Regulation of Protein Function

Question 1

What are some short term regulations of enzyme activity?

Answer 1

Substrate and product concentration.
Change enzyme conformation such as:
* Allosteric regulation
* Covalent modification
* Proteolytic cleavage

Question 2

What are some long term regulations of enzyme activity?

Answer 2

Change in rate of protein synthesis

Change in rate of protein degradation

Question 3

Where can you find hexokinase?

Answer 3

In muscles

Question 4

Where can you find glucokinase?

Answer 4

In liver

Question 5

What are isoenzymes?

Answer 5

Different forms of the same enzyme that have different kinetic properties.

Question 6

What is product inhibition? Give an example.

Answer 6

Accumulation of the product of a reaction inhibits the forward reaction. Glucose-6-phosphate inhibits hexokinase activity.

Question 7

What is allosteric regulation?

Answer 7

When an allosteric activator or allosteric inhibitor (small ligands) bind to a different site than the active site on an enzyme to either inhibit or promote activity.

Question 8

How does allosteric regulation manifest in a graph, and why?

Answer 8

It shows as a sigmoid curve (s-shaped) because it uses cooperative binding to promote a R state or a T state. Multi subunit enzymes are also used in allosteric regulation.

Question 9

How do allosteric activators work?

Answer 9

Bind onto the enzyme (not the active site) and increase the proportion of enzyme in the R state (high affinity). This is due to conformational change.

Question 10

How do allosteric inhibitors work?

Answer 10

Bind onto the enzyme (not the active site) and increase the proportion of enzyme in the T state (low affinity). This is due to conformational change.

Question 11

What is phosphofructokinase? What are the activators and inhibitors of it?

Answer 11

Thought to be the key regulator in glycolysis. It is allosterically regulated.
Activators:
*AMP
*Fructose-2,6-biphosphate
Inhibitors:
*ATP
*Citrate
*H+

Question 12

Give an example of covalent modification.

Answer 12

Phosporylation in the case of protein kinases. Trans the terminal phosphate from ATP to the -OH group of Ser, The, and Tyr.

Question 13

What do protein phosphatases do?

Answer 13

Reverse the effects of kinases by catalysing the hydrolytic removal of phosphoryl groups from proteins.

Question 14

Why is protein phosphorylation so effective?

Answer 14

It adds 2 negative charges.
A phosphoryl group can make hydrogen bonds
Rate of phosphorylation/dephosphorylation can be adjusted
Links energy status of the cell to metabolism through ATP
Allow for amplification effects
Conformational change due to its bulkiness (size)

Question 15

What are zymogens? Give an example and explain why they occur.

Answer 15

Inactive precursors of digestive enzymes. They are found in the stomach but mainly in pancreas. They are inactive and needs to undergo proteolytic cleavage in order to be activated. This is because you don’t want them to break down tissue in pancreas.

Question 16

Give other examples of proteolytic cleavage and why it is important.

Answer 16

Protein hormones such as insulin are synthesis as inactive precursors.
Blood clotting is mediated by a cascade of proteolytic activations that ensures a rapid and amplified response.
Tissue remodelling
Apoptosis
Many of these are to ensure a substance reaches its target without affecting anything on the way.

Question 17

What is chymotrypsinogen?

Answer 17

A zymogen

Question 18

Explain the process from chymotrypsinogen to alpha-chymotrypsin.

Answer 18

Trypsin activates chymotrypsinogen to form pi-chymotrypsin. Chymotrypsin then releases two dipeptides in order to become alpha-chymotrypsin.

Question 19

What is alpha1-antitrypsin?

Answer 19

A plasma protein that inhibits a range of proteases.

Question 20

Why is alpha1-antitrypsin clinically relevant?

Answer 20

Elastase is a protease which needs inhibition in alveoli. If there is a deficiency of alpha1-antitrypsin there is not enough of it to inhibit the action of elastase. So elastase destroy the elastin in the alveolar walls and emphysema occurs.

Question 21

Why is the amplication factor in blood clotting important?

Answer 21

This allows a very small amount of the initial factor to be needed in order to give a large response (cascade).

Question 22

What is the modular structure of prothrombin?

Answer 22

A the N’ terminal there is a Gla domain which target the prothrombin to appropriate sites for its activation.
There are two Kringle domains which help keep prothrombin inactive.
There is a protease function (serine protease) which is the thrombin part of the prothrombin which is contained in the C’ terminal domain.

Question 23

What is prothrombin?

Answer 23

A precursor to thrombin which itself is a serine protease which converts soluble fibrinogen to fibrin for coagulation.

Question 24

How does prothrombin turn into thrombin?

Answer 24

By proteolytic cleavage of Factor Xa and a cofactor Va.

Question 25

What are g-carboxylglutamate (Gla residues) and what are their role?

Answer 25

A protein with cooh groups that allows interaction with sites of damage and brings together clotting factors with calcium bridges.

Question 26

What is an important cofactor in the addition of cooh groups from the precursors of Gla residues in order to form gla residues?

Answer 26

Vitamin K

Question 27

What will a deficiency of vitamin K cause?

Answer 27

A reduced ability in clotting.

Question 28

What is the structure of fibrinogen?

Answer 28

2 sets of tripeptides, alpha, beta and gamma joined at N’terminal by disulphide bonds.
3 globular domains linked rods
They look like a crab.

Question 29

Why is the N’ terminal alpha and beta chains important in fibrinogen?

Answer 29

Because they are highly negatively charged they prevent aggregation of fibrinogen. So it doesn’t clot.

Question 30

What is used to turn fibrinogen into fibrin? Outline the process

Answer 30

Thrombin cleaves the alpha and beta chains at the N’ terminals by proteolytic cleavage. This allows aggregation of fibrin to form a clot.

Question 31

Outline the key control points in blood clotting.

Answer 31

1. Inactive zymogens present at low concentration
2. Proteolytic cleavage for activation
3. Amplification of initial signal by cascade
4. Clustering of clotting factors at site of damage
5. Feedback activation by thrombin ensures continuation of clotting
6. Termination of clotting by multiple mechanisms
7. Clot breakdown controlled by proteolytic activation