The structure of 26 s proteasome. The proteasome is a 26S enzyme complex that comprised of core 20S catalytic complex and 19S regulatory complex (A). The 20S proteasome core has chymotrypsin-like, trypsin-like, and peptidyl glutamyl-like activities that are associated with three distinct units: β5, β2, β1, respectively. Chymotrypsin-like activity at proteasome β5 subunit (PSMB5) is associated with the rate-limiting step of proteolysis. Proteasome inhibitor bortezomib (Bort) mainly inhibit the chymotrypsin-like activity at the PSMB5 (B).