4. Enzyme Activity

Question 1

What three things can increase the rate of reaction? (Briefly explain how they do this).

Answer 1

Increasing temperature- increases the number of molecules with the activation energy.
Increasing concentration- increases the chance of molecular collisions.
Enzymes- biological catalysts lower the activation energy.

Question 2

What are some important features of an enzyme?

Answer 2

Highly specific, unchanged after the reaction, don’t affect the equilibrium, increase rate of reaction, mostly protein and some may need cofactors.

Question 3

How can enzymes be looked at clinically?

Answer 3

Inheritable genetic disorders may have inadequate levels of particular enzymes, overactive enzymes can lead to disease, enzyme activity can be measured for diagnosis, some drugs inhibit enzyme activity.

Question 4

What is the active site?

Answer 4

The part of an enzyme where the substrate binds and the chemical reaction occurs.

Question 5

Briefly explain the lock and key hypothesis.

Answer 5

The active site of an enzyme is complementary in shape to that of the substrate.

Question 6

Briefly explain the induced fit hypothesis.

Answer 6

The active site only forms a complementary shape after binding of the substrate.

Question 7

Why must the binding between active and substrate be in balance?

Answer 7

If the bonds are too tight then the product won’t be released but the multiple non-covalent weak bonds have to hold the substrate in place strongly enough for the reaction to take place.

Question 7

Why can the rate of reaction only be calculated at t=0?

Answer 7

Because only here is the concentration of substrate known and the rate has to be calculated when the substrate concentration is known.

Question 8

What is the normal shape of graph for rate of reaction of an enzyme on a graph?

Answer 8

Hyperbola.

Question 9

What does the Michaelis-Menten equation predict?

Answer 9

That a plot of V0 (velocity at t=0) versus [S] will be a rectangular hyperbola.

Question 10

What does the Michaelis-Menten model for enzyme catalysis propose?

Answer 10

That a specific complex between the enzyme and the substrate is a necessary intermediate in catalysis

Question 11

What is the Michaelis-Menten equation?

Answer 11

V0 = Vmax [S]
____________
Km + [S]

Question 12

What does Vmax for stand for and mean?

Answer 12

Maximum velocity, it is the maximum rate of reaction when all enzyme active sites are saturated with substrate.

Question 13

What does Km for stand for and mean?

Answer 13

Michaelis constant, the substrate concentration that gives half maximal velocity.

Question 14

What does the following say about an enzyme’s affinity for its substrate?

a. Low Km
b. High Km

Answer 14

a. High affinity

b. Low affinity.

Question 15

On a Lineweaver-Burk plot, what does the following show?

a. The y intercept
b. The x-intercept
c. A very far left x-intercept

Answer 15

a. 1/Vmax
b. -1/Km
c. Low Km so a higher affinity for the substrate.

Question 16

What is a competitive inhibitor?

Answer 16

It resembles the substrate and binds to the active site of the enzyme so the proportion of enzymes bound to the substrate decreases.

Question 17

What affect does a competitive inhibitor have on Vmax and Km? (Give a brief reason explaining why).

Answer 17

It has no effect on Vmax as adding enough substrate will overcome the effect of the competitive inhibitor. But the Km will increase as it has a lower affinity for the substrate.

Question 18

What is a non-competitive inhibitor?

Answer 18

It binds to the enzyme at an alternative site and decreases the turnover number of the enzyme.

Question 19

What affect does a competitive inhibitor have on Vmax and Km? (Give a brief reason explaining why).

Answer 19

It lowers the Vmax as the enzyme will always act more slowly so maximum rate is lowered. Km is unaffected as the affinity of the enzyme for the substrate is unaffected.