We have studied the dynamic properties of human (h) and salmon (s) calcitonin (CT) in solution. For both hormones, distance geometry in torsion-angle space has been used to generate three-dimensional structures consistent with NMR data obtained in sodium dodecyl sulfate micelles. For sCT and hCT we used, respectively, 356 and 275 interproton distances together with hydrogen-bonds as restraints. To better characterize their flexibility and dynamic properties two fully unrestrained 1100-ps molecular dynamics (MD) simulations in methanol were performed on the lowest-energy structures of both hormones. Statistical analyses of average geometric parameters and of their fluctuations performed in the last 1000 ps of the MD run show typical helical values for residues 9-19 of sCT during the whole trajectory. For hCT a shorter helix was observed involving residues 13-21, with a constant helical region in the range 13-19. Angular order parameters S(phi) and S(psi) indicate that hCT exhibits a higher flexibility, distributed along the whole chain, including the helix, while the only flexible amino acid residues in sCT connect three well-defined domains. Finally, our study shows that simulated annealing in torsion-angle space can efficiently be extended to NMR-based three-dimensional structure calculations of helical polypeptides. Furthermore, provided that a sufficient number of NMR restraints describes the system, the method allows the detection of equilibria in solution. This identification occurs through the generation of 'spurious' high-energy structures, which, for right-handed alpha-helices, are likely to be represented by left-handed alpha-helices.
The 32 amino acid hormone human calcitonin was studied at pH 3.7 and 7.4 by multidimensional NMR spectroscopy in sodium dodecyl sulfate micelles at 310K. The secondary structure was obtained from nuclear Overhauser enhancement spectroscopy (NOESY), 3JHNalpha coupling constants, and slowly exchanging amide data. Three-dimensional structures consistent with NMR data were generated by using distance geometry calculations. A set of 265 interproton distances derived from NOESY experiments, hydrogen-bond constraints obtained from amide exchange, and coupling constants were used. From the initial random conformations, 30 distance geometry structures with minimal violations were selected for further refinement with restrained energy minimization. In micelles, at both pHs, the hormone assumes an amphipathic alpha-helix from Leu9 to Phel6, followed by a type-I beta-turn between residues Phel6 and Phel9. From His20 onward the molecule is extended and no interaction with the helix was observed. The relevance of the amphipathic helix for the structure-activity relationship, the possible mechanisms of interaction with the receptor, as well as the formation of fibrillar aggregates, is discussed.
A new derivative of avarol, monoacetyl avarol, has been isolated from the sponge Dysidea avara. The structural elucidation and biological activities are reported.
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