Crystal Structures of Salinosporamide A(NPI-0052)and B (NPI-0047)in Complex with the 20S Proteasome Reveal Important Consequences of beta-Lactone Ring Opening and a Mechanism for Irreversible Binding

摘要

The crystal structures of the yeast 20S proteasome core particle(CP)in complex with Salinosporamides A(NPI-0052;1)and B(4)were solved at <3 A resolution.Each ligand is covalently bound to Thr1O~gamma via an ester linkage to the carbonyl derived from the beta-lactone ring of the inhibitor.In the case of 1,nucleophilic addition to the beta-lactone ring is followed by addition of C-3O to the chloroethyl group,giving rise to a cyclic ether.The crystal structures were compared to that of the omuralide/CP structure solved previously,and the collective data provide new insights into the mechanism of inhibition and irreversible binding of 1.Upon opening of the beta-lactone ring,C-3O assumes the position occupied by a water molecule in the unligated enzyme and hinders deacylation of the enzyme-ligand complex.Furthermore,the resulting protonation state of Thr1NH_2 deactivates the catalytic N-terminus.