Intracellular Proteolysis

Question 1

What are the 4 terms for enzymes that break down proteins

Answer 1

Proteases, proteinases, peptidases, cathepsins

Question 2

What does it mean if a protease is a serine protease?

Answer 2

Has serine residue at its active site

Question 3

Difference between exo and endo peptidases?

Answer 3

Exo - End cleaving of amino acids
Endo - Middle cleaving

Question 4

What 3 terms are used to describe inactive proteins

Answer 4

Zymogen
Proprotein
Proenzyme

Question 5

How does proteolysis activate proteins with chymotrypsin

Answer 5

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

Question 5

How does proteolysis activate proteins with chymotrypsin

Answer 5

Chymotrypsin cleaved -> Trypsin -> Pi-chymotrypsin -> Final chymotrypsin

Question 6

Describe proteolysis with insulin

Answer 6

1. Preproinsulin carries signal sequence to RER during synthesis. Removal of sequence
2. Forms proinsulin
3. Cleaved again to release C peptide, forming mature insulin

Question 7

Describe the clotting cascade with protelysis

Answer 7

One serine protease cleaves another protease to convert zymogen to active protease. Many serine proteases are involved

Question 8

Which two factors in the clotting cascade are the cause of X-linked haemophilia

Answer 8

Factor VIII or IX
Mutations impair clotting

Question 9

What sequence is needed at splyce acceptor sites?

Answer 9

AG

Question 10

Name two famous cysteine proteases

Answer 10

Bromelain (from pineapple), papain (from papaya) (meat tenderisers)

Question 11

Describe HIV-1 Protease (aspartyl protease)

Answer 11

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

Question 12

What are two ways in which proteins are degraded (through compartmentalisation)

Answer 12

Lysosomes or ubiquitylation -> Proteasome

Both convert to amino acids

Question 13

Describe ubiquitin

Answer 13

Small protein 76 AAs
Highly conserved through evolution
Attachment of ubiquitin to target protein, marking them for degradation with the proteasome

Question 14

Describe the first step of protein ubiquitylation

Answer 14

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.

Question 15

Describe the second step of protein ubiquitylation

Answer 15

Ubiquitin transferred from E1 to E2 (ubiquitin-conjugating enzyme). From one cysteine to another, thioester bond

Question 16

Describe the third step of protein ubiquitylation

Answer 16

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.

Question 17

Why are some proteins short-lived and why are others long-lived

Answer 17

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

Question 18

Name some stabilising amino acids

Answer 18

Met, Gly, Ala, Ser, Thr, Val

Question 19

Name some destabilising amino acids

Answer 19

Ile, Gln, Tyr, Glu, Pro, Leu, Phe, Asp, Lys, Arg