Cg. Unson et al., MULTIPLE-SITE REPLACEMENT ANALOGS OF GLUCAGON - A MOLECULAR-BASIS FORANTAGONIST DESIGN, The Journal of biological chemistry, 269(17), 1994, pp. 12548-12551
Extensive structure activity analysis has allowed us to identify speci
fic residues in the glucagon sequence that are responsible for either
receptor recognition or signal transduction. For instance, we have dem
onstrated that aspartic acid 9 and histidine 1 are essential for activ
ation, and that an ionic interaction between the negative carboxylate
and the protonated imidazole may contribute to the activation reaction
at the molecular level. In the absence of the carboxylic group at pos
ition 9, aspartic 21 or aspartic 15 might furnish distal electrostatic
effects to maintain partial agonism. Further investigation establishe
d that each of the 4 serine residues in the hormone play distinct role
s. Serine 8 provides an important determinant of binding. Whereas neit
her serines 2, 11, nor 16 are required for receptor recognition. We ha
ve shown that serine 16 is essential for signal transduction and thus
have identified it to be the third residue in glucagon to participate
in a putative catalytic triad to gether with aspartic 9 and histidine
1, in the transduction of the glucagon response. In this work, we util
ized insights into the functional significance of particular residues
in the peptide appropriated from our structure-function assignments, a
s the basis of a molecular approach for the design of active-site dire
cted antagonists of glucagon. The importance as well as the accuracy o
f our findings are confirmed by the synthesis of a series of improved
glucagon antagonists based on replacements at positions 1, 9, 11, 16,
and 21. The inhibition index, (I/A)(50) of our best antagonist des-His
(1)[Nle(9)-Ala(11)-Ala(16)]glucagon amide, has been improved 10 fold o
ver the previous best glucagon inhibitor.