Enzymes

Question 1

What is an enzyme?

Answer 1

A globular protein which acts as a biological catalyst, controlling the rate of biological reactions.

Question 2

What is a cofactor?

Answer 2

Non protein component required for activity.

Question 3

What is a prosthetic group?

Answer 3

A non-protein component that is covalently bonded to or very tightly associated with the enzyme.

Question 4

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

Answer 4

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

Question 5

What is an apoenzyme?

Answer 5

The protein constituent of an enzyme to which the prosthetic group attaches.

Question 6

What is meant by the term holoenzyme?

Answer 6

This is the whole enzyme (Apoenzyme + prosthetic group)

Question 7

What is the active site of an enzyme?

Answer 7

The area on the enzyme to which the substrate attaches to and is acted upon.

Question 8

What are the six classes of enzymes?

Answer 8

1. Oxioreductases
2. Transferases
3. Lyases
4. Hydrolases
5. Ligases
6. Isomerases

Question 9

What type of reactions do oxioreducatases catalyse?

Answer 9

Transfer of electrons

Question 10

What type of reactions do transferases catalyse?

Answer 10

Group transfers

Question 11

What type of reactions do hydrolases catalyse?

Answer 11

Hydrolysis

Question 12

What type of reactions do Lyases catalyse?

Answer 12

Formation of /Addition to double bonds

Question 13

What type of reactions do Ligases catalyse?

Answer 13

Formation of C-C, C-S, C-N or C-O.

Question 14

What type of reactions do isomerases catalyse?

Answer 14

Transfers within molecules

Question 15

What three theories are used to explain enzymes reducing activation energy?

Answer 15

1. Induced fit
2. Desolvation
3. Entropy reduction

Question 16

What is meant by the induced fir theory of enzyme action?

Answer 16

Substrate binds to active site of enzyme
Enzyme undergoes conformational change so that active site is now specific in shape to substrate.
Reaction proceeds

Question 17

What is meant by the entropy reduction theory of enzyme action?

Answer 17

Enzyme binds substrate molecules in correct orientation and close enough together to allow the breaking/ making of bonds required for the reaction.
Remember, if the enzyme binds these substrate molecules, they are no longer free in solution and so there has been a reduction in entropy.

Question 18

What is meant by the desolvation theory of enzyme activity?

Answer 18

Weak bonds between the enzyme and the substrate replace Hydrogen bonds between the substrate and aqueous environment.

Question 19

What is meant by Vmax?

Answer 19

This is the maximum rate of reaction which can occur for a particular enzyme concentration.
It occurs when all of the reaction sites are saturated with substrate.
Increase in substrate conc. after this point will have no effect.

Question 20

What is Km?

Answer 20

The ratio of the rate constant of the conversion of ES to E + S to the rate constant for the conversion of E+S to ES.

Question 21

When does Km occur?

Answer 21

When the initial rate of reaction is exactly half of that of the maximum rate of reaction. Here it gives a value for the substrate conc.

Question 22

What does a large Km indicate?

Answer 22

Less stable ES complex, low affinity of enzyme to bind to substrate

Question 23

What are isozymes?

Answer 23

Different proteins which catalyse the same reaction

E.g. Hexokinase and Glucokinase

Question 24

What s the Michaelis- Menten equation?

Answer 24

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

Question 25

What is an allosteric enzyme?

Answer 25

An enzyme with many different subunits, each containing an active site. Substrate binds by cooperative binding.
E.g. Haemoglobin

Question 26

What is meant by cooperative binding?

Answer 26

Binding of a substrate to one sub-unit causes a conformational change which increases the ease of which a substrate binds to the next sub-unit and so in.

Question 27

What shape is the kinetics curve on graph of reaction velocity against substrate concentration for an allosteric enzyme?

Answer 27

Sigmoidal

Question 28

What are the models of action on which allosteric enzyme activity is based?

Answer 28

1. Concerted model

2. Sequential model

Question 29

Describe the concerted model of allosteric enzyme activity.

Answer 29

Each sub unit can exist in one of two different conformations
One conformation binds substrate molecules well, the other does not.
The subunits continuously flip IN CONCERT between the two different conformations
When a substrate molecule binds to one active site, it locks the other subunits in the binding conformation, making it easier for further subunits to bind.
Allosteric activators will stabilise the binding conformation. Allosteric inhibitors will stabilise the non-binding sub-unit confromation

Question 30

Describe the sequential model of allosteric enzyme activity.

Answer 30

No flipping between conformations
Sub-units exist in a conformation which may bind activators, inhibitors and substrate molecules.
Substrate binding causes a change in one subunit, which causes a change in the next subunit, making it more likely to bind substrate.
This process continues.

Question 31

Describe the action of competitive enzyme inhibitors.

Answer 31

These molecules resemble the substrate and bind to the active site (i.e. they compete with the substrate for the active site)

Question 32

How do competitive inhibitors affect Vmax and Km?

Answer 32

Competitive inhibitors reduce the affinity of the enzyme for the substrate and so increase Km.
Increasing [S] can overcome the problem so Vmax is unchanged.

Question 33

Describe the action of non-competitive enzyme inhibitors.

Answer 33

Non- competitive inhibitors bind non-covalently, away from the active site. They make the enzyme-substrate complex which forms inactive.

Question 34

How do non-competitive inhibitors affect Vmax and Km?

Answer 34

ES complexes can still form and so Km is unchanged.

Increasing [S] will have no effect and so Vmax is decreased.

Question 35

What are proproteins/ proenzymes?

Answer 35

These are inactive precursors to enzymes.
Cleavage by proteases generates the active form of the enzyme.
This is how digestive enzymes operate.

Question 36

Name three ways in which enzymes can be modulated.

Answer 36

1. Allosteric modulation
2. Covalent modulation
3. Proteolytic cleavage