A 1H NMR study of human calcitonin in solution.

Human calcitonin (hCT) has been investigated by NMR at 400 MHz in DMSOd6 and in an 85% DMSOd6-15% 1H2O (v/v) cryoprotective mixture. All backbone and side-chain resonances have been assigned, and the secondary structure has been determined in both solvents. In DMSOd6, the simultaneous presence of d alpha N, dNN, and some specific weak medium-range nuclear Overhauser effects, together with the amide temperature coefficients and the analysis of the NH-alpha CH spin-spin coupling constants, indicates that hCT is highly flexible but with three domains (comprising segments Asn3-Gly10, Gln14-Thr21, and Thr25-Ala31) in extended conformations which dynamically transform into isolated beta turns in the N- and C-terminal regions and into adjacent tight turns, resembling a 3(10) helix structure, in the central part. The DMSO-water mixture rigidifies the polypeptide chain, favoring an ordered, extended conformation. NOESY data indicate the presence of a short double-stranded antiparallel beta sheet in the central region made by residues 16-21 and connected by a two-residue hairpin loop formed by residues 18 and 19. Two tight turns, formed by residues 3-6 and 28-31, were also identified. The central beta sheet does not favor an amphipathic distribution of the residues as found for salmon calcitonin [Motta, A., Castiglione Morelli, M. A., Goud, N., & Temussi, P. A. (1989) Biochemistry 28, 7998-8002]. This is in agreement with the smaller tendency of hCT to form the amphipathic alpha helix, postulated to be responsible for the interaction of hCT with lipids. The possible role of the cis-trans isomerism of Pro is discussed.