Ubiquitin is the key protein involved in
Correct Answer: Protein degradation
Description: Ans. b (Protein degradation) (Ref. Harper's biochemistry 27th ed., Chapter 45)# Ubiquitin targets many intracellular proteins for degradation.# It is estimated that a minimum of 4 ubiquitin molecules must be attached to commit a target molecule (e.g., a misfolded form of CFTR-protein involved in cystic fibrosis) to degradation in a proteasome.UBIQUITIN: A KEY MOLECULE IN PROTEIN DEGRADATION# Many proteins are degraded by the ubiquitin proteasome pathway, the discovery of which earned Aaron Ciechanover, Avram Hershko, and Irwin Roose a Nobel Prize.# There are 2 major pathways of protein degradation in eukaryotes:- One involves lysosomal proteases and does not require ATP.- The other pathway involves ubiquitin and is ATP-dependent.# Ubiquitin is small protein that plays key role in marking proteins for subsequent degradation in proteasomes.# Ubiquitin is known to be involved in:y- Degradation of proteins, and is particularly in disposal of misfolded proteins- Cell cycle regulation (degradation of cyclins),- DNA repair;- Activation of NFB,- Muscle wasting,- Viral infections, etc# Ubiquitin can be cleaved from a target protein by deubiquitinating enzymes and the liberated ubiquitin can be reused.MISFQLDED PROTEINS UNDERGO ENDOPLASMIC RETICULUM-ASSOCIATED DEGRADATION# Chaperones present in the lumen of the ER and in the cytosol target misfolded proteins to proteasomes. Prior to entering proteasomes, most proteins are ubiquitinated and escorted to proteasomes by polyubiquitin-binding proteins.- Diseases caused by lack of a particular functioning protein, due to its degradation as a consequence of misfolding, include:* Cystic fibrosis (misfolded CFTR protein),Q* Marfan syndrome (misfolded fibrillin),* Fabry disease (misfolded alpha galactosidase),* Gaucher's disease (misfolded beta glucocerebrosidase) and* Retinitis pigmentosa 3 (misfolded rhodopsin).* Some cancers may be associated with misfolding, and hence ineffective functioning, of tumour suppressor proteins such as von Hippel Lindau protein or p53.- Many protein misfolding diseases are characterised not by disappearance of a protein but by its deposition in insoluble aggregates within the cell. E.g.:* Alzheimer's disease (deposits of amyloid beta and tau),* Type II diabetes (depositis of amylin),* Parkinson's disease (deposits of alpha synuclein),* Spongiform encephalopathies such as Creutzfeldt-Jakob disease (deposits of prion protein).Educational Points:# The GOLGI APPARATUS Is Involved in Glycosylation & Sorting of Proteins.
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Biochemistry
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