Endomorphins were subjected to a number of structural modifications in
a search for their bioactive conformations. The alicyclic beta-amino
acids cis-(1S,2R)ACPC/ACHC, cis-(IR,2S)ACPC/ACHC,
trans(IS,2S)ACPC/ACHC, and trans-(1R,2R)ACPC/ACHC were introduced into
endomorphins to examine the conformational effects on the bioactivity.
Use of a combination of receptor binding techniques, H-1 NMR, and
molecular modeling allowed the conclusion that Pro(2) substitution by
these residues causes changes in structure, proteolytic stability, and
pharmacological activity. It seems that the size of the alicyclic
beta-amino acids does not have marked influence on the receptor binding
affinities and/or selectivities. Among the new analogues, the
cis-(1S,2R)ACPC(2) and cis-(1S,2R)ACHC(2)-containing derivatives
displayed the highest binding potencies and efficacies in receptor
binding and ligand-stimulated [S-35]GTP gamma S functional experiments.
Molecular dynamic simulations and H-1 NMR studies of the
cis-ACPC/ACHC-containing analogues revealed that many conformations are
accessible, though it is most likely that these peptides bind to the
mu-opioid receptor in a compact, folded structure rather than extended.