Solution conformations of an insect neuropeptide: Crustacean cardioactive peptide (CCAP)

The solution structure of crustacean cardioactive peptide (CCAP), a cyclic amidated nonapeptide neurohormone, was studied using molecular dynamics techniques, with constraints derived from NMR studies in water and water/dodecylphosphocholine micellar medium. This peptide, found in various invertebrates, has the primary sequence Pro1 Phe2 Cys3 Asn4 Ala5 Phe6 Thr7 Gly8 Cys9 NH2, with an intramolecular disulfide bridge between the two cysteine residues. In aqueous solution the peptide was found to have a type(IV) β-turn between residues 5-8. In a water/decane biphasic medium a type(IV) β-turn between residues 3 and 6 and two classic γ-turns between residues 4-6 and 7-9, were found. Analysis of the 1H and 13C NMR chemical shifts data showed that the model free S2 order parameter of the residues varied between 0.65 and 0.9. The molecular dynamic root mean square fluctuations of structural ensembles of the backbone varied between 0.5 and 2.2 with the central residues showing the least fluctuations.