Systematic Comparison of Peptidic Proteasome Inhibitors Highlights the α‐Ketoamide Electrophile as an Auspicious Reversible Lead Motif

Abstract

The ubiquitin-proteasome system (UPS) has been successfully targeted by both academia and the pharmaceutical industry for oncological and immunological applications. Typical proteasome inhibitors are based on a peptidic backbone endowed with an electrophilic C-terminus by which they react with the active proteolytic sites. Although the peptide moiety has attracted much attention in terms of subunit selectivity, the target specificity and biological stability of the compounds are largely determined by the reactive warheads. In this study, we have carried out a systematic investigation of described electrophiles by a combination of in vitro, in vivo, and structural methods in order to disclose the implications of altered functionality and chemical reactivity. Thereby, we were able to introduce and characterize the class of α-ketoamides as the most potent reversible inhibitors with possible applications for the therapy of solid tumors as well as autoimmune disorders.

Noncovalent Fluorescent Probes of Human Immuno- and Constitutive Proteasomes

“…11 Recently, it was shown that the α-ketoamide motif is a peptide C-terminus electrophilic motif, leading to reversible inhibitors conversely to most of covalent inhibitors. 12 Neglected during a long period, noncovalent inhibitors introduce today a plausible alternative mechanism of inhibition to that of covalent inhibitors. 8,13 Inhibitor noncovalency favors deep solid tissue penetration, and noncovalent inhibitors can be more specific, stable, and less reactive than the covalent ones.…”

Rational Design of Proteasome Inhibitors as Antimalarial Drugs

“…[8] They first determined the substrate preference of the parasite proteasome by analyzing the degradation pattern of 228 divergent peptides. [9] Upon comparison with the EM structure of the human constitutive proteasome, [10a] it became clear that the b2b inding pocket is broader and more open in the parasite than in humans,t hus enabling an easier fit of the tryptophan at the P3 position. Since at ryptophan residue at these positions is favored in protozoa, three peptide-based vinyl sulfones (WLW-vs,WLL-vs,and LLW-vs) were employed and their activities evaluated in vitro with purified P. falciparum CP.R emarkably,t he three compounds predominantly displayed activity towards the parasite b2s ubunit, whereas the human b2site was hardly affected.…”

Selective Inhibition of the Immunoproteasome by Ligand‐Induced Crosslinking of the Active Site

“…[11] Recently, extensive research on the optimization of peptide backbones by incorporation of unnatural amino acids resulted in b5i-and b5c-specific epoxyketones. [14][15][16] Notably, PSF compounds block the CP activity in the low nanomolar range, [16] albeit they are the only peptidic CP inhibitors known so far whose electrophilic headgroup is shifted by a methylene unit, which demands an exceptional binding mode. [14][15][16] Notably, PSF compounds block the CP activity in the low nanomolar range, [16] albeit they are the only peptidic CP inhibitors known so far whose electrophilic headgroup is shifted by a methylene unit, which demands an exceptional binding mode.…”