Peptidyl vinyl sulphones are a novel class of extremely potent and specific cysteine protease inhibitors. They are highly active against the therapeutically important cathepsins O2, S and L. The highest kinact/Ki values exceed 107 M-1·s-1 for cathepsin S and 105 M-1·s-1 for cathepsins O2 and L. To study the primary specificity site of the novel human cathepsin O2 and the effectiveness of this novel class of inhibitors, a series of peptidyl vinyl sulphones with variations in the P2 residue was synthesized. Leucine in the P2 position was proven to be the most effective residue for cathepsin O2 and also for cathepsins S and L. Cathepsins O2 and S share a decreased accessibility towards P2 hydrophobic non-branched residues such as aminohexanoic acid (norleucine), methionine and oxidized methionine, but are distinguished by their different affinity towards phenylalanine in the P2 position. In contrast, cathepsin S accepts a broader range of hydrophobic residues in its S2 subsite than cathepsins O2 and L. The primary specificity-determining subsite pocket S2 in cathepsin O2 appears to be spatially more restricted than those of cathepsins S and L.

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