Development and validation of opioid ligand-receptor interaction models: The structural basis of mu vs. delta selectivity

Opioid receptor binding conformations for two structurally related, conformationally constrained tetrapeptides, JOM-6 (mu receptor selective) and JOM-13 (delta receptor selective), were deduced using conformational analysis of these ligands and analogs with additional conformational restrictions. Docking of these ligands in their binding conformations to opioid receptor structural models, based upon the published rhodopsin X-ray structure, implicates specific structural features of the mu and delta receptor ligand binding sites as forming the basis for the mu selectivity of JOM-6 and the delta selectivity of JOM-13. In particular, the presence of E229 in the mu receptor (in place of the corresponding D210 of the delta receptor) causes an adverse electrostatic interaction with C-terminal carboxylate-containing ligands, resulting in the observed preference of ligands with an uncharged C-terminus for the mu receptor. In addition, the requirement that the Phe(3) side chain of JOM-13 assume a gauche orientation for optimal delta binding, whereas the Phe(3) side chain of JOM-6 must be in a trans orientation for high-affinity it binding can be largely attributed to the steric effect of replacement of L300 of the delta receptor by W318 of the mu receptor. Testing this hypothesis by examining the binding of JOM-6 and several of its key analogs with specific it receptor mutants is described. Our initial results are consistent with the proposed ligand-receptor interaction models.