AB-INITIO STUDY OF ASPARTIC AND GLUTAMIC-ACID - SUPPLEMENTARY EVIDENCE FOR STRUCTURAL REQUIREMENTS AT POSITION-9 FOR GLUCAGON ACTIVITY

Our previous work established that position 9 aspartic acid in glucago n was a critical residue for transduction of the hormone response. An uncoupling of the binding interaction from adenylate cyclase activatio n was demonstrated by the observation that amino acid replacements at position 9 resulted in peptides that had no measurable adenylate cycla se activity yet were still recognized by the glucagon receptor. It was also later shown that His1 played a major role in activation, and it was suggested that an electrostatic interaction between the aspartic a cid carboxylate and the histidine imidazole occurred as part of the ac tivation mechanism. This did not preclude intermolecular interactions of this aspartic acid with other residues within the receptor binding site. The observation that a conservative substitution of glutamic aci d for aspartic acid at position 9 was sufficient to result in the pote nt antagonist, des-His1d[Glu9]glucagon amide, implied that even glutam ic acid possessed the minimum properties necessary for inhibition, and that the precise position of the carboxyl group at position 9 in gluc agon was an absolute requirement for full agonist activity. The presen t investigation was conducted with ab initio calculations and molecula r modeling to shed some light on the source of this phenomenon.