MOLECULAR SIMULATION OF DYNORPHIN A-(1-10) BINDING TO EXTRACELLULAR LOOP-2 OF THE KAPPA-OPIOID RECEPTOR - A MODEL FOR RECEPTOR ACTIVATION

The structure of the second extracellular loop region (EL2) of the kap pa-opioid receptor has been explored in an effort to understand the st ructural basis far dynorphin A binding and selectivity. Application of secondary structure prediction methods and homology modeling resulted in a turn-helix motif for the N-terminal region of kappa-EL2. A simil ar motif was not predicted for EL2 of either the delta or mu opioid re ceptors. The EL2 helix was further shown to be amphiphilic and complem entary to the helical component of dynorphin A, Using a model of the K -receptor (Metzger et al. Neurochem. Res. 1996, 21, 1287-1294), includ ing the newly predicted ELZ turn-helix domain, a binding mode is propo sed based on helix-helix interactions between hydrophobic residues of EL2 and the helical component of dynorphin A-(1-10), Molecular simulat ions of the receptor-ligand complex yielded structures in which the ty ramine moiety or opioid ''message'' of dynorphin is bound within a con served aromatic pocket in the transmembrane domain while the helical p ortion contacted residues in EL2 and in the extracellular end of trans membrane helices 6 and 7, The model is in general agreement with site- directed mutagenesis data and chimera studies that have identified bin ding domains in both the EL2 and transmembrane regions to dynorphln A, The results confirm the importance of the opioid ''message'' displaye d. by many opioid, ligands but also suggest a potential mechanism of r eceptor activation that may be mediated by EL2 through interactions wi th the ''address'' component of dynorphin A.