Enzymes

Question 1

What are enzymes?

Answer 1

Catalysts that speed up chemical reactions

Question 2

Do enzymes alter the equilibrium of a reaction?

Answer 2

No, but they do accelerate the time it takes to reach equilibrium

Question 3

What are a few roles of enzymes in living organisms?

Answer 3

Aid:

• respiration
• digestion
• Muscles and nerve function

Question 4

What is the role of Alkaline phosphatase?

Answer 4

Found throughout the body, involved in mineralisation of tissue and bone

Question 5

What is the role of Amylase?

Answer 5

Found in saliva, converts starch into sugars

Question 6

What is the role of Maltase?

Answer 6

Found in saliva, breaks maltose into glucose

Question 7

What is the role of Lysozyme?

Answer 7

Antimicrobial, breaks down peptidoglycan layer in bacterial cell wall’s

Question 8

What is the enzymes ‘lock and key’ model?

Answer 8

Enzymes active site complimentary in shape to that of a specific substrate

Question 9

What is the enzymes ‘indices-fit’ model?

Answer 9

• When the active site and substrate do not fit together exactly but the enzyme changes shape when the substrate binds to become a complimentary shape to the substrate

Question 10

What does Sucrase break down into?

Answer 10

Glucose and Fructose

Question 11

What are most enzymes made of?

Answer 11

Protein

Question 12

What is enzyme activity dependent on?

Answer 12

The maintenance of their 3D structure

Question 13

What is enzyme activity affected by?

Answer 13

Temperature and pH

Question 14

What happens to enzymes at extremes of temperature or pH?

Answer 14

They will denature and become totally inactive

Question 15

What is the difference between energies of reactants and products called?

Answer 15

Standard free energy change

Question 16

What is the energy input required to initiate a reaction called?

Answer 16

The activation energy

Question 17

Is activation energy higher or lower for a catalysed reaction compared to an uncatalyzed reaction?

Answer 17

Lower

Question 18

Can enzymes alter the free energy change?

Answer 18

No

Question 19

What 6 factors affect the rate of an enzyme reaction?

Answer 19

1. Temperature
2. pH
3. Enzyme concentration
4. Substrate concentration
5. Inhibitors and activators
6. Covalent modification (the alteration of a synthesised protein facilitated by a catalyst)

Question 20

What is Thermodynamic activation?

Answer 20

Increase in free energy

Question 21

What is Thermodynamic inactivation?

Answer 21

Protein denaturation - enzyme rapidly loses activity

Question 22

What effect will an increase in the enzyme concentration have on the reaction rate?

Answer 22

Will increase the reaction rate:
- more enzymes = faster reaction
so the equilibrium will be reached more rapidly

Question 23

Once equilibrium is reached in a reaction what will happen to the concentrations of the product and substrate?

Answer 23

They will not change

Question 24

What is Vmax?

Answer 24

The maximum rate of reaction (enzyme is saturated with substrate)

Question 25

What is Km?

Answer 25

The Michaelis constant, an inverse measurement of affinity - the higher the Km value, the lower the affinity of the enzyme for its substrate The Km is the concentration of substrate at which the rate of reaction is half its maximum value

Question 26

What does a low Km indicate?

Answer 26

The enzyme is normally saturated with substrate, acts at a fairly constant rate regardless of variations in substrate concentration

Question 27

What does a high Km indicate?

Answer 27

The enzyme is not normally saturated with substrate, its activity will vary as substrate concentration varies - rate of product formation depends on substrate availability

Question 28

What is the Michaelis-Menten equation?

Answer 28

V = Vmax[S]/(Km + [S]) - Km is the substrate concentration at which V = 0.5Vmax and is a measure of the affinity of the enzyme for its substrate: the lower the Km, the higher the affinity - Vmax and hence Km can be estimated from the graph of V against [S]

Question 29

What is the Lineweaver-Burk plot?

Answer 29

A more accurate estimate of Vmax and Km by plotting 1/V and 1/[S] - gives a straight line for most enzymes

Question 30

What is an irreversible inhibitor?

Answer 30

- damages enzymes beyond repair | - Completely inactivates permanently

Question 31

What is a reversible inhibitor?

Answer 31

- full enzyme activity is regained when the inhibitor is removed - Most natural enzyme inhibitors fall into this category

Question 32

What is feedback inhibition?

Answer 32

A product of a pathway acting as an inhibitor of an earlier step in the pathway

Question 33

What is a competitive inhibitor?

Answer 33

- Where a substrate and inhibitor compete for the same active binding site - Inhibition can be overcome with large quantities of substrate

Question 34

What is a non-competitive inhibitor?

Answer 34

- Binds to an allosteric site on the enzyme i.e. one other than the active site - Inhibition cannot be reversed by adding large quantities of substrate

Question 35

Do competitive inhibitors alter the Vmax of an enzyme?

Answer 35

No, as a large quantity of substrate can outcompete the inhibitor

Question 36

Do competitive inhibitors alter the Km of an enzyme?

Answer 36

Yes, the Km is increase, as inhibitor competes with substrate for binding to active site, affinity of substrate will appear to decrease (Km is an inverse measure of affinity)

Question 37

Do non-competitive inhibitors alter the Vmax of an enzyme?

Answer 37

Yes, Vmax is decreased, a proportion of the enzyme molecules will be inactive at any time due to binding by the inhibitor

Question 38

Do non-competitive inhibitors alter the Km of an enzyme?

Answer 38

No, Km is unchanged as binding of a substrate can still occur

Question 39

What are allosteric enzymes?

Answer 39

In addition to active sites they also possess allosteric sites to which non-substrate modulators bind, creating a conformational change in subunits

Question 40

What do positive modulators do?

Answer 40

Increase the affinity for substrate at active site

Question 41

What do negative modulators do?

Answer 41

Decrease the affinity for substrate at active site

Question 42

Many enzymes can be covalently modified by other enzymes to modify their activity. What is a common modification?

Answer 42

The addition of a phosphate group - The enzyme which brings this about is phosphorylase kinase, which in turn is activated indirectly by adrenaline and glucagon - The effect is reversed by phosphorylase phosphatase, which removes the phosphate group