Development and testing of protocols for computer-aided design of peptide drugs, using oxytocin

Considerable clinical interest in neuropeptides and peptide hormones has stimulated recent research and development of peptide-based drugs. This process differs from most classical drug discovery procedures because peptide molecules have considerable inherent flexibility. In the present paper, to identify lowest energy and metastable conformers for drug design, and to develop protocols for such studies, conformational search algorithms, incorporating empirical energy calculations, have been applied in the analysis of the peptide oxytocin. Energy minimization in torsion angle space was carried out from a variety of starting conformations, including published structures, in all-atom mode and all with distance constraints for disulphide bond formation. The energy-minimized conformations have been further optimized by a mapping method. Complementary simulations have been performed in united-atom mode and a model representing the effects of water using dummy sites has been developed and tested for this representation. Several of the preferred conformers together with de novo conformations have been used as starting points in molecular dynamics simulations; 28 low potential energy conformations were located at a temperature of 4 K. Conformations are analysed to identify hydrogen bonds, phi-psi angle distributions and the RMS values relative to the X-ray structure of deamino-oxytocin. The modelled structure of lowest energy in the molecular mechanics calculations was also that of least RMS deviation from the crystal structure; whilst structures of lower energy but larger deviation were identified by molecular dynamics techniques. A metastable structure has been identified which satisfies existing criteria for the "active form", and this model is tested by a theoretical residue-substitution technique, to provide clues on the agonist/antagonist relationship at the atomic level.