SOLUTION CONFORMATIONS OF POTENT BICYCLIC ANTAGONISTS OF OXYTOCIN BY NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY AND MOLECULAR-DYNAMICS SIMULATIONS

The solution conformations of two potent bicyclic antagonists of oxyto cin (3)-Gln(4)-Asn(5)-Cys(6)-Pro(7)-Leu(8)-Gly(9)-NH2, OT), [Mpa(1),cy clo(Glu(4),Lys(8))]OT, and [dPen(1), cyclo(Glu(4),Lys(8))]OT were stud ied by a combined use of H-1 and C-13 NMR spectroscopy in DMSO and mol ecular dynamics (MD) simulations. NMR data have suggested a model for the three-dimensional (3D) structure of the bicyclic analogues of OT ( OT-BC) with a beta-turn at the Tyr(2) and Ile(3) residues, and with a cis amide bond between Cys(6) and Pro(7). A 3D structure containing a type III beta-turn at Tyr(2)-Ile(3) has been shown to be consistent wi th NMR data. This structure was proposed as a model of the solution co nformation of OT-BC and extensively tested by MD simulations with the AMBER force field. MD simulations at 300 K with NMR derived distance a nd phi torsion angle constraints demonstrated the consistency of this model with NMR data, and its stability was further demonstrated by non -constrained MD simulations. Dynamic properties of the 3D structure we re explored by high-temperature MD at 500 K. Conformational transition s induced by a constrained rotation around the S-S bond revealed relat ively low potential energy barriers (30 to 50 kJ/mol) between equilibr ium left-handed and right-handed conformers of the disulfide bridge in OT-BC. A dynamic model of the solution structure of OT-BC with the re latively stable backbone conformation and a fast conformational equili brium in the disulfide bridge and lactam bridge moieties was proposed as a result of the extensive MD simulations. The solution structure of OT-BC is consistent with structure-activity relations of peptide and non-peptide antagonists of OT. In particular, a beta-turn at Tyr(2)-Il e(3) seems to be the common feature responsible for antagonist interac tion with the uterine receptor of OT. On the other hand, the 3D struct ure of OT-BC differs considerably from the crystal and solution struct ures of OT analogues with agonist activity. Therefore, this study supp orts the hypothesis of different modes of receptor binding for agonist s and antagonists of OT. The model of 3D structure of OT-BC proposed i n this study may be used as a template for the rational design of pept ide and nonpeptide antagonists of oxytocin.