Session 3 -- Protein Regulation

Question 1

What are the short term methods of maintaining enzyme activity?

Answer 1

1. Concentrations of product and substrate

2. Changes in enzyme conformation

Question 2

Give examples of regulation by changes in enzyme conformation

Answer 2

1. Allosteric regulation
2. Covalent Modification
3. Proteolytic Cleavage

Question 3

Give the two main long term methods of maintaining enzyme activity

Answer 3

Rate of protein synthesis

Rate of protein degradation

Question 4

Describe allosteric regulation

Answer 4

Enzymes have multiple active sites
Binding of one substrate enhances substrate binding to other sites
Activators increase activity by converting T to R state
Inhibitors decrease activity by converting R to T state

Question 5

What type of curves to allosteric enzymes show?

Answer 5

Sigmoidal curve

Question 6

Give an example of an allosteric enzyme

Answer 6

Phosphofructokinase – sets the pace of glycolysis
Activated by AMP and fructose - 2,6 -bisphosphate
Inhibited by ATP, citrate and H+

Question 7

Describe covalent modification

Answer 7

The addition of a group to proteins via amino acids

Question 8

Give the main example of covalent modification

Answer 8

Phosphorylation – the addition of a phosphate group to -OH groups, converting ATP to ADP

Question 9

What are the enzymes which catalysed the attachment and removal of phosphate groups?

Answer 9

Kinases catalyse attachment of phosphate groups

Phosphatases catalyse the removal of phosphate groups

Question 10

Describe proteolytic activation

Answer 10

Zymogens are activated by the removal of part of the polypeptide chain in order to transport enzymes to their site of action without causing damage on the way (proteases)

Question 11

How is the rate of protein synthesis controlled?

Answer 11

By changing the rate of mRNA transcription

Question 12

How is the rate of protein degradation controlled?

Answer 12

Proteins can be tagged for destruction by the addition of ubiquitin, a small protein molecule

Question 13

What are the main ways in which metabolic pathways are controlled?

Answer 13

Feedback inhibition
Feedforward activation
Counter regulation of pathways

Question 14

What is the intrinsic pathway for blood clotting?

Answer 14

Damaged endothelial lining of blood cells promotes the binding of factors 9 and 11 which are targeted to membrane by Gla domains. This pathway maintains the thrombin activation.

Question 15

What is the extrinsic pathway for blood clotting?

Answer 15

Vascular damage releases factor 3 which leads to the activation of factor 7, activating factor 10 …

Question 16

Describe the structure of prothrombin

Answer 16

Thrombin part is contained in C-terminal domain
Two kringle domains maintain the inactivated form
Gla domains allow for the targeting of the enzyme to its site of action

Question 17

What are Gla residues and why are they important?

Answer 17

Are additions of COOH groups to glutamate residues forming carboxyglutamate (Gla). Occurs by post-translational modification in the liver.
Allows for the interaction with sites of damage, bringing together clotting factors

Question 18

What is fibrinogen and describe its structure

Answer 18

Inactive zymogen, has highly negative charges on ends to prevent aggregation of fibrinogen. Made up of 2 sets of tripeptides joined by disulphide bonds

Question 19

How is fibrin converted to fibrinogen?

Answer 19

Thrombin cleaves A and B fibrinopeptides, allowing globular domains to interact with exposed sequences forming a fibrin mesh. The mesh is stabilised by amide bonds formed between different monomers, catalysed by transglutaminase (activated by thrombin by protransglutaminase)

Question 20

What occurs in haemophilia?

Answer 20

A defect occurs in factor 8 which stimulates the activity of factor 9 by positive feedback and amplifying the clotting signal. Treatment is with a recombinant factor 8.

Question 21

How is the clotting process terminated?

Answer 21

1. Clotting factor are diluted by blood flow and removal by the liver
2. Digestion of clot by proteases (t-PA and streptokinase)
3. Binding by inhibitor molecules (antithrombin 3)

Question 22

What are the main regulation methods for the blood clotting cascade?

Answer 22

1. Inactive zymogens are present at low concentrations
2. The initial signal is amplified
3. Gla domains limit activation to site of damage
4. Thrombin activates multiple factors
5. Termination by numerous processes