Functional association of the N-terminal residues with the central region in glucagon-related peptides.

GLP-1 is an incretin peptide involved in the regulation of glucose metabolism and the glucose-dependent stimulation of insulin secretion. Ex-4 is a paralog of GLP-1 that has comparable GLP-1R potency but extended physiological action. GLP-1 and Ex-4 are helical peptides that share ∼50% sequence homology but differ at several residues, notably the second amino acid which controls susceptibility to DPP-IV cleavage. This single amino acid difference yields divergent receptor potency when studied in the context of the two hormone sequences. Ex-4 uniquely tolerates Gly2 through select amino acid differences in the middle region of the peptide that are absent in GLP-1. We report that substitution of Ex-4 amino acids Glu16, Leu21, and Glu24 to the GLP-1 sequence enabled Gly2 tolerance. The coordination of the N-terminus with these central residues shows an interaction of substantial importance not only to DPP-IV stability but also to receptor activation. Extension of this observation to glucagon-based co-agonist peptides showed different structural requirements for effective communication between the N-terminus and the mid-section of these peptides in achieving high potency agonism at the GLP-1 and GCGRs.