(NOV 2020-11)
77. In regulating the quantity of enzyme, its degradation plays a pivotal role. Following
statements are made to represent the degradation of enzymes in the 26S proteasome.
A. The active sites of proteolytic subunits face exterior of the proteasome cylinder
B. The active sites of proteolytic subunits face interior of the proteasome cylinder
C. Degrading enzymes are targeted to exterior of proteasome by covalent attachment of one or
more molecules of ubiquitin
D. Degrading enzymes are targeted to interior of proteasome by covalent attachment of one or
more molecules of ubiquitin
Which one of the following combinations of statements represent correct mode of enzyme
degradation?
(1) A and B (2) B and C
(3) B and D (4) A and C
The correct answer is (2) B and C.
Introduction
The 26S proteasome is a large ATP-dependent protease complex essential for regulated degradation of proteins, including enzymes, within eukaryotic cells. Its function ensures protein quality control, clearance of damaged proteins, and metabolic regulation. Two key mechanistic details are critical for understanding its operation: the orientation of the proteolytic active sites and the targeting mechanism of substrates via ubiquitination. This article clarifies these aspects and identifies correct statements regarding enzyme degradation by the proteasome.
26S Proteasome Structure and Active Site Orientation
-
The proteasome core is the 20S core particle, a barrel-shaped structure composed of four stacked rings.
-
Proteolytic active sites are located inside the central chamber of the proteasome cylinder, facing inward (Statement B is true).
-
This interior localization protects non-target proteins from unspecific degradation and ensures controlled protein processing.
-
Substrates access the catalytic chamber via narrow gated pores regulated by the 19S regulatory particle.
Ubiquitin-Mediated Targeting of Substrates
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Target proteins (degrading enzymes) are tagged with ubiquitin molecules covalently attached to lysine residues.
-
This polyubiquitin chain serves as a degradation signal recognized by ubiquitin receptors on the exterior of the proteasome (Statement C is true).
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Ubiquitinated proteins are then unfolded and translocated into the proteasome’s interior chamber for degradation.
-
Thus, substrates are targeted to the exterior, not interior, of the proteasome.
Explanation of Statements
| Statement | Content | Correct/Incorrect |
|---|---|---|
| A | Active sites face exterior of proteasome cylinder | Incorrect |
| B | Active sites face interior of proteasome cylinder | Correct |
| C | Degrading enzymes targeted to exterior via ubiquitin attachment | Correct |
| D | Degrading enzymes targeted to interior via ubiquitin attachment | Incorrect |
Biological Importance
-
The inward-facing active sites prevent uncontrolled proteolysis.
-
Ubiquitin labeling confers specificity for degradation.
-
Proteasome activity regulates key processes like cell cycle, signaling, and stress response.
Summary Table
| Feature | Location | Role |
|---|---|---|
| Proteolytic Active Sites | Inside proteasome cylinder | Catalyze substrate degradation |
| Ubiquitin Labeling | Substrate exterior targeting | Marks proteins for degradation |
Conclusion
The 26S proteasome degrades enzymes through a mechanism where the proteolytic active sites face inward within the proteasome cylinder (Statement B), and protein substrates targeted by ubiquitin are recognized at the proteasome exterior (Statement C). Therefore, the correct combination is (2) B and C.
20 Comments
Kirti Agarwal
September 12, 2025Opt b and c
Neelam Sharma
September 12, 2025B. The active sites of proteolytic subunits face interior of the proteasome cylinder
C. Degrading enzymes are targeted to exterior of proteasome by covalent attachment of one or
more molecules of ubiquitin
B and C
Because proteasome enzyme k interior side activity site h takt damage and misfold protein ka degradation android kr ske eske ley bhar se kis protein ko degrade Krna use ubiquetin tag Kiya jate h phir enzyme k sath bind hota h or proteasome ese andr ki tarf kinch leta h degrade kr deta h and ubiquetin ki binding covalent hoti proteasome k sath
Varsha Tatla
September 13, 2025Option B,c will be correct
Savita Garwa
September 13, 2025B. The active sites of proteolytic subunits face interior of the proteasome cylinder
C. Degrading enzymes are targeted to exterior of proteasome by covalent attachment of one or
more molecules of ubiquitin
Rishita
September 13, 2025Option b and c is right answer
Asha Gurzzar
September 13, 2025B and c will be correct answer
Aakansha sharma Sharma
September 13, 2025The 26S proteasome degrades enzymes through a mechanism where the proteolytic active sites face inward within the proteasome cylinder (Statement B), and protein substrates targeted by ubiquitin are recognized at the proteasome exterior (Statement C). Therefore, the correct combination is (2) B and C.
Vijay kumar Meena
September 14, 2025Okay 👍
Santosh Saini
September 14, 2025The 26S proteasome degrades enzymes through a mechanism where the proteolytic active site are located inside the central chamber of proteasome cylinder, degrading enzymes targeted by ubiquitin are recognised at the proteasome exterior
Meera gurjar
September 14, 2025B and c
Pallavi Ghangas
September 14, 2025The proteolytic site of proteosome is present interior to the cylinder whereas the substrate are targeted to the exterior
Dharmpal Swami
September 14, 2025Option B and C are correct
Lokesh Kumawat
September 14, 2025Option b and c is correct
Priya dhakad
September 14, 2025The proteolytic site of proteosome is present interior to the cylinder and substrate targeted by ubiquitin are recognised at the proteosome exterior.
Parul Yadav
September 15, 2025Degradation of enzymes in the 26S proteasome:
The active sites of proteolytic subunits face interior of the proteasome cylinder.
Degrading enzymes are targeted to the exterior of the proteasome by covalent attachment of one or more molecules of ubiquitin.
Soniya Shekhawat
September 15, 2025The active sites of proteolytic subunits face interior of the proteasome cylinder.
Degrading enzymes are targeted to the exterior of the proteasome by covalent attachment of one or more molecules of ubiquitin this is degradation of 26S proteasome: enzyme
priya khandal
September 17, 2025option b and c is correct sir
Muskan Yadav
September 17, 2025The 26S proteasome breaks down proteins using a mechanism in which the proteolytic (protein-cutting) active sites are located on the inside of its cylindrical structure (Statement B). Meanwhile, proteins that have been marked for degradation by ubiquitin are recognized on the outer surface of the proteasome (Statement C). Hence, the correct answer is option (2), which includes statements B and C.
Tanvi Panwar
September 17, 2025The proteolytic sites are located inside of the proteasome to prevent degradation of non-targeted proteins and ubiquitin are attached to the targeted protein as a signal for degradation whose receptors are present at the exterior of the proteasome . So option B and C are correct.
Aafreen Khan
September 23, 2025The active sites are within the central cavity of the 20S core particle which is part of 26 S proteasome.
Degrading enzymes targeted to exterior via ubiquitin attachment
So B and C are the correct answer