Importance of the C‐terminal α‐helical structure for glucagon's biological activity

The synthetic glucagon analogues [Glu²¹]glucagon, 2, and [Lys^17.18.Glu²¹]glucagon, 3, were designed using Chou‐Fasman calculations for the purpose of enhancing the probability for the formation of a C‐terminal amphipathic α‐helical conformation. Circular dichroism indicates increased α‐helical content for these analogues in solution relative to glucagon. Analogues 2 and 3 also exhibit a 3‐fold and 5‐fold increase in receptor binding potency, respectively. The adenylate cyclase stimulating potencies of 2 and 3 relative to glucagon are 2.1 and 7 times greater, respectively. Attempts were made at further α‐helical enhancement by further substitutions in the 10–13 region of glucagon. as represented by the glucagon analogues [Phe¹³,Lys^17.18 Glu²¹]glucagon, 4, and [Phe^10.13, Lys^17.18,Glu²¹]glucagon. 5. These latter substitutions resulted in lowered receptor binding and adenylate cyclase potencies for 4 and 5 relative to 3 despite increased α‐helical content in solution as observed by circular dichroism spectroscopy.