Membrane-Induced Secondary Structures of Neuropeptides:  A Comparison of the Solution Conformations Adopted by Agonists and Antagonists of the Mammalian Tachykinin NK1 Receptor

We present what we believe to be the first documented example of an inducement of distinctly different secondary structure types onto agonists and antagonists selective for the same G-coupled protein receptor using the same membrane-model matrix wherein the induced structures are consistent with those suggested to be biologically active by extensive analogue studies and conventional binding assays. 1H NMR chemical shift assignments for the mammalian NK1 receptor-selective agonists α-neurokinin (NKA) and β-neurokinin (NKB) as well as the mammalian NK1 receptor-selective antagonists [d-Pro2,d-Phe7,d-Trp9]SP and [d-Arg,d-Pro,d-Phe7,d-His9]SP have been determined at 600 MHz in sodium dodecyl sulfate (SDS) micelles. The SDS micelle system simulates the membrane−interface environment the peptide experiences when in the proximity of the membrane-embedded receptor, allowing for conformational studies that are a rough approximation of in vivo conditions. Two-dimensional NMR techniques were used to assign proton resonances, and interproton distances were estimated from the observed nuclear Overhauser effects (NOEs). The experimental distances were used as constraints in a molecular dynamics and simulated annealing protocol using the modeling package DISCOVER to generate three-dimensional structures of the two agonists and two antagonists when present in a membrane-model environment to determine possible prebinding ligand conformations. It was determined that (1) NKA is helical from residues 6 to 9, with an extended N-terminus; (2) NKB is helical from residues 4 to 10, with an extended N-terminus; (3) [d-Pro2,d-Phe7,d-Trp9]SP has poorly defined helical properties in the midregion and a β-turn structure in the C-terminus (residues 6−9); and (4) [d-Arg1,d-Pro2,d-Phe7,d-His9]SP has a helical structure in the midregion (residues 4−6) and a well-defined β-turn structure in the C-terminus (residues 6−10). Attempts have been made to correlate the observed conformational differences between the agonists and antagonists to their binding potencies and biological activity.