Characterization of amphiphilic secondary structures in neuropeptide Y through the design, synthesis, and study of model peptides

Neuropeptide Y (NPY) has the potential to form two amphiphilic secondary structures: a polyproline II-like helix in residues 1-8, and an α-helix in residues 13-32. NPY dimerizes in aqueous solution and forms stable monolayers at the air-water interface, suggesting that these amphiphilic conformations are stabilized at interfaces. Furthermore, the negative molar ellipticity of monomeric NPY at 222 nm (-8500 degree cm²/dmol), suggests that hydrophobic interactions with the NH₂-terminal amphiphilic structure may stabilize the α-helix in residues 13-32 before it binds to cell surfaces, even at physiological concentrations. In order to investigate the role of these amphiphilic structures, five NPY models with multiple substitutions in position 13-32 have been synthesized and studied. Our data demonstrate that the surfactant properties of NPY result from its potential to form amphiphilic secondary and tertiary structures and not from specific amino acid sequences in this region. However, specific residues on the hydrophilic face of the amphiphilic α-helix that have been substituted in the models appear to be required to reproduce the full potency of NPY in our pharmacological assays. A possible role for the amphiphilic structures in NPY in presenting such specific determinants to cell surface receptors in the correct conformation is suggested.