MGD S3 - Enzyme Activity: Kinetics and Inhibiton

Question 1

What is an enzyme?

Answer 1

A biological catalyst that increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, facilitate formation of the transition state

Question 2

Define Km

Answer 2

Concentration of substrate at half Vmax

Question 3

What effect does increasing affinity for substrate have on Km?

Answer 3

Increase affinity results in decreased Km

Question 4

Define Vmax

Answer 4

Maximum rate of reaction when all the enzyme’s active sites are saturated

Question 5

What is a non-competitive enzyme inhibitor?

Answer 5

• Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity of the enzyme and binds equally well to the enzyme whether or not it has already bound the substrate
• Vmax lowers whilst Km remains the same

Question 6

What is the Michaelis-Menten equation?

Answer 6

Vo= Vmax x [S] / Km + [S]

Question 7

Define the international unit of enzyme activity

Answer 7

The enzyme unit is a unit for the amount of a particular enzyme. One unit is defined as the amount of enzyme that produces a conversion of 1 micro mole of substrate a minute

Question 8

What would be the axis on a Lineweaver-Burk plot

Answer 8

Y axis- 1/V X axis- 1/S

Question 9

How would Km and Vmax be found from a Linewaver-Burk plot

Answer 9

1/Km- where the graph cuts the X axis

1/Vmax- where the graph cuts the Y axis

Question 10

What would be the effect of a competitive inhibitor on enzyme kinetics?

Answer 10

Increase Km - lower affinity for substrate Max stays the same

Question 11

List 5 mechanisms of enzyme control

Answer 11

1) Allosteric
2) Covalent modification Eg phosphorylation
3) Substrate/product concentration
4) Proteolytic cleavage
5) Long term - Change in rate of protein synthesis/degradation

Question 12

Describe the effect of an allosteric inhibitor

Answer 12

• Binds to the allosteric site (other than the active site) that brings about a conformational change that decreases its affinity for a substrate
• This CAN NOT be overcome by increasing substrate concentration

Question 13

What type of enzymes undergo allosteric regulation?

Answer 13

Multi-subunit enzymes

Question 14

What is the function of kinase enzymes?

Answer 14

Phosphorylates molecules by transfer of the terminal gamma-phosphate from ATP to the OH group of Ser, The, Tyr Eg hexokinase in step 1 of glycolysis

Question 15

Define the term zymogen

Answer 15

Inactive precursor of an enzyme which is activated by proteolytic cleavage

Question 16

What are the intrinsic and extrinsic pathways of blood clotting

Answer 16

Intrinsic - activated by membrane damage, trauma inside the vascular system

Extrinsic - Activated by external trauma to tissue, outside the vascular system

Question 17

List the factors involved in the intrinsic pathway of the blood clotting cascade

Answer 17

XI, IX, X (VII, V)

Question 18

List the factors involved in the extrinsic pathway of the blood clotting cascade

Answer 18

III, VII, X

Question 19

How is positive feedback involved in the blood clotting cascade?

Answer 19

Prothrombin is cleaved to thrombin which feeds back to factors XI, VIII and V which continues activation of the intrinsic pathway

Question 20

How are gla domains formed?

Answer 20

Glutamate residues on factors II, VII IX and X are carboxylated in the liver to form gla domains

Question 21

What are the functions of gla domains?

Answer 21

• Allow interaction with sites of damage as prothrombin binds calcium via Gla residues
• Brings together clotting factors as only prothrombin next to damaged site will be activated

Question 22

How is a fibrin clot stabilised?

Answer 22

Peptide bonds form between lysine and glutamine residues catalysed by transglutaminase

Question 23

Describe the process of fibrinolysis?

Answer 23

Breakdown of the fibrin clot by conversion from plasminogen to plasmin (fibrin fragment) by streptokinase and t-PA when proteolytically activated

Question 24

How is the clotting cascade process stopped?

Answer 24

• Localisation of prothrombin by diluting factors by blood flow to liver
• Digestion by proteases Eg protein C
• Specific inhibitors Eg Antithrombin III

Question 25

Name three methods of regulation if the blood clotting cascade

Answer 25

Any from:

• Inactive zymogens at low concentrations
• Amplification of signal by cascade mechanism
• Feedback activation by thrombin
• Multiple termination mechanisms
• Clustering factors at site of damage
• Clot breakdown controlled by proteolytic activation

Question 26

What is plasmin?

Answer 26

A serine protease that acts to dissolve fibrinblood clots