Intracellular Proteolysis
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What are the 4 terms for enzymes that break down proteins
Proteases, proteinases, peptidases, cathepsins
What does it mean if a protease is a serine protease?
Has serine residue at its active site
Difference between exo and endo peptidases?
Exo - End cleaving of amino acids Endo - Middle cleaving
What 3 terms are used to describe inactive proteins
Zymogen Proprotein Proenzyme
How does proteolysis activate proteins with chymotrypsin
Chymotrypsin (inactive) cleaved -> Trypsin (active) -> Pi-chymotrypsin (active) -> Final chymotrypsin (active) Trypsinogen is also cleaved to trypsin used in the above. All stages involve cleaving
How does proteolysis activate proteins with chymotrypsin
Chymotrypsin cleaved -> Trypsin -> Pi-chymotrypsin -> Final chymotrypsin
Describe proteolysis with insulin
• Preproinsulin carries signal sequence to RER during synthesis. Removal of sequence • Forms proinsulin • Cleaved again to release C peptide, forming mature insulin
Describe the clotting cascade with protelysis
One serine protease cleaves another protease to convert zymogen to active protease. Many serine proteases are involved
Which two factors in the clotting cascade are the cause of X-linked haemophilia
Factor VIII or IX Mutations impair clotting
What sequence is needed at splyce acceptor sites?
AG
Name two famous cysteine proteases
Bromelain (from pineapple), papain (from papaya) (meat tenderisers)
Describe HIV-1 Protease (aspartyl protease)
Virus expresses large precusor protein, to be cleaved to produce protein products. HIV-1 Protease helps cleave to make the virus active for the production of new viral particles
What are two ways in which proteins are degraded (through compartmentalisation)
Lysosomes or ubiquitylation -> Proteasome Both convert to amino acids
Describe ubiquitin
Small protein 76 AAs Highly conserved through evolution Attachment of ubiquitin to target protein, marking them for degradation with the proteasome
Describe the first step of protein ubiquitylation
Ubiquitin attached to E1 (ubiquitin-activating enzyme), reacts with carboxy terminal (of ubiquitin) through thio group on the cysteine residue of the E1. The reaction requires ATP, and generates a thioester bond.
Describe the second step of protein ubiquitylation
Ubiquitin transferred from E1 to E2 (ubiquitin-conjugating enzyme). From one cysteine to another, thioester bond
Describe the third step of protein ubiquitylation
Ubiquitin transferred to lysine residue of a target protein, with the help of E3 (ubiquitin ligase). There are many E3 ligases that confer specificity and recognise the specific substrate.
Why are some proteins short-lived and why are others long-lived
Very first amino acid at end terminus of proteins. The first methionine is quickly removed, the second amino acid’s nature has a big effect on the stability and therefore half-life of the protein as it is recognised by the E3 ligase The ligase has greater affinity for destabilising proteins. N-end
Name some stabilising amino acids
Met, Gly, Ala, Ser, Thr, Val
Name some destabilising amino acids
Ile, Gln, Tyr, Glu, Pro, Leu, Phe, Asp, Lys, Arg