Solution structure of nociceptin peptides

Abstract

Peptides embedded in the sequence of pre-pro-nociceptin, i.e. nociceptin, nocistatin and orphanin FQ2, have shed light on the complexity of the mechanisms involving the peptide hormones related to pain and have opened up new perspectives for the clinical treatment of pain. The design of new ligands with high selectivity and bioavailability, in particular for ORL1, is important both for the elucidation and control of the physiological role of the receptor and for their therapeutic importance. The failure to obtain agonists and antagonists when using, for nociceptin, the same substitutions that are successful for opioids, and the conformational flexibility of them all, justify systematic efforts to study the solution conformation under conditions as close as possible to their natural environment. Structural studies of linear peptides in solution are hampered by their high flexibility. A direct structural study of the complex between a peptide and its receptor would overcome this difficulty, but such a study is not easy since opioid receptors are membrane proteins. Thus, conformational studies of lead peptides in solution are still important for drug design. This review deals with conformational studies of natural pre-nociceptin peptides in several solvents that mimic in part the different environments in which the peptides exert their action. None of the structural investigations yielded a completely reliable bioactive conformation, but the global conformation of the peptides in biomimetic environments can shed light on their interaction with receptors.

The Interaction of Highly Helical Structural Mutants with the NOP Receptor Discloses the Role of the Address Domain of Nociceptin/Orphanin FQ

“…[38,39] Notwithstanding, the NOESY spectra of N/OFQ in water, dimethyl sulfoxide (DMSO), in mixtures of water with DMSO or in several hydroalcoholic mixtures show a limited spread of the NH resonances corresponding to little ordering of the peptide. [29] The same behavior was shown by analogues containing Pro in key positions, [29] including those studied in the present paper, namely [Pro 6 ]-N/OFQ-amide, [Pro 7 ]-N/OFQamide and [Pro 11 ]-N/OFQ-amide (data not shown) consistently with the low helix-inducing property of this residue.…”

“…The other two Pro analogues, that is, [Pro 6 ]-N/OFQ-amide and [Pro 7 ]-N/OFQ-amide, while still consistent with the role of a helical conformation in the address domain, shed light on the importance of the conformation of the hinge region. As discussed at length in a previous paper, [29] only a b-turn centered on Gly 6 -Ala 7 is consistent with NOP activity. This observation is particularly relevant for the relative orientation of the two domains inside the receptor (vide infra).…”

“…[29] Table 1 shows the ten analogues of N/OFQ, synthesized according to published methods by using standard solidphase synthesis techniques [34] with a Milligen 9050 synthesizer as described in detail in the Experimental Section.…”

“…The decrease is most pronounced for [Pro 5 ]-N/OFQ(1-13)-amide and [Pro 7 ]-N/OFQ(1-13)-amide, whereas [Pro 6 ]-N/ OFQ(1-13)-amide retains an activity comparable to that of other active analogues. [29] This result was interpreted as an indication that the angle between the two domains ought to be similar to that imposed by a b-turn centered on Gly 6 -Ala 7 . This hypothesis is of limited validity because both domains are very flexible from a conformational point of view.…”

“…This hypothesis is of limited validity because both domains are very flexible from a conformational point of view. However, while it is well known that the N-terminal domain is intrinsically flexible, like that of opioids, [25,29] the address domain does have conformational tendencies that can be enhanced by environmental conditions and/or specific substitutions. Accordingly, we sought the best conditions to reveal its intrinsic conformational tendencies.…”