Enzymes

Question 1

What is the difference between a cofactor and a prosthetic group?

Answer 1

A prosthetic group IS a cofactor (non protein component required for activity) that is covalently bonded/ tightly associated with the enzyme.
Cofactor may not need to be attached to enzyme to aid with enzyme activity.

Question 2

How do enzymes work?

Answer 2

Lower activation energy
Increase the rate of a spontaneous reaction
Accelerate movement toward reaction equilibrium

Question 3

What are the three theories of enzyme action?

Answer 3

Desolvation
Induced fit
Entropy reduction

Question 4

How does desolvation work?

Answer 4

Weak bonds between the enzyme and substrate replace all H bonds between substrate and aq solution

Question 5

How does induced fit work?

Answer 5

Conformational change occurs in the protein as the substrate binds, making it enzymatically active.

Question 6

How does entropy reduction work?

Answer 6

Enzyme forces the substrates to bind in the correct orientation and in close enough contact for reaction to occur.

Question 7

When does Vmax occur?

Answer 7

When the highest initial rate of enzyme controlled reaction is occurring. It means that all the enzyme active sites are occupied.

Question 8

What is the Michaelis- Menten equation?

Answer 8

Vo= (Vmax)([S])/ (Km + [S])

Question 9

What is Km?

Answer 9

Km is the substrate concentration at which the initial reaction rate is half of the maximum reaction rate.

Question 10

What would a high Km indicate?

Answer 10

A less stable enzyme- substrate complex

Question 11

What is the similarity(s) between glucokinase and hexokinase?

Answer 11

They both catalyse the reaction:

Glucose + ATP-> Glucose-6- Phospate + ADP

Question 12

What is the difference between glucokinase and hexokinase?

Answer 12

Glucokinase operates in the liver, whereas Hexokinase operates in the rest of the body tissue.
Glucokinase has a high Vmax and high Km, whereas Hexokinase has a low Vmax and low Km.
Hexokinase can operate at relatively high rates even when [blood glucose] is low. Glucokinase will operate at very high rates when [blood glucose] is high.

Question 13

What are Isoenzymes?

Answer 13

Different enzymes which catalyse the same reaction

Question 14

How do competitive inhibitors affect Vmax and Km?

Answer 14

They increase Km and have no effect on Vmax.

Question 15

How do non-competitive inhibitors affect Vmax and Km?

Answer 15

They decrease Vmax but will not alter Km

Question 16

What is meant by the term allosteric enzyme?

Answer 16

An enzyme which changes its conformational ensemble upon binding to an effect, and so has an altered binding affinity at a different ligand binding site

Question 17

What are the two theories of allosteric enzyme activity?

Answer 17

Concerted and Sequential

Question 18

Describe concerted enzyme activity.

Answer 18

Each binding site of an enzyme exists in one of two conformations. One allows substrate to bind, one doesn’t.
With no substrate to bind, the sites flip between the two conformations in concert.
Once one substrate binds, it locks the other sites into place, increasing their affinity for further substrate to bind.
Allosteric activators will stabilise the OPEN conformation, whereas allosteric inhibitors will stabilise the CLOSED conformation.

Question 19

Describe sequential enzyme activity.

Answer 19

Binding sites exist in a state that may be able to bind substrate.
Binding the substrate causes a conformational change in one binding site.
This causes a conformational change in another binding site, allowing it to increase its affinity to bind substrate and so on.

Question 20

What is the similarity between Concerted and Sequential binding models of enzymes?

Answer 20

In both models, binding of one substrate sensitises the enzyme to bind to more substrates.

Question 21

What is the main difference between Concerted and Sequential binding models of enzymes?

Answer 21

In the sequential model, there is no flipping between different conformational states.

Question 22

Name three ways in which enzymes can be modulated?

Answer 22

Allosteric modulation
Covalent modification
Proteolytic Cleavage