Question

Ubiquitin Proteasome System

A.) What is ubiquitin? How many Kd large is Ubiquitin?

B.) The UPS utilizes three proteins to place ubiquitin on a target. What are those three proteins?

C.) What step requires ATP?

D.) Describe the overall process (three steps)

E.) What is the role of lysine residues in the process?

F.) What is the proteasome?

G.) Describe the role of the proteasome in cancer

Answer 1

1. Ubiquitin (Ub) is a residue protein that exists within the cell either free or in conjugation with some other proteins. Its molecular weight is 8.5kDa

2. The three main proteins are basically the enzymes that are used for placement of ubiquitin on a target and these are as follows

a) E1 ubiquitin-activating enzyme

b) E2 ubiquitin-conjugating enzyme

c) E3 ubiquitin ligase

3. The Ub dependent pathways involve degradation of Ub-conjugated protein by an ATP-dependent protease i.e 26S proteasome

4. Three main steps are

Step 1 – Ubiquitin is activated by the E1 by utilizing the energy provided by ATP to form E1-ubiquitin thioester. The E1-ubiquitin thioester is formed by utilizing a) C-terminal carboxyl group of ubiquitin and b) cysteine (C) residue on the E1.

Step 2 - Ubiquitin is then transferred from E1 to active site of cysteine residue on the E2 to again form a E2-ubiquitin thioester–linked intermediate

Step 3- Ubiquitin is transferred from E2 usually to substrate residue i.e internal lysine (K) residue