 310, 324, or 337 of the human V1R s equence dramatically altered OPC-21268 affinity for the receptor, whereas b inding of AVP, the peptide V1R antagonist d(CH2)(5)Tyr(Me)AVP, and the nonp eptide V1R antagonist SR49059 was not altered by these mutations. Computer modeling explained the mutagenesis results. Docking of OPC-21268 onto a hom ology-built model of the V1R receptor yielded a model for the bound ligand in which the hydrophobic part is deeply embedded in the transmembrane regio n, whereas the polar part is located on the surface of the extracellular si de. The increased affinity of the G337A mutant is due to two additional van der Waals contacts of the alanine methyl group with carbon atoms on the an tagonist. The I310V mutant reduces the hydrophobicity in the vicinity of th e polar oxygen atom of the antagonist. The I224V mutant relieves overcrowdi ng in a hydrophobic binding pocket involving the aromatic residues Trp(175) , Phe(179), Phe(307), and Trp(304). Finally, the E324D mutant enables the f ormation of a hydrogen bond of the carboxylate side chain with the amide si de chain of Gln(311), which in turn forms a hydrogen bond with the N57 nitr ogen atom of OPC-21268. Thus, a few residues, distinct from those involved in agonist binding, control interspecies selectivity toward OPC-21268 nonpe ptide antagonist binding.