Figure 2: Proteasome inhibition by bortezomib. Bortezomib is a modified dipeptidyl boronic acid inhibitor that selectively and potently inhibits chymotryptic threonine protease activity, the rate-limiting proteolytic step in the proteosome. The boronic acid specifically, and with high affinity, fits the active sites (shown by stars) of the proteasome. The boronic acid group forms a complex with the threonine hydroxyl group (Thr1) in the chymotrypsin-like active site and acts as a reversible inhibitor of the chymotryptic-like activity of the proteasome, which is sufficient to inhibit proteolysis.