The mammalian 26S proteasome is a 2500 kDa multi-catalytic complex involved in intracellular protein degradation. We describe the synthesis and properties of a novel series of non-covalent di-peptide inhibitors of the proteasome used on a capped tri-peptide that was first identified by high-throughput screening of a library of approx. 350000 compounds for inhibitors of the ubiquitin–proteasome system in cells. We show that these compounds are entirely selective for the β5 (chymotrypsin-like) site over the β1 (caspase-like) and β2 (trypsin-like) sites of the 20S core particle of the proteasome, and over a panel of less closely related proteases. Compound optimization, guided by X-ray crystallography of the liganded 20S core particle, confirmed their non-covalent binding mode and provided a structural basis for their enhanced in vitro and cellular potencies. We demonstrate that such compounds show low nanomolar IC50 values for the human 20S β5 site in vitro, and that pharmacological inhibition of this site in cells is sufficient to potently inhibit the degradation of a tetra-ubiquitin–luciferase reporter, activation of NFκB (nuclear factor κB) in response to TNF-α (tumour necrosis factor-α) and the proliferation of cancer cells. Finally, we identified capped di-peptides that show differential selectivity for the β5 site of the constitutively expressed proteasome and immunoproteasome in vitro and in B-cell lymphomas. Collectively, these studies describe the synthesis, activity and binding mode of a new series of non-covalent proteasome inhibitors with unprecedented potency and selectivity for the β5 site, and which can discriminate between the constitutive proteasome and immunoproteasome in vitro and in cells.

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We identified N,C-capped dipeptides that are selective for the Mycobacterium tuberculosis proteasome over human constitutive and immunoproteasomes. Differences in S3 and S1 binding pockets appeared to account for species-selectivity. The inhibitors are able to penetrate mycobacteria and kill non-replicating M. tuberculosis under nitrosative stress.

We have identified short N,C-capped peptides that selectively inhibit the proteasome of the malaria-causing pathogen Plasmodium falciparum. These compounds are highly potent in culture with no toxicity in host cells. One cyclic biphenyl ether compound inhibited intraerythrocytic growth of P. falciparum with an IC50 of 35 nM, and we show that even a pulse treatment with this cyclic peptide induced parasite death due to proteasome inhibition. These compounds represent promising new antimalarial agents that target the essential proteasomal machinery of the parasite without toxicity toward the host.

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The syntheses of new redox-active crown ethers 8a, 8b, 9a, and 9b containing cis-and trans-conjugated olefinic linkages between the ferrocene redox center and respectively benzo-15-crown-5 and /V-phenylaza-15-crown-5 are described. The sodium cation forms 1:1 stoichiometric complexes with the trans ferrocenyl ionophores 8b and 9b, whereas potassium produces 1:2 intermolecular sandwich complexes with the same ligands, which were also observed by fast atom bombardment mass spectrometry. Electrochemical investigations reveal the binding of Na+, K+, and Mg2+ guest cations at the respective crown ether coordinating sites results in shifts of the ferrocene oxidation wave to more positive potentials if a conjugated -electron system links the heteroatoms of the ionophore to the redox center. The magnitude and type (one or two waves) of the anodic shift are related to the charge:radius ratio of the cationic guest, Mg2+ producing the largest value and K+ the smallest.

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