Implications of replacing peptide bonds in the COOH-terminal B chain domain of insulin by the psi (CH2-NH) linker.

To evaluate more thoroughly the importance of main-chain structure and flexibility in ligand interactions with the insulin receptor, we undertook to synthesize analogues with reduced peptide bonds in the COOH-terminal B chain domain of the hormone (a stable, but adjustable beta-strand region). By use of solid-phase, solution-phase and semisynthetic methods, analogues were prepared in which ArgB22 of des-octapeptide(B23-B30)-insulin was extended by the sequences Gly-Phe-psi (CH2-NH)-Phe-NH2, Gly-Gly-psi(CH2-NH)-Phe-Phe-NH2, Gly-Phe-psi (CH2-NH)-Phe-Phe-Thr-Pro-Ala-Thr-OH, and Gly-Phe-Phe-psi (CH2-NH)-Phe-Thr-Pro-Ala-Thr-OH, and were studied with respect to their abilities both to interact with the hepatocyte insulin receptor and to form soluble anion-stabilized hexamers in the presence of Co2+ and phenol. Additional analogues of des-pentapeptide(B26-B30)-insulin were also examined. Overall, our results show that, whereas all analogues retain considerable ability to form organized metal ion-coordinated complexes in solution, the reduction of peptide bonds both proximal and distal to the critical side chain of PheB25 results in analogues with severely diminished receptor binding potency. We conclude that the peptide carbonyls from both PheB24 and PheB25 are important for insulin-receptor interactions and that the structural organization of the region when insulin is bound to its receptor differs from that occurring during simple monomer-monomer and higher-order interactions of the hormone.