A Selective AT₂ Receptor Ligand with a γ-Turn-Like Mimetic Replacing the Amino Acid Residues 4−5 of Angiotensin II

Abstract

Three angiotensin II (Ang II) analogues encompassing a benzodiazepine-based gamma-turn-like scaffold have been synthesized. Evaluation of the compounds in a radioligand binding assay showed that they had no affinity to the rat liver AT(1) receptor. However, one of the compounds displayed considerable affinity to the pig uterus AT(2) receptor (K(i) = 3.0 nM) while the other two lacked affinity to this receptor. It was hypothesized that the reason for the inactivity of one of these analogues to the AT(2) receptor was that the guanidino group of the Arg(2) residue and/or the N-terminal end of the pseudopeptide could not interact optimally with the receptor. To investigate this hypothesis, a conformational analysis was performed and a comparison was carried out with the monocyclic methylenedithioether analogue cyclo(S-CH(2)-S)[Cys(3,5)]Ang II which is known to bind with high affinity to the AT(2) receptor (K(i) = 0.62 nM). This comparison showed that, in the compounds with high AT(2) receptor affinity, the guanidino group of the Arg(2) residue and the N-terminal end could access common regions of space that were not accessible to the inactive compound. To examine the importance of the guanidino group for binding, the Arg side chain was removed by substituting Arg(2) for Ala(2) in the analogue having the high affinity. This analogue lacked affinity to AT(2) receptors, which supports the role of the guanidino group in receptor binding.

A Single β-Amino Acid Substitution to Angiotensin II Confers AT 2 Receptor Selectivity and Vascular Function

“…For example, ␥-turn scaffolds have been introduced around residues 4 and 5 to produce AT 2 R-selective compounds. 30 In other studies, analogs with tyrosine-functionalized bicyclic dipeptides replacing the Tyr 4 -Ile 5 residues presented an extended backbone conformation with heightened AT 2 R-to-AT 1 R selectivity. 31 The majority of the aforementioned studies relied on binding affinity estimates derived from displacement of iodinated Ang II from AT 1 R in rat liver membranes and AT 2 R in pig uterine membranes.…”

“…Substitution of His 6 by 4-NH 2 -Phe 6 in Ang II produced a peptide with high AT 2 R-to-AT 1 R selectivity in binding assays. 29 Hallberg and colleagues 30 have also performed a number of modifications to Ang II, principally around the Tyr 4 -Ile 5 residues, resulting in a number of highly AT 2 Rselective peptidomimetics. For example, ␥-turn scaffolds have been introduced around residues 4 and 5 to produce AT 2 R-selective compounds.…”

Modeling binding modes of angiotensin II and pseudopeptide analogues to the AT2 receptor

“…2) [18][19][20][21] was performed in the Ang II binding site of the AT 2 receptor model. The ligand-receptor complex corresponding to ligand conformation 3 in Table 1 was used as a template for building the pseudopeptides.…”

“…The binding conformation of Ang II when interacting with the AT 2 receptor has been investigated to a lesser extent, but monoand bi-cyclizations, including turn mimicking structures, in the 3-5 region of Ang II have produced analogues with retained affinity [7,[17][18][19][20][21]. An extended receptor-bound conformation of Ang II has also been suggested in the case of AT 2 binding [16].…”

“…We have previously derived models of the binding mode of several constrained pseudopeptide Ang II analogues to the AT 2 receptor using a ligand-based approach [18][19][20][21]. In the present study we aim to explore the ligand binding mode of both the pseudopeptides and the native ligand in the environment of the receptor.…”