The epoxyketone microbial metabolites epoxomicin (115;
Scheme 20) and eponemycin (116; Scheme 20) exhibited
cytotoxic activities as a result of proteasome inhibition
being the most selective proteasome inhibitors reported to
date. There are reports of other natural products active as
proteasome inhibitors but with different mechanisms to those
described previously. Thus, the cyclic peptide TMC-95-A
(117; Scheme 20), isolated from Apiospora montagnei is a
potent chymotrypsin-like inhibitor, but with activity against
the other sites as well,318 apparently binding noncovalently
to active sites through an array of hydrogen bonds. (-)-
Epigallocatechin 3-gallate (118; Scheme 20) is a potent
covalent inhibitor of the 20S proteasome, apparently due to
acylation of the active site threonines through threonine
cleavage of the ester linkage in EGCG