Solvent-accessibility of discrete residue positions in the polypeptide hormone glucagon by 19F-NMR observation of 4-fluorophenylalanine

The amino acid 4-fluoro-L-phenylalanine (4F-Phe) was introduced at the positions of Phe6 and Phe22 in the 29-residue polypeptide hormone glucagon by expressing glucagon in E. coli in the presence of an excess of 4F-Phe. Glucagon regulates blood glucose homeostasis by interaction with the glucagon receptor (GCGR), a class B GPCR. By referencing to the 4F-Phe chemical shifts at varying D2O concentrations, the solvent exposure of the two Phe sites along the glucagon sequence was determined, showing that 4F-Phe6 was fully solvent exposed and 4F-Phe22 was only partially exposed. The incorporation of fluorine atoms in polypeptide hormones paves the way for novel studies of their interactions with membrane-spanning receptors, specifically by differentiating between effects on the solvent accessibility, the line shapes, and the chemical shifts from interactions with lipids, detergents and proteins. Studies of interactions of GCGR with ligands in solution is at this point of keen interest, given that recent crystallographic studies revealed that an apparent small molecule antagonist actually binds as an allosteric effector at a distance of ~20 Å from the orthosteric ligand binding site (Jazayeri et al., in Nature 533:274-277, 2016).