Intracellular Proteolysis

Question 1

Define proteolysis

Answer 1

Break down of proteins by proteases

Question 2

What is the role of statins?

Answer 2

Statins block cholesterol synthesis by inhibiting HMG-CoA reductase

Question 3

Which enzymes are responsible for proteolysis?

Answer 3

• peptidases
• proteinases
• proteases
• cathepsins

Question 4

What does the N end rule state?

Answer 4

The N end terminal amino acids (after methionine cleaved off by methionine peptidases) determine a proteins half life

Question 5

Name the enzymes involved in protein ubiquitylation

Answer 5

E1 - Ubiquitin activating enzyme
E2 - Ubiquitin conjugating enzyme
E3 - Ubiquitin ligase

Question 6

What is the consequence of statins inhibiting cholesterol biosynthesis?

Answer 6

Leads to the upregulation of LDL receptors
LDL removed from bloodstream - less cholesterol in blood
- so cells take up cholesterol from blood
- upregulating gene transcription

Question 7

How is ubiquitin attached to the substrate protein?

Answer 7

Covalently polymerased to the protein

Question 8

How does the liver use cholesterol?

Answer 8

Liver converts cholesterol into bile acids which are secreted throughout the bile ducts

Question 9

What is the function of metalloproteases?

Answer 9

Cleave off metal proteins

Question 10

Explain what is meant by the N end rule?

Answer 10

Rule governing rate of protein degradation by recognising the residue of proteins at the N end terminal

Question 11

What do endopeptidases do?

Answer 11

Cleave amino acids off the middle of polypeptide chains

Question 12

Which TF activates the genes required for the cholesterol biosynthesis pathway?

Answer 12

SREBP

Question 13

What is a proteosome?

Answer 13

Large protein complex

Question 14

Explain what happens when there is high cholesterol levels?

Answer 14

1. SREBP + SCAP = tight complex
2. SREBP remains in ER
   - doesn’t go to nucleus - transcription not activated
3. INSIG anchors complex to ER
4. Cholesterol binds to SCAP (allosteric protein)
5. SCAP(with cholesterol) binds to INSIG

Question 15

How are proteins categorised based on their half lives?

Answer 15

1. short lived

2. long lived

Question 16

Why does SREBP form complex with SCAP?

Answer 16

By binding to SCAP, SREBP can be transported to the golgi in vesicles

Question 17

Name examples of exopeptidases

Answer 17

Aminopeptidases

Carboxypeptidases

Question 18

Outline the process of protein ubiquitylation using enzymes

Answer 18

1. COOH on ubiquitin forms a thioester bond with E1 Cysteine using ATP
2. Ubiquitin molecule transferred to the target protein
   from E2 to E3
3. E3 enzymes are specific to target proteins

Question 19

Give an example of non specific proteolysis?

Answer 19

Ubiquitin Protease Pathway - significant for protein degradation

Question 20

Explain how the Cholesterol biosynthesis pathway deals with low cholesterol?

Answer 20

1. SCAP + INSIG dissociate
   - low cholesterol reverses the conformational changes
   occured (no longer complementary)
2. SCAP and INSIG move to Golgi via vesicular transport
3. Proteases activate SREBP via proteolysis
4. Activated SREBP leaves golgi
5. In Nucleus activates genes for cholesterol biosynthesis
   and LDL receptors

Question 21

What is the role of exopeptidases?

Answer 21

Cleaves off amino acids at the end of polypeptide chains

Question 22

What is SREBP?

Answer 22

Sterol regulated element binding proteins

Membrane bound transcription factors synthesised on the ER membranes

Question 23

How does non specific proteolysis occur?

Answer 23

Compartmentalised as acidic proteases present in lysosomes

Question 24

What is the benefit of protein ubiquitylation?

Answer 24

Marks the protein for degradation via the proteosome

Question 25

What causes X linked Haemophillia?

Answer 25

Deficiencies of Factor VIII or IX in clotting cascade pathway mutated gene found on X chromosome Haemophilia B caused by single nucleotide base change in clotting Factor IX gene - incorrect RNA splicing - non functional, truncated protein formed

Question 26

What is a half life?

Answer 26

The time it takes to degrade half the amount of protein at the start of the measurement

Question 27

Where are cysteine proteases found?

Answer 27

In fruits - pineapple, papaya, fig and kiwi

Question 28

Which proteins require specific proteolysis in order to be activated?

Answer 28

1. Digestive enzymes | 2. Clotting factors

Question 29

What are the uses of cysteine proteases?

Answer 29

Meat tenderiser | - digests collagen and extracellular matrix

Question 30

What is specific proteolysis?

Answer 30

When proteolytic enzyme recognises a specific base sequence to activate a protein - a protein needs cleaving to be activated from a proprotein

Question 31

What is the function of HIV-1 protease?

Answer 31

HIV-1 Protease is essential to the life cycle of HIV

Question 32

What is non specific proteolysis

Answer 32

The degradation of proteins

Question 33

What is a significant aspartyl protease?

Answer 33

HIV- 1 protease

Question 34

How does HIV-1 cause infection?

Answer 34

1. HIV protein generates 2 proteins GAG and POL 2. These are processed by proteolysis 3. HIV-1 proteolysis cleaves proproteins GAG and POL -> activated 4. Able to infect cells

Question 35

Explain how digestive enzymes undergo specific proteolysis

Answer 35

1. Chymotrypsinogen (precursor of chymotrypsin) activated by trypsin enzyme 2. Trypsin cleaves the precursor at Arg15+16 to activate it 3. Chymotrypsin undergoes autolysis - cleaves itself at Ser14 - Arg15 and Thr147-Asn148 4. Produces 3 chains connected by disulphide bonds

Question 36

Give an example of clotting factors

Answer 36

Serine proteases

Question 37

Where are digestive enzymes produced?

Answer 37

Generated by exocrine pancreas | Secreted to small intestine

Question 38

How are clotting factors activated via specific proteolysis?

Answer 38

In a cascade pathway one activated (cleaved enzyme) activates the next e.g. E9 activated by the activation of E12