Proteolysis Flashcards
What are the different classes of proteases?
Metallo proteases
Serine Proteases
Cysteine Proteases
Aspartyl Proteases
What is an endopeptidase?
Cleaves substrate protein in the middle of the chain
What is an exopeptidase?
Cleave substrate protein at the end of the chain
What are examples of exopeptidases?
Amino and carboxypeptidases - this is because this is the C-terminal and the N-terminal of the amino acid
What is specific proteolysis?
Protein activation the protease will activate a certain protein e.g preproinsulin to proinsulin
What is non-specific proteolysis?
Protein degradation enzymes do not need to be specific. e.g stomach enzymes to digest the proteins in ingestion
What are inactive forms of a protein known as?
Zymogen
Proenzyme
Proprotein
What is the digestive example of protein activation?
Chymotrypsinogen and trypsinogen is converted to chymotrypsin and trypsin
What is an example of protein activation in clotting factors?
Prothrombin is converted to thrombin
What us the famous HIV-1 aspartyl protease
The HIV genome produces two proteins called Gag and Pol however they are inactive and need to be activated by specific proteolysis. The activator is the HIV-1 aspartyl protease. HIV protease inhibitors were produced to prevent the production of these two proteins.
How is protein degradation compartmentalised?
The breakdown can be targeted to different areas for example lysosomes or by ubiquitylation to the proteasome which then breaks the protein down into amino acids
What is the proteasome?
Proteasome is a large protease complex where the active sites will point into an inner cavity. The ubiquitin-substrate complex is targeted to this cavity and degraded into amino acids and small peptides
What does ubiquitin do?
Target proteins for degradation and guides them to the proteasome
What are the enzymes that are involved in ubiquitylation?
E1 = ubiquitin activating E2 = ubiquitin conjugating E3 = ubiquitin ligase
What is the process of ubiquitylation
1) Ubiquitin -COOH terminus forms a thioester bond with a cysteine residue on E1 this activates ubiquitin. This process requires ATP. Ubiquitin is conjugated to E1.
2) Activated ubiquitin will be transferred onto a cysteine molecule on E2.
3) Ubiquitin is then transferred from E2 to lysine on the target protein using E3.
What amino acid does ubiquitin bind to on the target protein? (Last stage of ubiquitylation)
Lysine
What is the N-end rule?
E3 Ubiquitin Ligase will have a poor affinity for the N -terminal of stabilizing amino acid residues therefore they are less likely to be guided to the proteasome
How does the nature of an amino acid determine its half life?
Amino acids can either be stabalizing or destabalizing. Destabalzing amino acid residues will have a higher affinity for E3 resulting in a greater interaction between the N-terminal and E3. This means that the protein is more likely to be targeted to the proteasome and broken down resulting in a shorter half life.
Example of destabilizing amino acid
Isoleucine, Tyrosine, Leucine, Arginine, Phenylalanine, Aspartic acid
Examples of stabilizing amino acids
Methionine, Glycine, Alanine, Serine, Threonine
What is an example of protein activation?
When blood cholesterol levels are too low
What is the SREBP pathway in the regulation of cholesterol?
SREBP is a transcription factor involved in the regulation of cholesterol.
At low levels cholesterol binds to SCAP causing it to undergo a conformational change binding to INSIG which will anchor SCAP and SREBP into the ER.
At low levels cholesterol will no longer bind to SCAP causing the conformational change to be reversed and SCAP and SREBP will leave the complex and go to the Golgi. At the Golgi there will be proteases which will cleave SREBP by N-terminal proteolysis. This forms N-SREBP which will then travel to the nucleus and activate specific genes e,g LDL receptors and cholesterol biosynthesis pathway
How do statins work?
Statins will lower blood cholesterol levels by blocking the enzyme HMG-CoA reductase involved in the synthesis of AcetylCoA to cholesterol. This will lower the blood cholesterol concentration and the SREBP pathway is activated to increase levels to normal if necessary
