NMR and Alanine Scan Studies of Glucose-dependent Insulinotropic Polypeptide in Water*

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone that stimulates the secretion of insulin after ingestion of food. GIP also promotes the synthesis of fatty acids in adipose tissue. Therefore, it is not surprising that numerous literature reports have shown that GIP is linked to diabetes and obesity-related diseases. In this study, we present the solution structure of GIP in water determined by NMR spectroscopy. The calculated structure is characterized by the presence of an α-helical motif between residues Ser¹¹ and Gln²⁹. The helical conformation of GIP is further supported by CD spectroscopic studies. Six GIP-(1–42)Ala1–7 analogues were synthesized by replacing individual N-terminal residues with alanine. Alanine scan studies of these N-terminal residues showed that the GIP-(1–42)Ala⁶ was the only analogue to show insulin-secreting activity similar to that of the native GIP. However, when compared with glucose, its insulinotropic ability was reduced. For the first time, these NMR and modeling results contribute to the understanding of the structural requirements for the biological activity of GIP.