Proteasomes are large
multi-enzyme complexes that digest endogenous proteins...
transcription factors, cell cycle cyclins, virus coded proteins, improperly folded proteins due
to translation errors (made by faulty genes) and proteins damaged by cytosol molecules...
to short peptides, followed by --> hydrolysis of these peptides via cytoplasmic exopeptidases
Protein Digestion... begins when cells add small polypeptide (ubiquitin) to a protein to be digested;
addition of uniquitin targets a protein's entry into a Proteasome complex.
Proteasome is a barrel-shaped structure made of a lid, a base and 4 stacked protein rings with
trypsin, chymotrypsin, and caspace proteolytic activity.
|At left is a side view of the 20S
proteasome. Active sites are formed at the N-terminals of
β1, β2, and β5, each of which has different substrate preferences.
At right is a cutaway stereoview showing how the active sites (yellow) are sequestered within the catalytic chamber.
(M. Rechsteiner, C.P. Hill, Trends Cell Biol, 15:27–33, 2005):
from The Scientist: Vol. 9, Issue 9, page 26, May 9, 2005.
Mechanism of Action*
|Following processing by an ubiquitin
C-terminal hydrolase (UCH), Ub is conjugated to a specific
substrate as a chain by the ubiquitin-activating (E1),
-conjugating (E2), and -ligating (E3) enzymes. The chain targets
the substrate to the 26S proteasome, which comprises one 20S
proteolytic complex and two 19S regulatory complexes.
(from C.A. Ross, C.M. Pickart, Trends Cell Biol, 14:703–11, 2004)
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