BIO C HE MIA I – ĆW ICZ E NIA

L I S T A 5

do wykładu dr. hab. inż. P. Dobryszyckiego

E n z y m y : p o d s t a w o w e p o j ę c i a i k i n e t y k a

1. The combination of an apoenzyme with a cofactor forms what? What are the two types of

cofactors? What distinguishes a prosthetic group from a cosubstrate?

2. Which of the following statements is correct? The free energy change of a reaction

(a) if negative, enables the reaction to occur spontaneously.

(b) if positive, enables the reaction to occur spontaneously.

(c) is greater than zero when the reaction is at equilibrium.

(d) determines the rate at which a reaction will attain equilibrium.

3. If the standard free-energy change (∆ G°) for a reaction is zero, which of the following statements about the reaction are true?

(a) The entropy (∆ S°) of the reaction is zero.

(b) The enthalpy (∆ H°) of the reaction is zero.

(c) The equilibrium constant for the reaction is 1.0.

(d) The reaction is at equilibrium.

(e) The concentrations of the reactants and products are all 1 M at equilibrium.

4. The enzyme triose phosphate isomerase catalyzes the following reaction:

k 1

Dihydroxyacetone phosphate glyceraldehyde 3-phosphate

k -1

The ∆ G°' for this reaction is 1.83 kcal/mol. In light of this information, which of the following statements are correct?

(a) The reaction would proceed spontaneously from left to right under standard conditions.

(b) The rate of the reaction in the reverse direction is higher than the rate in the forward direction at equilibrium.

(c) The equilibrium constant under standard conditions favors the synthesis of the compound on the left,

dihydroxyacetone phosphate.

(d) The data given are sufficient to calculate the equilibrium constant of the reaction.

(e) The data given are sufficient to calculate the left-to-right rate constant ( k 1).

5. Glycogen phosphorylase, an enzyme involved in the metabolism of the carbohydrate polymer

glycogen, catalyzes the reaction:

Glycogen

n + phosphate glucose 1-phosphate + glycogenn-1

K'eq= 0,088

Based on these data, which of the following statements are correct?

(a) Because glycogen phosphorylase normally degrades glycogen in cellular metabolism, there is a paradox in that the equilibrium constant favors synthesis.

(b) The ∆ G°' for this reaction at 25°C is 1.43 kcal/mol.

(c) The phosphorolytic cleavage of glycogen consumes energy, that is, it is endergonic.

(d) If the ratio of phosphate to glucose 1-phosphate in cells is high enough, phosphorylase will degrade

glycogen.

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6. The transition state of an enzyme-catalyzed reaction that converts a substrate to a product

(a) is a transient intermediate formed along the reaction coordinate of the reaction.

(b) has higher free energy than either the substrates or products.

(c) is the most populated species along the reaction coordinate.

(d) is increased in concentration because the enzyme binds tightly to it.

(e) determines the velocity of the reaction.

7. Which of the following statements is true? Enzyme catalysis of a chemical reaction

(a) decreases ∆ G' so that the reaction can proceed spontaneously.

(b) increases the energy of the transition state.

(c) does not change ∆ G°', but rather changes the ratio of products to reactants at equilibrium.

(d) decreases the entropy of the reaction.

(e) increases the forward and reverse reaction rates.

8. Which of the following statements regarding an enzyme-substrate complex (ES) is true?

(a) The heat stability of an enzyme frequently changes upon the binding of a substrate.

(b) At sufficiently high concentrations of substrate, the catalytic sites of the enzyme become filled and the reaction rate reaches a maximum.

(c) An enzyme-substrate complex can usually be isolated.

(d) Enzyme-substrate complexes can usually be visualized by x-ray crystallography.

(e) Spectroscopic changes in the substrate or the enzyme can be used to detect the formation of an enzyme-substrate complex.

9. Which of the following statements regarding simple Michaelis-Menten enzyme kinetics are

correct?

(a) The maximal velocity V max is related to the maximal number of substrate molecules that can be

"turned over" in unit time by a molecule of enzyme.

(b) K M is expressed in terms of a reaction velocity (e.g., mol s-1).

(c) K M is the dissociation constant of the enzyme-substrate complex.

(d) K M is the concentration of substrate required to achieve half of V max.

(e) K M is the concentration of substrate required to convert half the total enzyme into the enzyme-substrate complex.

10. Explain the relationship between KM and the dissociation constant of the enzyme-substrate

complex KES.

11. From the plot of velocity versus substrate

concentration shown in Figure 8.1, obtain the

following parameters. (The amount of enzyme

in the reaction mixture is 10-3 µmol.)

(a) K M

(b) Vmax

(c) k2 / KM

(d) Turnover number

12. What is the significance of kcat/KM?

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13. Which of the following statements is correct? The turnover number for chymotrypsin is

100 s-1, and for DNA polymerase it is 15 s-1. This means that

(a) chymotrypsin binds its substrate with higher affinity than does DNA polymerase.

(b) the velocity of the chymotrypsin reaction is always greater than that of the DNA polymerase reaction.

(c) the velocity of the chymotrypsin reaction at a particular enzyme concentration and saturating

substrate levels is lower than that of the DNA polymerase reaction under the same concentration

conditions.

(d) the velocities of the reactions catalyzed by both enzymes at saturating substrate levels could be made

equal if 6.7 times more DNA polymerase than chymotrypsin were used.

14. The enzyme hexokinase catalyzes the following reaction:

Glucose + ATP glucose 6-phosphate + ADP

For this reaction, ∆G°' = -4.0 kcal/mol.

(a) Calculate the change in free energy ∆G' for this reaction under typical intracellular conditions using

the following concentrations:

glucose, 55 mM;

ATP, 5.0 mM;

ADP, 1.0 mM;

and

glucose 6-phosphate, 0.1 mM. Assume that the temperature is 25°C.

(b) In the typical cell, is the reaction catalyzed by hexokinase close to equilibrium or far from equilibrium? Explain.

15. Which of the following statements about the different types of enzyme inhibition are correct?

(a) Competitive inhibition occurs when a substrate competes with an enzyme for binding to an inhibitor

protein.

(b) Competitive inhibition occurs when the substrate and the inhibitor compete for the same active site

on the enzyme.

(c) Noncompetitive inhibition of an enzyme cannot be overcome by adding large amounts of substrate.

(d) Competitive inhibitors are often similar in chemical structure to the substrates of the inhibited enzyme.

(e) Noncompetitive inhibitors often bind to the enzyme irreversibly.

16. The kinetic data for an enzymatic reaction in

the presence and absence of inhibitors are

plotted in Figure 8.2. Identify the curve that

corresponds to each of the following:

(a) no inhibitor

(b) noncompetitive inhibitor

(c) competitive inhibitor

(d) mixed inhibitor

17. Draw approximate Lineweaver-Burk plots for each of the inhibitor types in question 16.

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