Question

Which of the following enzyme finally transfers ubiquitin to the target protein

• A. 26S protease complex
• B. E2
• C. E3
• D. Secretary vesicles

Hint:

E3 ligases provide specificity to the system. While cells contain only a few types of E1 and E2 enzymes, they contain hundreds of distinct E3 ligases, each recognizing a specific set of target proteins.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The ubiquitin-proteasome system (UPS) is the primary pathway for targeted protein degradation in eukaryotic cells. This pathway regulates processes such as hormone signaling, cell cycle progression, and stress responses.

Step 2: Detailed Explanation:
The attachment of ubiquitin to a target protein involves a cascade of three enzymes:
1. E1 (Ubiquitin-activating enzyme): Activates ubiquitin in an ATP-dependent reaction, forming a thioester bond between the C-terminus of ubiquitin and a cysteine residue on E1.
2. E2 (Ubiquitin-conjugating enzyme): Receives the activated ubiquitin from E1 via a transesterification reaction.
3. E3 (Ubiquitin-protein ligase): Binds both the activated E2-ubiquitin complex and the specific target protein. It then catalyzes the final transfer of ubiquitin from E2 to a lysine residue on the target protein, targeting it for degradation by the 26S proteasome.
Therefore, the E3 ligase is the enzyme that finally transfers ubiquitin to the substrate.

Step 3: Final Answer:
The enzyme that finally transfers ubiquitin to the target protein is E3.