Acylation of the alpha-amino group in neuropeptide Y(12-36) increases binding affinity for the Y2 receptor.

Competition assays using three series of analogs of neuropeptide Y (NPY) ([Xaa11]NPY(11-36), [Xaa12]NPY(12-36), and [Xaa13]NPY(13-36) revealed that the binding affinity for the Y2 receptor was considerably lowered by truncation of residue 11. Upon acetylation or succinylation of the alpha-amino group, the binding affinity of [Xaa12]NPY(12-36) recovered to a level similar to that of [Xaa11]NPY(11-36). No significant difference was observed between the increases caused by acetylation and those caused by succinylation, suggesting that the increase in binding affinity cannot be explained by the change in the net charge at the N-terminus as a consequence of the modification. The scattered data points on a plot of the alpha-helix content vs. IC50 of all these analogs revealed the absence of any apparent relationship, an indication that prior formation of the alpha-helix is not necessary for binding to the Y2 receptor. It has been widely accepted that fewer than 12 residues from the C-terminus are directly involved in binding of NPY to the Y2 receptor, while the remaining part of NPY only assists in the adoption of a favorable conformation by the C-terminal hexapeptide for recognition by the receptor. However, the present results suggest that the region around residue 12 does not project from the Y2 receptor.