Conformational analysis of possible biologically active (receptor-bound) conformations of peptides derived from cholecystokinin, cerulein and little gastrin and the opiate peptide, Met-enkephalin

Possible biologically active (receptor-bound) conformations of peptides derived from cholecystokinin (CCK) have been deduced using conformational analysis combined with comparative studies of their biological specificities. Two peptides, the completely active carboxyl terminal heptapeptide from CCK (CCK-7), whose sequence is Tyr-Met-Gly-Trp-Met-Asp-Phe-NH2, and the carboxyl terminal heptapeptide from cerulein (CER-7) which has the same sequence as for CCK-7 except for replacement of Met 2 with a Thr 2, both stimulate peripheral receptors in gall bladder, pancreas, and pylorus in the gastrointestinal system. In contrast, two other very similar peptides, the last four residues of CCK (CCK-4) whose sequence is Trp-Met-Asp-Phe-NH2, and the carboxyl terminal hexapeptide of little gastrin (LGA-6, Tyr-Gly-Trp-Met-Asp-Phe-NH2, i.e., residue 2 deleted relative to CCK-7 and CER-7 sequences), interact specifically with gastrin receptors and not at all or very weakly with peripheral receptors. All of these peptides react with CCK receptors in the central nervous system, especially in forebrain. The results in the GI tract suggest that the peptides active on peripheral receptors adopt structures that are significantly different from those of the peptides that interact with gastrin receptors. We have generated all of the many low energy conformations for each of these peptides. By retaining only the conformations that are the same for peptides within the same group and then rejecting those resulting conformations that are the same for the peptides in the two different groups, we can greatly reduce the possible active conformations for the peptides within each class. Recently, a set of rigid indole derivatives of benzodiazepine has been synthesized and found to bind exclusively to peripheral receptors and are excellent antagonists of CCK-7 (and CER-7). The lowest energy common structure for CCK-7 and CER-7, a beta bend, is uniquely superimposable on these derivatives, thus making this structure a likely one for receptor recognition. This conclusion of course is predicated on the assumption that the peptide and non-peptide analog bind in the same way to the same receptor. In addition, we find that there are only two common conformations for CCK-4 and LGA-6: alpha-helical and a bent helical structure. In both of these structures, the indole of the Trp residue lies on the same "face" and near the benzene ring of the Phe residue. In contrast, the indole ring of the CCK-7 and CER-7 points away from the two benzene rings of Tyr 1 and Phe 7. The latter two rings lie close and parallel to one another. The proposed structure of CCK-7 (CER-7) is strikingly similar to one determined for Met-enkephalin (whose sequence, Tyr-Gly-Gly-Phe-Met, is homologous to that for the first five residues of CCK-7 in another, independent study. We find that Met-enkephalin is a strong antagonist of the action of CCK-7 (IC50=1 micromolar). This result provides support for the deduced active conformation for CCK-7 and CER-7 and for the active conformation of Met-enkephalin in the gastrointestinal tract. The analysis used in predicting the active structures of the CCK peptides in the gastrointestinal system can also be used to deduce the active structures for these peptides in the central nervous system.