Lecture 10 - Enzymes as Catalysts

Question 1

What are the six categories of enzymes?

Answer 1

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

Question 2

Serine proteases

Answer 2

Proteolytic enzymes that catalyze peptide bond hydrolysis (they include pancreatic enzymes)

Question 3

True or false: The catalytic amino acid residues are usually the same as the substrate-binding residues

Answer 3

False

Question 4

Function of the catalytic triad

Answer 4

To make the serine particularly reactive in order to cleave the peptide bond of the substrate

Question 5

Steps of serine protease mechanism

Answer 5

1. The correct substrate is bound in the correct orientation
2. Attack by a serine
3. Transition state stabilization
4. Product release by water

Question 6

True or false: The oxyanion formed in the transition state is stabilized by the enzyme.

Answer 6

True

Question 7

What is involved in Gibb’s free energy?

Answer 7

1. change in heat

2. change in entropy

Question 8

Spontaneous reactions have what kind of change in Gibb’s free energy?

Answer 8

Negative

Question 9

When you remove product, the reaction is driven…

Answer 9

Forward

Question 10

What does stabilization of a transition state by a catalyst do?

Answer 10

It lowers the activation barrier, or the free energy between the substrate and the transition state, thus increasing the rate at which equilibrium is reached

Question 11

Do catalysts change the equilibrium ratio of [product]/[substrate] or the free energy of the substrate and product?

Answer 11

No

Question 12

What do enzymes couple?

Answer 12

ATP (or GTP or CTP) hydrolysis and bond synthesis

Question 13

What are the substrates of hexokinase?

Answer 13

ATP and glucose

Question 14

What does hexokinase do?

Answer 14

It phosphorylates glucose

Question 15

What is the velocity of an enzyme?

Answer 15

The amount of product formed per unit time

Question 16

What is the Km?

Answer 16

The substrate concentration at 1/2 vmax

Question 17

What are the axes on a graph following Michaelis-Menten kinetics in terms of measuring enzyme activity?

Answer 17

Reaction velocity v. [substrate]

Question 18

True or false: Michaelis-Menten kinetic analysis requires the use of pure enzyme.

Answer 18

False

Question 19

To measure enzyme with M-M kinetics…

Answer 19

Use saturating amounts of substrate (»Km) or measure at vmax

Question 20

To measure substrate with M-M kinetics…

Answer 20

Use low substrate levels (lower or at Km)

Question 21

True or false: Often, substrate concentration fluctuates around Km, controlling enzyme activity.

Answer 21

True

Question 22

What does Km reflect?

Answer 22

The ability of the enzyme to bind substrate

Question 23

Kcat

Answer 23

Vmax/[enzyme]

Question 24

What does Kcat measure?

Answer 24

How many substrate molecules can be used per second

Question 25

Anabolic reactions

Answer 25

Build or synthesize

Question 26

Catabolic reactions

Answer 26

Eat or break down

Question 27

Enzymes help obtain equilibrium more quickly by…

Answer 27

stabilizing the transition state

Question 28

What is vmax?

Answer 28

Maximal activity for a sample

Question 29

What can cause a deficiency of enzyme activity?

Answer 29

1. Lack of enzyme
2. Defective enzyme
3. Lack of substrate

Question 30

True or false: When the change in free energy is zero, the substrate and product are in equilibrium.

Answer 30

True