Nociceptin is a neuropeptide that binds to and activates the opioid receptor-like ORL1 receptor. In order to explore the structural elements necessary for receptor recognition and activation, we designed and synthesized a series of nociceptin analogues, in which nonpolar amino acid residues such as Gly6, Ala7, Ala11, Leu14, and Ala15 were substituted respectively by Trp. [Trp6]- and [Trp7]-nociceptins exhibited rather weak activities (5-15% of nociceptin), and [Trp11]- and [Trp15]-nociceptins also showed reduced activities (40-50%). These results suggested that the space available for these particular residues are relatively restricted. By contrast, the Trp/Leu-substitution at position 14 retained full receptor binding activity, and the resulting [Trp14]nociceptin exhibited an increased biological activity in the functional assay using [35S]GTPγS. This suggested that the receptor residue interacting with nociceptin-Leu14 is the aromatic amino acid.
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