Question

Describe ubiquitin driven degradation pathways in cells. (500 words)

Answer 1

In all tissues, majority of intracellular protiens are degraded by Ubiqitin- Proteosome Pathway(UPP). A group of enzymes accelerates the degradation pathway.These enzymes link chains of polypeptides co-factors, Ub(ubiquitin) on to the protiens to mark them for degradation. By this tagging protiens are recognized by 26s proteosome, which is a very large multicatalytic protease complex.This complex degrades the ubiqutinated protiens into small peptides. Three enzymes help in attaching Ub to protiens and subjects  the protiens for degradation. They are

1) E1 (Ub-activating enzyme)- molecular weight is 110kD

2) E2s (Ub- carrier or conjugating protiens) - Prepares ubiqitin for conjugation. Molecular weight is 16kD.

3) E3 (Ub- protien ligase) - It is the key enzyme because it recognizes a specific protien substrate and catalyzes the reaction by transferring of Ub on to it.

Ub is made up of 76 aminoacids. Its C-terminus is composed of glycine which helps in conjugation and internally it contains lyceine helps in formation of polyubiquitin chains.

In the initial step Ub is activated at its C- terminus by ubiquitin activating enzyme E1. This enzyme uses ATP to produce Ub thiolester which is a highly reactive form of Ub. Once activated it is transferred to sulfhydryl group of E2s. This E2s contains cysteine which forms thiolester linkage with activated Ub.E3 catalyzes the transfer of activated Ub from E2, initially to a lysine in the target protein and subsequently to the lysines in the Ub. Ubiquitin is attached to lysine on target protein by isopeptide bond. E3 ligases interacts with both the target protiens and E2, imparting substrate specificity to E2. Commonly E3 polyubiquitinates their substrate by Lys48 linked chains of ubiquitin, targeting the substrate for degradation in proteosome. Ubiquitination by E3 ligase regulates many activites in cells like trafricking, DNA repair and signaling. E3 ligases are also key players of cell cycle control, mediating cyclin degradation and CDK inhibitor protiens.