Deck 9: Enzymes and Enzyme Kinetics

Question 1

You are studying the effects of a new drug on the activity of your favorite enzyme. The results of your assays indicate that the drug acts as a noncompetitive inhibitor. Which of the following changes to the Km and Vmax allowed that determination to be made regarding the drugs actions?

A) K_m decreased, V_max increased
B) K_m decreased, V_max unchanged
C) K_m increased, V_max decreased
D) K_m unchanged, V_max decreased
E) K_m unchanged, V_max unchanged

Answer 1

D
EXPLANATION: Noncompetitive inhibitors bind to enzyme or ES complex other than at the catalytic site and thus have no effect on substrate binding. However, the resultant ESI complex cannot form products. This type of inhibition cannot be reversed by the addition of more substrate. Under these conditions K_m appears unaltered, while V_max is decreased proportionately to inhibitor concentration.

Question 2

The drug acetazolamide is used in the treatment of glaucoma since it decreases the production of aqueous fluid, thereby reducing intraocular pressures. This drug is a noncompetitive inhibitor of carbonic anhydrase. Which of the following statements most accurately relates to the actions of this type of inhibitor?

A) decreases both V_max and apparent K_m
B) decreases apparent K_m
C) decreases V_max
D) increases apparent K_m
E) increases V_max

Answer 2

C
EXPLANATION: Noncompetitive inhibitors bind to enzyme or ES complex other than at the catalytic site and thus have no effect on substrate binding. However, the resultant ESI complex cannot form products. This type of inhibition cannot be reversed by the addition of more substrate. Under these conditions K_m appears unaltered while V_max is decreased proportionately to inhibitor concentration.

Question 3

Drugs that inhibit enzymes in a competitive manner will have which of the following effects on the Michaelis-Menten equation derived parameters:V_max and/or K_m ?

A) decrease both V_max and K_m
B) decrease K_m but no effect on V_max
C) decrease V_max but no change on K_m
D) increase K_m but no change in V_max
E) increase V_max but no increase in K_m

Answer 3

D
EXPLANATION: Competitive inhibitors bind enzyme at the catalytic site, thereby competing with substrate for binding in a dynamic equilibrium-like process. This type of inhibition is reversible by the addition of higher concentrations of substrate. With competitive inhibitors the maximum velocity (V_max) of the reaction is unchanged, while the apparent affinity of the substrate to the binding site is decreased. In other words, K_m, as defined by the substrate concentration required for half maximal activity, is increased.

Question 4

Drugs that inhibit enzymes in a noncompetitive manner will have which of the following effects on the Michaelis-Menten equation derived parameters: V_max and/or K_m ?

A) decrease both V_max and K_m
B) decrease K_m but no effect on V_max
C) decrease V_max but no change on K_m
D) increase K_m but no change in V_max
E) increase V_max but no increase in K_m

Question 5

Drugs that inhibit enzymes in an uncompetitive manner will have which of the following effects on the Michaelis-Menten equation-derived parameters: V_max and/or K_m ?

A) decrease both V_max and K_m
B) decrease K_m but no effect on V_max
C) decrease V_max but no change on K_m
D) increase K_m but no change in V_max
E) increase V_max but no increase in K_m

Question 6

Which of the following best explains why noncompetitive inhibitors of enzymes generally make better pharmaceuticals?

A) their inhibitory activity is unaffected by substrate concentration
B) they affect only K_m and not V_max
C) they bind to the same site as the substrate
D) they increase the rate of degradation of the target enzyme
E) they irreversibly inhibit the enzyme

Question 7

The standard free-energy change (DG0′) for the hydrolysis of phosphoenolpyruvate (PEP) is −14.8 kcal/mol.' The DG0' for the hydrolysis of
ATP is −7.3 kcal/mol. What is the ΔG^0' for the following reaction? Phosphoenolpyruvate + ADP → pyruvate + ATP

A) −22.1 kcal/mol
B) −14.8 kcal/mol
C) −7.5 kcal/mol
D) −7.3 kcal/mol
E) +7.3 kcal/mol

Question 8

Which of the following indicates whether the reaction, A → B, is thermodynamically favorable under standard state conditions at near physiological pH?

A) ΔG^'
B) ΔG^0'
C) K_eq
D) K_m
E) V_max

Question 9

Which of the following relates to allosteric enzymes?

A) they are inactive on their substrates in the absence of accessory proteins
B) they can bind more than one substrate simultaneously
C) they can convert multiple different substrates to product
D) they possess binding sites for regulatory molecules in addition to substrate-binding sites
E) they require additional cofactor-binding sites for subsequent binding of substrate

Question 10

Assessment of enzyme activity under standard condition and plotted as a linear Lineweaver-Burk plot gives the green line (x). The red line (y) was most likely obtained by addition of which of the following to this enzyme assay?

A) allosteric activator
B) competitive inhibitor
C) double the amount of enzyme
D) half the amount of enzyme
E) noncompetitive inhibitor

Question 11

Kinases are a class of enzymes that incorporate a phosphate onto their substrates. The catalytic activity of kinases classifies them as members of which of the following enzyme families?

A) hydrolases
B) isomerases
C) ligases
D) oxidoreductases
E) transferases

Question 12

You are studying the effects of the addition of a potential pharmaceutical compound on the activity of your enzyme of interest. You find that the addition of the compound results in an increase in the K_m of the reaction but does not affect the V_max . Which of the following defines the inhibitory action of the compound?

A) competitive
B) noncompetitive
C) suicide
D) uncompetitive

Question 13

If the DG0′ of the reaction A → B is −40 kJ/mol, under standard conditions which of the following
Is correct?

A) the reaction is at equilibrium
B) ΔG^0' will never reach equilibrium
C) ΔG^0' will not occur spontaneously
D) ΔG^0' will proceed at a rapid rate
E) ΔG^0' will proceed spontaneously from left to right

Question 14

For the reaction A → B, ΔG^0' = −60 kJ/mol. The reaction is started with 10 mmol of A and no B is initially present. After 24 hours, analysis reveals the presence of 2 mmol of B and 8 mmol of A. Which of the following is the most likely explanation?

A) A and B have reached equilibrium concentrations
B) an enzyme has shifted the equilibrium toward A
C) B formation is kinetically slow; equilibrium has not been reached by 24 hours
D) formation of B is thermodynamically unfavorable
E) the result described is impossible, given the fact that ΔG^0' is −60 kJ/mol

Question 15

When a mixture of 3-phosphoglycerate and 2-phosphoglycerate is incubated at 25°C with phosphoglycerate mutase until equilibrium is reached, the final mixture contains 6 times as much 2-phosphoglycerate as 3-phosphoglycerate. Which one of the following statements is most nearly correct, when applied to the reaction as written? (R = 8.315 J/mol·K; T = 298 K) 3-Phosphoglycerate → 2-phosphoglycerate

A) ΔG^0' is −4.44 kJ/mol
B) ΔG^0' is zero
C) ΔG^0' is +12.7 kJ/mol
D) ΔG^0' is incalculably large and positive
E) ΔG^0' cannot be calculated from the information given

Question 16

When a mixture of glucose 6-phosphate and fructose 6-phosphate is incubated with the enzyme phosphohexose isomerase (which catalyzes the interconversion of these 2 sugars) until equilibrium is reached, the final mixture contains twice as much glucose 6-phosphate as fructose 6-phosphate. Which one of the following statements is best applied to this reaction outlined below? (R = 8.315 J/mol·K; T = 298 K) Glucose 6-phosphate → fructose 6-phosphate

A) ΔG^0' is incalculably large and negative
B) ΔG^0' is -1.72 kJ/mol
C) ΔG^0' is zero
D) ΔG^0' is +1.72 kJ/mol
E) ΔG^0' is incalculably large and positive

Question 17

Hydrolysis of 1 M glucose 6-phosphate catalyzed by glucose 6-phosphatase is 99% complete at equilibrium (ie, only 1% of the substrate remains). Which of the following statements is most nearly correct? (R = 8.315 J/mol·K; T = 298 K)

A) ΔG^0' = −11 kJ/mol
B) ΔG^0' = −5 kJ/mol
C) ΔG^0' = 0 kJ/mol
D) ΔG^0' = +11 kJ/mol
E) ΔG^0' cannot be determined from the information given

Question 18

The reaction A + B → C has a ΔG^0' of −20 kJ/mol at 25°C. Starting under standard conditions, one can predict which of the following?

A) at equilibrium, the concentration of B will exceed the concentration of A
B) at equilibrium, the concentration of C will be less than the concentration of A
C) at equilibrium, the concentration of C will be much greater than the concentration of A or B
D) C will rapidly break down to A + B
E) when A and B are mixed, the reaction will proceed rapidly toward formation of C

Question 19

Which of the following reactions has the largest negative value for the standard free energy-change (ΔG^0')?

A) fructose 1,6-bisphosphate → fructose 6-phosphate
B) glucose 6-phosphate → fructose 6-phosphate
C) glycerol 3-phosphate → dihydroxyacetone phosphate
D) glycerol → glycerol 3-phosphate
E) phosphoenolpyruvate → pyruvate

Question 20

For the following reaction, ΔG^0' = +29.7 kJ/mol l-Malate + NAD+ → oxaloacetate + NADH + H⁺ Which of the following correctly describes the characteristics of the reaction as written?

A) can never occur in a cell
B) can occur in a cell only if it is coupled to another reaction for which ΔG^0' is positive
C) can occur only in a cell in which NADH is converted to NAD+ by electron transport
D) cannot occur because of its large activation energy
E) may occur in cells at some concentrations of substrate and product

Question 21

In glycolysis, fructose 1,6-bisphosphate is converted to 2 products with a standard free-energy change (ΔG^0') of 23.8 kJ/mol. Under what conditions, encountered in a normal cell, will the freeenergy change be negative, enabling the reaction to proceed spontaneously to the right?

A) under standard conditions, enough energy is released to drive the reaction to the right
B) the reaction will not go to the right spontaneously under any conditions because the ΔG^0' is positive
C) the reaction will proceed spontaneously to the right if there is a high concentration of products relative to the concentration of fructose 1,6-bisphosphate
D) the reaction will proceed spontaneously to the right if there is a high concentration of fructose 1,6-bisphosphate relative to the concentration of products

Question 22

The standard free-energy changes for the reactions below are given.
Phosphocreatine → creatine + P_i
ΔG^0' = −43.0 kJ/mol
ATP → ADP + P_i
ΔG^0' = −30.5 kJ/mol
What is the overall DG0′ for the following reaction?
Phosphocreatine + ADP → creatine + ATP

A) −73.5 kJ/mol
B) −12.5 kJ/mol
C) +12.5 kJ/mol
D) +73.5 kJ/mol
E) ΔG^0' cannot be calculated without Keq'

Question 23

The ΔG^0' values for the 2 reactions shown below are given:
Oxaloacetate + acetyl-CoA + H2O → citrate + CoASH ΔG^0' = −32.2 kJ/mol
Oxaloacetate + acetate → citrate ΔG^0' = −1.9 kJ/mol
What is the ΔG^0' for the hydrolysis of acetyl-CoA?
Acetyl-CoA + H₂ O → acetate + CoASH + ⁺

A) −34.1 kJ/mol
B) −32.2 kJ/mol
C) −30.3 kJ/mol
D) +61.9 kJ/mol
E) +34.1 kJ/mol

Question 24

A 57-year-old man has just returned from an overseas trip and reports having had severe substernal chest pain 3 days ago. Which of the following is the most appropriate laboratory test to order for this patient?

A) aspartate aminotransferase
B) creatine kinase, MB fraction
C) creatine kinase, total
D) lactate dehydrogenase, LD1 fraction
E) troponin I

Question 25

Enzymes are efficient catalysts because they can do which of the following?

A) catalyze reactions that otherwise would not occur
B) decrease the free energy of activation of reactants
C) decrease the standard free-energy change (ΔG^0') of reactions
D) prevent the conversion of product to substrate
E) shift the equilibrium of reactions toward more complete conversion to product

Question 26

Which of the following is the mechanism by which allosteric effectors influence enzymatic activity?

A) binding at the catalytic site
B) changing the conformation of the enzyme
C) forming high-energy complexes with substrate
D) increasing hydration of the active site
E) protecting against degradation of the enzyme

Question 27

Which of the following conditions is most likely if the reaction shown in the diagram is at equilibrium?

A) all of A is consumed
B) all of B is consumed
C) both A and B are present, but the concentration of A is greater than that of B
D) both A and B are present, but the concentration of B is greater than that of A
E) the concentrations of A and B are equal

Question 28

A newborn screening test for galactosemia is positive in your patient. Genetic studies demonstrate the infant harbors a particular mutation in the GALT gene that impairs the activity of galactose-1-phosphate uridyl transferase. The impaired enzyme cannot convert galactose 1-phosphate and UDP-glucose to UDP-galactose and glucose 1-phosphate. Galactose 1-phosphate accumulates and affects the activity of UTP-dependent glucose-1-phosphate pyrophosphorylase (UGP). Which of the following actions by galactose 1-phosphate proves that it
Is a competitive inhibitor of UGP?

A) decreases the apparent K_m for glucose 1-phosphate
B) decreases both the V_max and the apparent Km for glucose 1-phosphate
C) decreases the V_max for glucose 1-phosphate
D) increases the apparentKmfor glucose 1-phosphate
E) increases the V_max for glucose 1-phosphate

Question 29

You are studying the effects of compounds that inhibit the activity of succinate dehydrogenase, SDH. You demonstrate that malonate is a competitive inhibitor of this enzyme. Which of the following sets of findings would most likely provide ideal conditions for maximum inhibition of SDH by malonate?

A) Succinate (mM) 1
K_m (mM) 1
Malonate (mM) 1
K_i (mM) 1
B) Succinate (mM) 1
K_m (mM) 1
Malonate (mM) 10
K_i (mM) 10
C) Succinate (mM) 1
K_m (mM) 10
Malonate (mM) 1
K_i (mM) 1
D) Succinate (mM) 1
K_m (mM) 10
Malonate (mM) 10
K_i (mM) 1

Question 30

Given the table of values for the hydrolysis of some phosphorylated compounds, which of the following best describes the DG0′ for the following reaction? phosphoenolpyruvate + ADP → pyruvate + ATP

A) −22.1 kcal/mol
B) −14.8 kcal/mol
C) −7.8 kcal/mol
D) −7.0 kcal/mol
E) −5.0 kcal/mol

Question 31

In the absence of an enzyme, the conversion of X→ Y exhibits a DG0′ of +5.0 kJ/mol. Which of the following best describes the ΔG^0' of this reaction in the presence of an enzyme that accelerates the reaction 100 fold?

A) −500 kJ/mol
B) −50 kJ/mol
C) −5 kJ/mol
D) +5 kJ/mol
E) +50 kJ/mol

Question 32

The hydrolysis of ATP to ADP and inorganic phosphate proceeds with a ΔG^0' of −7kcal/mol. The hydrolysis of phosphocreatine has a ΔG^0' of −10kcal/mol. In the reaction catalyzed by creatine kinase (creatine phosphokinase), in the direction of phosphocreatine formation, which of the following is the overall ΔG^0' in kcal/mol?

A) −17
B) 17 × TΔS
C) −3
D) −3 × TΔS
E) +3

Question 33

Which of the following is most likely to increase as a result of the binding of an activating effector to the regulatory site of a multi-subunit, allosterically regulated enzyme?

A) 1/V_max
B) acetylation of inactive subunits
C) K_m
D) proportion of subunits in an active conformation
E) ribosylation of inactive subunits