Solution structure by 2D 1H-NMR of a chimeric peptide recognized by galanin and neuropeptide Y receptors

The 25 amino acid residue chimeric peptide M32, galanin(1-13)-neuropeptide Y(25-36)-amide, was synthesized. The peptide was found to be recognized by both galanin and NPY receptors. The solution structure in 30% (v/v) 1,1,1,3,3,3-hexafluoro-2-propanol was examined by 2-D 1H-NMR and by CD. Proton resonance assignments were made, and structures were calculated using DIANA and refined by restrained energy minimization and molecular dynamics. The obtained structures contain an alpha-helical part in the NPY portion of the peptide including residues 13-20, and in some structures it continues to the C-terminal Tyr25. The more flexible N-terminal portion of the peptide has the freedom to approach the C-terminal alpha-helix, via a reverse turn or a nascent alpha-helix, which permits the N-terminus with Trp2 to come into close contact with the C-terminus with Tyr25. Among the ten NMR structures with lowest energy, there are structures reminiscent of the horseshoe shape of aPP, a close relative of NPY with known crystal structure. It appears that the strong alpha-helical character of the NPY (25-36) amide fragment of M32 helps to stabilize structural features in the galanin-derived part of the peptide. It is noteworthy that this rigid NPY portion of M32 does not prevent the recognition of the peptide by galanin receptors; rather, the peptide has unusually high affinity: IC50 = 0.1 nM at galanin receptors. The chimeric peptide M32 is also recognized by NPY receptors with submicromolar affinity (IC50 = 0.25 microM). The availability of a solution structure for peptide M32, which is recognized by two peptide receptors that are both members of the family of G-protein-coupled receptors, may be useful in understanding peptide receptor-ligand interactions and in designing new galanin and NPY receptor ligands.