**5.1.2 Boronic acid peptides**

Boronate inhibitors are much more potent than their structurally analogous peptide aldehydes [Adams et al., 1998]. These includes MG262 (Z-Leu-Leu-Leu-boronate; analogous to MG132) and PS-341 (pyrazylcarbonyl-Phe-Leu-boronate; analogous to the aldehyde PS-402). MG262 is a cell permeable and reversible inhibitors of the chymotrypsin like activity of the proteasome. PS-341 is clinically the most advanced proteasome inhibitor and inhibits the chymotrypsin like active site of the proteasome β-subunit. Its boronic acid group binds the active site threonine in the proteasome with high affinity and specificity (Fig 5).
