Fig. 23.26 The proteasome, a cellular shredding machine, proteolytically cleaves ubiquitinylated proteins selectively into small oligopeptides that have between 3 and 20 amino acids. The crystal structure of the 20S proteasome from yeast (subunits are shown in different colors) is shown at the left. Six of these units are inhibited by bortezomib (yellow). The boronic acid derivative bortezomib 23.53 (right, gray) reacts with the N-terminal Thr1 and forms a covalent boronic acid ester complex.