Structure–Activity Study on the Spatial Arrangement of the Third Aromatic Ring of Endomorphins 1 and 2 Using an Atypical Constrained C Terminus

Abstract

The discovery of endomorphins (EMs) has opened the possibility of searching for new analgesics. However, the design of peptide analgesics has proven to be very difficult as a result of their conformational flexibility and a lack of clarity in structure-activity relationships (SAR). In EMs, the amino acid side chains exhibit considerable conformational flexibility, especially in the third aromatic ring, which is free to adopt a bioactive conformation. To resolve these problems, a series of C terminus EM analogues, [Xaa(4)-R]EMs, modified through the substitution of Phe(4) with nonaromatic residues and termination with benzyl groups, were designed to generate conformational constrains of the third aromatic ring by amide bond and torsion angles (phi(4) and psi(4)) of Xaa(4). Introduction of (S)-alpha-methyl or (S)/(R)-alpha-carboxamide on the methylene unit of the benzyl group was designed to produce an atypical topographical constraint (phi(5)) of the third aromatic ring rotation. Interestingly, some EM derivatives, with elimination of the C-terminal carboxamide group and significant changes in the address sequence (Phe(4)-NH(2)), still exhibited higher mu-opioid receptor (MOR) affinity than unmodified EMs. In contrast, some analogues with incorrectly constrained C termini displayed very low affinity and pharmacological activities. Thus, our results indicate that these EM analogues, with atypical constrained C termini, provide model compounds with potent MOR agonism. They also give evidence that the proper spatial orientation and conformational restriction of the third aromatic ring are crucial for the interaction of EMs with MOR.

Characterization of opioid activities of endomorphin analogs with C-terminal amide to hydrazide conversion

“…Thus, we concluded that EM-1-NHNH 2 showed an agonist property relative to EM-1, while EM-2-NHNH 2 probably acted as a partial agonist relative to its parent EM-2. Thus, C-terminal amidation would largely influence the molecular conformation of a bioactive peptide and its interactions with receptor Yu et al, 2007a). Moreover, the antinociceptive effects of these compounds were inhibited by naloxone, indicating an opioid mechanism.…”

Synthesis and evaluation of new endomorphin analogues modified at the Pro2 residue

“…The much lower activity shown by 5 , as compared to the Trp3 containing analogue [3Aze 2 ]EM-1 ( 4 ), can be interpreted on the basis of the well recognized role of the aromatic residues on ligand-receptor interactions. Several studies on EM-1 and EM-2 clearly show that aromatic side chains with different size, distinct noncovalent interactions and H-bonding capability, can highly influence conformations43 and binding 21,44,45. Results of ligands 4 and 5 indicate that the beneficial effect on the activity, constantly observed in EM analogues containing the indole ring system at position 3, gives results that are particularly efficient as compared to the Phe3 residue in models containing the achiral 3Aze residue.…”

Conformational Similarities and Dissimilarities Between the Stereoisomeric Forms of Endomorphin‐2

“…Nevertheless, not only the afore-mentioned L-D isomerism produces an effect on the structural properties and bioactivity of EM2, but the cis-trans isomerism affects also the conformational features and affinity of this tetrapeptide. Namely, as the results obtained from the earlier studies pointed out, the EM2 and its structurally modified derivatives existed as an equilibrium mixture of the cis-and trans-isomers regarding the Tyr (14). Although the previous studies supported the presence either of the trans or of the cis Tyr 1 -Pro 2 peptide bond in the biologically active form of EMs, it is important to note that the isomerization state of this peptide bond concerning the bioactive conformation of EM1 and EM2 is still obscure (5).…”

“…Based on the three ranges of tc region, it could be deduced that three different groups of cis-trans stereoisomer pairs could be distinguished, which were characterized by high, moderate, and low similarities, respectively, as follows. The first group with the range of 0.74-0.81 included four different types of peptide pairs containing certain tripeptide units, i.e., (i) the trans-(Y Table 2), [14]). For the former case, the g ()) rotamer was mainly favored (i.e., a g (+) ⁄ g ()) ⁄ trans ratio of circa 20% ⁄ 60% ⁄ 20%), while for the latter one, the g (+) rotamer was mostly preferred (i.e., a g (+) ⁄ g ()) ⁄ trans ratio of circa 60% ⁄ 20% ⁄ 20%), regarding the side chain of Y 1 ⁄ y 1 amino acid.…”

“…Moreover, Cterminal c-and inverse c-turns were detected in the case of both cis-and trans-isomers of the stereoisomeric forms of EM2 (see Table 4). N-terminal c-turn was observed for the peptides possessing the p 2 amino acid (i.e., S [3], S [6], S [9], S [10], S [12], S [13], S [15] and S [16]) (see Figure 7A), while N-terminal inverse c-turn appeared for the molecules having the P 2 residue (i.e., S [1], S [2], S [4], S [5], S [7], S [8], S [11] and S [14]) (see Figure 7B). Taking into account the c-turns located at the C-terminal tripeptide region (see Figure 7C,D), it could be concluded that in the case of stereoisomers with the F 3 amino acid (i.e., S [1], S [2], S [3], S [5], S [6], S [8], S [10] and S [13]), the populations of inverse c-turns were larger than those of c-turns; however, in the case of stereoisomeric forms with the f 3 residue (i.e., S [4], S [7], S [9], S [11], S [12], S [14], S [15] and S [16]), the relationship between the ratios of two types of the c-turn structures was the opposite.…”