Glucagon and glucagon-like peptide-1 (GLP-1)are two structurally related hormones that acutely regulate glucose control in opposite directions through homologous receptors. The molecular basis for selectivity between these two hormones and their receptors is of physiological and medicinal importance. The application of co-agonists to enhance body weight reduction and correct multiple abnormalities associated with the metabolic syndrome has recently been reported. Substitution of amino acids 16, 18, and 20 in glucagon with those found in GLP-1 and exendin-4 were identified as partial contributors to balanced, high potency receptor action. The amidation of the C-terminus was an additional glucagon-based structural change observed to be of seminal importance to discriminate recognition by both receptors. In this work, the molecular basis for receptor selectivity associated with differences in C-terminal peptide sequence has been determined. A single charge inversion in glucagon and GLP-1 receptor sequence at position 68* was determined to significantly alter hormone action. Changing E68* in GLP-1R to the corresponding Lys of GCGR reduced receptor activity for natural GLP-1 hormones by eightfold. The enhanced C-terminal positive charges in GLP-1 peptides favor the native receptor's negative charge at position 68*, while the unfavorable interaction with the C-terminal acid of native glucagon is minimized by amidation. The extension of these observations to other glucagon-related hormones such as oxyntomodulin and exendin, as well as other related receptors such as GIPR, should assist in the assembly of additional hormones with broadened pharmacology.
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