L10- Enzymes as Catalysts

Question 1

What enzyme catalyzes the hydrolysis of peptides on the C-terminal side of phenylalanine, tyrosine and tryptophan?

Answer 1

Chymotrypsin.

Question 2

What amino acids make up the catalytic triad in a serine protease?

Answer 2

Aspartate, histidine and serine.

Question 3

Do enzymes change their structure after catalysis?

Answer 3

No. At the end of the reaction cycle they are the same as when they started.

Question 4

When the Gibbs free energy (G) of a reaction is negative, that reaction is ______.

Answer 4

Spontaneous.

Question 5

When the Gibbs free energy (G) of a reaction is positive, that reaction is ______.

Answer 5

Non-spontaneous.

Question 6

When the Gibbs free energy (G) of a reaction is zero, that reaction is ______.

Answer 6

At equilibrium.

Question 7

True or False. All exothermic reactions are spontaneous.

Answer 7

False. Delta G can be negative (indicating a spontaneous reaction) even when delta H is positive. This occurs when delta S (the change in entropy) is positive.

Question 8

True or False. Reactions in the body are in equilibrium.

Answer 8

False.

Question 9

How do enzymes affect the energy of reactions?

Answer 9

Enzymes lower the activation energy. They accelerate the rate of reactions, but do not change the potential energy of the substrates or products.

Question 10

How does the activation energy affect the speed of a reaction?

Answer 10

In general, the larger the activation energy the slower the reaction.

Question 11

Define catabolism.

Answer 11

Burning fuel to make ATP.

Question 12

Define anabolism.

Answer 12

Burning energy (usually ATP) for biosynthetic processes, active transport and/or mechanical work.

Question 13

How can one determine reaction velocity?

Answer 13

By measuring the amount of product formed per unit time.

Question 14

What kinetic parameters can be extracted from a Michaelis-Menten kinetics graph?

Answer 14

Vmax, the maximum velocity of the catalyzed reaction, and Km, the substrate concentration required to achieve half the maximum velocity.

Question 15

When is Km equal to the substrate concentration in a Michaelis-Menten Kinetics graph?

Answer 15

When the substrate concentration is at half of Vmax.

Question 16

Delta G for ATP hydrolysis (ATP + H2O -> ADP + Pi) is -7.3 kcal/mole. How much energy is required to perform the reverse reaction?

Answer 16

+7.3 kcal/mole.

Question 17

What is a cofactor?

Answer 17

Cofactors are typically small molecules that help promote catalysis. Many vitamins act as cofactors.

Question 18

Which enzyme is deficient in individuals with methylmalonic acidemia?

Answer 18

Methylmalonyl mutase.

Question 19

What is the cofactor for methylmalonyl mutase?

Answer 19

Vitamin B12.

Question 20

What are the substrates and products of methylmalonyl mutase?

Answer 20

The enzyme converts methylmalonyl CoA to succinyl CoA.

Question 21

True or False. Generally, the atoms in an enzyme that bind to a substrate are different than the atoms that participate in the catalysis of a reaction.

Answer 21

True.

Question 22

What are the four steps, in order, of a reaction catalyzed by serine proteases?

Answer 22

1. Binding of substrate, 2. attack by serine, 3. stabilization of transition state, and 4. release of products.

Question 23

What is an oxyanion hole and what is its function?

Answer 23

It is a structural pocket in an enzyme that stabilizes an oxyanion (i.e. stabilizes the transition state).

Question 24

The initial enzyme velocity proportional to what?

Answer 24

The amount of substrate.

Question 25

The y-intercept in Lineweaver-Burk plots corresponds to what kinetic parameter?

Answer 25

NAME?

Question 26

The x-intercept in Lineweaver-Burk plots corresponds to what kinetic parameter?

Answer 26

NAME?

Question 27

The slope in Lineweaver-Burk plots corresponds to what kinetic parameter?

Answer 27

Km/Vmax.