Intracellular Proteolysis

Question 1

What is proteolysis?

Answer 1

The opposite of protein synthesis- break down of proteins using hydrolysis reactions

Question 2

What’s a synonym for proteases?

Answer 2

Peptidases

Question 3

What do serine proteases contain at their active sites?

Answer 3

Serine

Question 4

What do cysteine and aspartyl proteases have at their active sites?

Answer 4

Aspartic acid residues

Question 5

What must metalloproteases form a compound with in order to be activated?

Answer 5

Specific metal ions

Question 6

What’s the difference between endopeptidases and exopeptidases?

Answer 6

Endopeptidases are used in the middle of the protein chains and exopeptidases are used on the ends of protein chains

Question 7

If the sequence of proteolysis is specific, what happens to the protein?

Answer 7

It is activated

Question 8

If the sequence of proteolysis is non-specific, what happens to the protein?

Answer 8

Degraded

Question 9

What gives rise to multiple peptide hormones?

Answer 9

Proteolytic processing of propiomelanocortin in the Golgi apparatus

Question 10

Give examples of cysteine proteases

Answer 10

Bromelain (pineapples)

Papain (papaya)

Question 11

How do cysteine proteases work as meat tenderisers?

Answer 11

meat is Mostly protein and proteases help break down the tough fibres

Question 12

Give an example of an aspartyl protease

Answer 12

HIV-1 protease (AKA retropepsin) catalysed the cleaving of the long precursor chain on HIV which activates it

Question 13

Why is non-specific protein degradation compartmentalised?

Answer 13

To stop it causing lots of damage in other parts of the cell

Question 14

What are the two pathways proteins could follow to be degraded?

Answer 14

Lysosomal or ubiquitinproteasome

Question 15

How are proteases formed in the lysosomes designed to minimise damage?

Answer 15

They only operate at low pHs

Question 16

What are the steps in the protein degradation pathway of ubiquitinproteasome?

Answer 16

• protein is first covalently modified with a chain of ubiquitin molecules
• it is then targeted to the proteasome structure

Question 17

What molecule attaches to a molecule to mark it for proteasomal degradation?

Answer 17

Ubiquitin

Question 18

What is the 3 step pathway for protein ubiquitylation

Answer 18

1- ubiquitin is attached to E1 which reacts with the COOH via the cysteine residue. This requires ATP and forms a thioester bond
2- ubiquitin is attached to E2 also via a thioester bond
3- the ubiquitin-E2 complex has E3 added to it. Then it is transferred from the cysteine to the lysine residue.

Question 19

Which residues have to be polyubiquitinated for them to be marked for proteasomal degradation

Answer 19

K11 or k48

Question 20

Why do some proteins have a long half life and some have a short half life?

Answer 20

Depends on the amino acid at the N-terminus of the amino acid. Stabilising amino acids are not well recognised by the ligases so will survive longer and vice versa