Inhibitor SourceBook™ Second Edition

Proteases
Proteasome and Ubiquitination Inhibitors
Proteasomes are large multi-subunit complexes,
localized in the nucleus and cytosol that selectively
degrade intracellular proteins. A protein marked for
degradation is covalently attached to multiple molecules
of ubiquitin (Ubq). Four or more Ubq are required to
set a protein for degradation by the proteasome. Ubq
is a highly conserved 76-amino acid (8.6 kDa) protein,
which escorts proteins for rapid hydrolysis to the multicomponent
enzymatic complex, the 26S proteasome. The
proteolytic core of this complex, the 20S proteasome,
contains multiple peptidase activities and functions
as the catalytic machine. This core is composed of 28
subunits arranged in four heptameric, tightly stacked,
rings (a 7 , b 7 , b 7 , a 7 ) to form a cylindrical structure. The
a-subunits make up the two outer and the b-subunits the
two inner rings of the stack. The entrance of substrate
proteins to the active site of the complex is guarded
by the a-subunits that allow access only to unfolded
and extended polypeptides. The proteolytic activity is
confined to the b-subunits. The 19S complex “caps” at
each end of the 20S proteasome help in unfolding protein
substrates. The 19S cap contains subunits with ATPase
activity, subunits that recognize and bind polyubiquitin
chains and putative unfoldases that unfold protein
chains and translocate them into the 20S proteasome.
In the Ubq-proteosome degradation pathway, Ubq is
first covalently ligated to target proteins by a multienzymatic
system consisting of Ubq-activating (E1),
Ubq-conjugating (E2), and the Ubq-ligating (E3)
enzymes. The E1 activates a Ubq monomer at its Cterminal
cysteine residue to a high-energy thioester
bond which is then transferred to a reactive cysteine
residue of the E2 enzyme. The final transfer of Ubq
to the h-amino group of a reactive lysine residue of
substrate proteins is brought about by the E3 enzyme.
Ubiquitinated protein is then escorted to the 26S
proteasome where it undergoes final degradation and
the ubiquitin is released and recycled. The ubiquitinproteasome
system plays a major role in the degradation
of many proteins involved in cell cycle, proliferation,
and apoptosis. Proteasomes also breakdown abnormal
proteins that result from oxidative stress and
mutations that might otherwise disrupt normal cellular
homeostasis. This pathway has been implicated in
several forms of malignancy, in the pathogenesis of
several genetic diseases, and in the pathology of muscle
wasting. It is also involved in the destruction of proteins
that participate in cell cycle progression, transcription
control, signal transduction, and metabolic regulation.
Several distinct groups of compounds, designed to act as
selective proteasome inhibitors, have helped immensely
in understanding the biological role and importance of
the ubiquitin-proteasome pathway. These compounds are
designed to block proteasome function in cancer cells
without significantly affecting biological processes in
the normal cell.
References:
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