Intracellular Proteolysis Flashcards
How can protein activity be regulated?
- Active site and allosteric regulators
- Pro-proteins and zymogens
- Phosphorylation
- pH and protein degradation
How quickly are abnormal variants turned over?
Quickly (more quickly than normal proteins)
When do cells increase rate of protein turnover?
- Times of starvation
- Breakdown of muscle protein
What is the N-terminal rule?
Half-life of a protein is dependent on which amino acid on the N-terminal
How was the N-terminal rule tested for?
By mutating the N-terminal residue of Beta-galactosidase and half life is observed
What are the features of caspases?
- Cysteine dependent aspartate-directed proteases
- Mediators of apoptosis
- Regulated post-trans via pro-enzyme activation
What are the features of cathepsins?
- Major lysosomal protease family
- Cysteine proteases optimally active at low pH
What are the features of calpains?
- Ca2+ dependent non-lysosomal proteases
- Res
What are the features of pro-protein convertases?
- Ca2+ dependent serine proteases
- Cleave various proteins (e.g hormones and neurotransmitters)
- Activated by growth factors
What are the features of proteasomes?
- Only example of threonine protease family
- Major site of intracellular protein degradation
- Ubiquitin dependent
What are the pathways used to degrade proteins?
- Lysosomal pathway
- Ubiquitin-mediated
Where are the end products of lysosomal degradation exported to?
The cytosol
What are the 4 ways of delivery of substrate proteins to the lysosome?
- Endocytosis
- Autophagy
- Phagocytosis
- Chaperone-mediated autophagy
How does autophagy delivery substrate proteins to lysosome?
Macromolecules and/or organelles are engulfed by ER-dervied membrane and delivered to the lysosome
What do acid hydrolases require to ensure they are transported to the lysosome?
Mannose-6-phosphate tag added at the golgi
How are mannose-6-phosphate tags added to an N-linked oligosaccharide?
- Addition of GlcNAc to an N-linked oligosaccharide in cis-golgi
- Phosphodiesterase removes GlcNAc, leaving just the M6P tag
Why are incorrectly folded amino acids not tagged with Mannose-6-phosphate?
If not properly folded then the amino acids are not in the correct place for the signal patch so it won’t bind
What recognises the M6P tag?
Receptor proteins in trans-golgi network
How is the M6P tag recognised to allow acid hydrolase to be transported to lysosome?
- M6P tagged acid hydrolase binds to M6P receptor in TGN
- Clathrin coasted vesicle is transported to early/late endosome and then to the lysosome
- Low pH causes M6P tagged acid hydrolase ro dissociate from the receptor
How is the M6P recycled?
What can lysosomal storage diseases result in?
The formation of inclusion bodies
What causes the formation of inclusion bodies in lysosomal storage diseases?
- Problems with GlcNAc phosphotransferase
- Substrates gets delivered but enzymes do not so substrates are not degraded
What is the structure of ubiquitin?
- small 76 residue protein
- Lysine residues exposed on surface that are critical to function
How does ubiquitin attach to proteins?
Via isopeptide bond between ubituitin’s C-terminal carboxylate and lysine residue on substrate