Conformational, receptor interaction and alanine scan studies of glucose-dependent insulinotropic polypeptide

Glucose-dependent insulinotropic polypeptide (GIP) is an insulinotropic incretin hormone that stimulates insulin secretion during a meal. GIP has glucose lowering abilities and hence is considered as a potential target molecule for type 2 diabetes therapy. In this article, we present the solution structure of GIP in membrane-mimicking environments by proton NMR spectroscopy and molecular modelling. GIP adopts an α-helical conformation between residues Phe6–Gly31 and Ala13–Gln29 for micellar and bicellar media, respectively. Previously we examined the effect of N-terminal Ala substitution in GIP, but here eight GIP analogues were synthesised by replacing individual residues within the central 8–18 region with alanine. These studies showed relatively minor changes in biological activity as assessed by insulin releasing potency. However, at higher concentration, GIP(Ala16), and GIP(Ala18) showed insulin secreting activity higher than the native GIP (P < 0.01 to P < 0.001) in cultured pancreatic BRIN-BD11 cells. Receptor interaction studies of the native GIP with the extracellular domain of its receptor were performed by using two different docking algorithms. At the optimised docking conformation, the complex was stabilised by the presence of hydrophobic interactions and intermolecular hydrogen bonding. Further, we have identified some potentially important additional C-terminal interactions of GIP with its N-terminal extracellular receptor domain.

Graphical abstractStructural studies of GIP and the receptor interaction studies of GIP with the extracellular domain of its receptor were performed by using two different docking algorithms. Docking model displays molecular interactions of GIP ligand with its N-terminal extra cellular domain at the receptor binding pocket.Figure optionsDownload full-size imageDownload high-quality image (155 K)Download as PowerPoint slideResearch highlights► Solution structural studies of GIP hormone in micellar and bicellar media. ► GIP adopts an alpha helical conformation in membrane mimicking environments. ► N-terminal receptor interaction studies of GIP receptor. ► Advanced docking studies of GIP with its N-terminal domain. ► Molecular recognition studies of GIP ligand and its receptor interactions.