Chemotype-selective modes of action of κ-opioid receptor agonists.

The κ-opioid receptor (KOR) is activated by diverse agonists through both shared and distinct molecular mechanisms. By docking four chemically different ligands (dynorphin A, U-69593, salvinorin A, and an octahydroisoquinolinone carboxamide) into the antagonist-bound KOR crystal structure and testing 18 mutated positions, two classes of mutations were identified: those impairing receptor function mainly by reducing ligand binding, and those impairing function without strongly affecting binding. Mutations of the latter type were located at the binding site periphery and did not interact strongly with ligands. These “functional” residues, together with water molecules seen in the crystal structure, likely help transmit the agonist binding signal to conserved rotamer switches that trigger receptor activation.

Study at a glance

Abstract

The crystal structures of opioid receptors provide a novel platform for inquiry into opioid receptor function. The molecular determinants for activation of the κ-opioid receptor (KOR) were studied using a combination of agonist docking, functional assays, and site-directed mutagenesis. Eighteen positions in the putative agonist binding site of KOR were selected and evaluated for their effects on receptor binding and activation by ligands representing four distinct chemotypes: the peptide dynorphin A(1-17), the arylacetamide U-69593, and the non-charged ligands salvinorin A and the octahydroisoquinolinone carboxamide 1xx. Minimally biased docking of the tested ligands into the antagonist-bound KOR structure generated distinct binding modes, which were then evaluated biochemically and pharmacologically. Our analysis identified two types of mutations: those that affect receptor function primarily via ligand binding and those that primarily affect function. The shared and differential mechanisms of agonist binding and activation in KOR are further discussed. Usually, mutations affecting function more than binding were located at the periphery of the binding site and did not interact strongly with the various ligands. Analysis of the crystal structure along with the present results provide fundamental insights into the activation mechanism of the KOR and suggest that "functional" residues, along with water molecules detected in the crystal structure, may be directly involved in transduction of the agonist binding event into structural changes at the conserved rotamer switches, thus leading to receptor activation.