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Lecture 30

BIOC 3300 Lecture Notes - Lecture 30: Isopeptide Bond, Ubiquitin, Proteasome

Biochem & Molecular Biology
Course Code
BIOC 3300
Mc Leod Roger

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1. Why is protein catabolism
necessary in the cell?
- to eliminate damaged
- to remove unnecessary
2. Where in biological pathways are
the most rapidly degraded
proteins found?
At important points of
metabolic control
3. Is lysosomal protein degradation
selective or non-selective?
4. Inside lysosomes, what
compounds degrade proteins,
and what lysosomal cell state is
optimal for their function?
acidic optimum
5. How does chloroquine inactivate
cathepsins inside the lysosome?
It increases the pH of the
6. In diabetic patients, what differs
in terms of lysosomal protein
lysosomal protein
degradation increases to
meet nutritional
requirements of the cell
7. What must be added to a protein
so that it is marked for
degradation by the proteasome?
8. What is ubiquitin's function? To covalently bind to
proteins so that they are
marked for degradation
by the proteasome
9. What is the difference between
E1, E2, and E3 protein?
E1: ubiquitin activating
E2: ubiquitin
conjugating enzyme
E3: ubiquitin protein
10. What are the three steps
involved in ubiquitin
conjugation to a protein?
1. Ubiquitin C-terminus is
linked with a thioester to E1
protein (using 1 ATP)
2. Thioester is transferred to
E2 protein
3. ubiquitin is transferred to ε-
amino group of Lys residue
in the target protein, forming
isopeptide bond
11. What is the bond called
between ubiquitin and the
target protein?
isopeptide bond
12. What is the difference
between the number of
different E1/E2/E3 proteins
present in an organism?
Only 1 E1 protein
10-20 different E2 proteins
MANY different E3 proteins
13. Why are there so many E3
So that ubiquitin can
specifically target a protein
that has an E3 protein
associated with it
14. What is the usual structure
of ubiquitin monomers on
a protein that is destined to
be degraded?
Chains of ubiquitin residues
(up to 50)
15. What are the three domains
of the proteasome called?
one 20S (cylindrical core)
two 19S (end caps)
16. Where does proteolysis
occur in the proteasome?
Occurs in the 20S cylindrical
17. What is the overall function
of the 19S caps of the
They identify and unfold the
18. True or false: ubiquitin is
degraded in the
FALSE: ubiquitin is recognized
and cleaved by the 19S end
caps and cleaved before they
can enter the protein
Lecture 30 - Protein Catabolism
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