Proteasome epoxomicin

X-ray crystallographic analysis demonstrated that the epoxyketone pharmacophore of epoxomicin forms a covalent adduct as a morpholino ring [50] with the amino terminal threonine of the 20S proteasome. Epoxomicin draws its specificity from the uniqueness of the proteasomal N-terminal threonine non-proteasomal proteases lack an N-terminal nucleophilic residue and thus cannot form a stable covalent morpholino adduct with the epoxomicin epoxyketone pharmacophore [50],... 

Whereas standard proteases use serine, cysteine, aspartate, or metals to cleave peptide bonds, the proteasome employs an unusual catalytic mechanism. N-terminal threonine residues are generated by self-removal of short peptide extensions from the active yS-subunits and act as nucleophiles during peptide-bond hydrolysis [23]. Given its unusual catalytic mechanism, it is not surprising that there are highly specific inhibitors of the proteasome. The fungal metabolite lactacystin and the bacterial product epoxomicin covalently modify the active-site threonines and in-... 

Fig. 10.7. Inh ibitor binding to individual active sites of the yeast 20S proteasome. The inhibitors lactacystin (A), epoxomicin (B) and TMC95A (C) are colored green and are shown in stereo mode together with their unbiased electron densities. The active-site Thrl is highlighted in black. (A) Covalent binding of the Streptomyces metabolite lactacystin to the active site of 5. The SI pockets of the active subunits and differ from that of 5 and are not suitably constructed to bind the inhibitor. As discussed in the text, Met45 (black), which is located at the bottom of the 5-Sl pocket, makes the difference for inhibitor...

Elofsson, M., Sin, N., and Crews, C. M. Epoxomicin, a potent and selective proteasome inhibitor, exhibits in vivo antiinflammatory activity. Proc. Natl. Acad. Sci. USA 1999, 96, 10403-10408. 

Sin N, Kim K, Elofsson M, Meng L, Auth H, Crews CM (1999) Total synthesis of the potent proteasome inhibitor epoxomicin a useful tool for understanding proteasome biology. Bioorg Med Chem Lett 9 2283-2288... 

Crews, Crystal structure of epoxomicin 20S proteasome reveals molecular basis for selectivity of or, /T-epoxyketone proteasome inhibitors, J. Am. Chem. Soc. 2000,... 

In recent years, a number of structurally distinct compounds targeting the proteasome have reached the clinic, amongst others the covalent and irreversible inhibitor carfilzomib [5]. The structure of carfilzomib is based on that of the natural product, epoxomicin, that also features the epoxyketone electrophilic trap. Indeed, numerous natural product proteasome inhibitors with a distinguishing electrophile grafted onto a peptidic core have been described over the years, including lactacystin, syringolin A (SylA), and fellutamide B. An important class of synthetic covalent proteasome inhibitors is represented by the peptide vinyl sulfones, whereas numerous noncovalent proteasome inhibitors have been discovered as well (e.g., TMC 95A). 

Figure 2.3 Serine protease and hydrolase ABPs. (A) Reaction of a general serine hydrolase probe containing a fluorophosphonate (FP) reactive electrophile. This class of probes has been used extensively to label various classes of serine hydrolases including proteases, esterases, lipases and others. (B) The peptide diphenyl phosphonate (DPP) reacts with the serine nucleophile in the active site of serine proteases. This probe is much less reactive than the FP class of probes but is more selective towards serine proteases over other types of serine hydrolases.(C) The natural product epoxomicin contains a keto-epoxide that selectively reacts with the catalytic N-terminal threonine of the proteasome P-subunit. This reaction results in the formation of a stable six-membered ring. This class of electrophile has been used in probes of the proteasome.

Wei D, Lei B, Tang M, Zhan C-G (2012) Frmdamental reaction pathway and fiee energy profile for inhibition of proteasome by epoxomicin. J Am Chem Soc 134(25) 10436—10450. 

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