Structure-function studies on positions 17, 18, and 21 replacement analogues of glucagon: The importance of charged residues and salt bridges in glucagon biological activity

We have designed and synthesized eight compounds 2-9 which incorporate various amino acid residues in positions 17, 18, and 21 of the glucagon molecule: 2, [Lys¹⁷]glucagon amide; 3, [Lys¹⁸]glucagon amide; 4, [Nle¹⁷,Lys¹⁸,Glu²¹]glucagon amide; 5, [Orn^17,18 Glu²¹]glucagon amide; 6, [D-Arg¹⁷]glucagon; 7, [D-Arg¹⁸]glucagon; 8, [D- Phe¹⁷]glucagon; and 9, [D-Phe¹⁸]glucagon. Compared to glucagon (IC₅₀ = 1.5 nM), analogues 2-9 were found to have binding affinity IC₅₀ values (in nM) of 0.7, 4.1, 1.0, 2.0, 5.0, 25.0, 43.0, and 32.0, respectively. When these compounds were tested for their ability to stimulate adenylate cyclase (AC) activity, they were found to be full or partial agonists having maximum stimulation values of 100, 100, 100, 100, 87, 78, 94, and 100%, respectively. On the basis of the X-ray crystal structure of [Lys^17,18,Glu²¹]glucagon amide reported here, the ability to form a salt bridge between Lys¹⁸ and Glu²¹ is probably key to their increased binding and second messenger activities. Among the eight analogues synthesized here, only analogue 4 preserves the ability to form a salt bridge between Lys¹⁸ and Glu²¹. However, since these modifications are minor they do not seem to change the amphiphilic character of the C-terminus, allowing these analogues to reach 78-100% stimulation in the adenylate cyclase assay. Biological data from analogues 6-9 supports the idea that position 18 of glucagon may influence binding only, while position 17 may influence both receptor recognition and transduction.