ROLES OF ASPARTIC-ACID-15 AND ASPARTIC-ACID-21 IN GLUCAGON ACTION - RECEPTOR ANCHOR AND SURROGATES FOR ASPARTIC-ACID-9

The discovery of aspartic acid at position 9 in glucagon to be a criti cal residue for transduction has spurred renewed efforts to identify o ther strategic residues in the peptide sequence that dictate either re ceptor binding or biological activity. It also became apparent from fu rther studies that Asp(9) operates in conjunction with His(1) in the a ctivation mechanism that follows binding to the glucagon receptor. Ind eed, it was later demonstrated that the protonatable histidine imidazo le is important for transduction. It is likely that the interaction of a positively charged histidine 1 with a negatively charged aspartic a cid 9 might be part of the triggering step at the molecular level. Two other aspartic acid residues in glucagon are capable of assuming a si milar role, namely that of contributing to an electrostatic attraction with histidine via a negative carboxylate. These studies were conduct ed to investigate the role of aspartic acid 15 and 21 in glucagon acti on. Evidence reported here, gathered from 31 replacement analogs, supp orts the idea that in the absence of the requisite carboxyl group at p osition 9, histidine utilizes Asp(21) or Asp(15) as a compensatory sit e. Asp(15) was also found to be indispensable for binding and may serv e to tether the hormone to the receptor protein at the binding site. I t is also demonstrated that these new findings promote the design of b etter glucagon antagonists.