Quiz Enzymes

Question 1

Which one of the following is not among the six internationally accepted classes of enzymes?
A) Hydrolases
B) Ligases
C) Oxidoreductases 
D) Polymerases
E) Transferases

Answer 1

D) Polymerases

Question 2

One of the enzymes involved in glycolysis, aldolase, requires Zn2+ for catalysis. Under conditions of zinc deficiency, when the enzyme may lack zinc, it would be referred to as the:
A) apoenzyme.
B) coenzyme.
C) holoenzyme.
D) prosthetic group.
E) substrate.

Answer 2

A) apoenzyme.

Question 3

Enzymes are potent catalysts because they:
A) are consumed in the reactions they catalyze.
B) are very specific and can prevent the conversion of products back to substrates.
C) drive reactions to completion while other catalysts drive reactions to equilibrium.
D) increase the equilibrium constants for the reactions they catalyze.
E) lower the activation energy for the reactions they catalyze.

Answer 3

) lower the activation energy for the reactions they catalyze.

Question 4

The role of an enzyme in an enzyme-catalyzed reaction is to:
A) bind a transition state intermediate, such that it cannot be converted back to substrate.
B) ensure that all of the substrate is converted to product.
C) ensure that the product is more stable than the substrate.
D) increase the rate at which substrate is converted into product.
E) make the free-energy change for the reaction more favorable.

Answer 4

increase the rate at which substrate is converted into product.

Question 5

Which one of the following statements is true of enzyme catalysts?
A) TheircatalyticactivityisindependentofpH.
B) They are generally equally active on D and L isomers of a given substrate.
C) They can increase the equilibrium constant for a given reaction by a thousand fold or more.
D) Theycanincreasethereactionrateforagivenreactionbyathousandfoldormore.
E) To be effective, they must be present at the same concentration as their substrate.

Answer 5

Theycanincreasethereactionrateforagivenreactionbyathousandfoldormore.

Question 6

A) They bind to substrates, but are never covalently attached to substrate or product.
B) They increase the equilibrium constant for a reaction, thus favoring product formation.
C) They increase the stability of the product of a desired reaction by allowing ionizations,
resonance, and isomerizations not normally available to substrates.
D) They lower the activation energy for the conversion of substrate to product.
E) To be effective they must be present at the same concentration as their substrates.

Answer 6

They lower the activation energy for the conversion of substrate to product.

Question 7

Which of the following statements is false?
A) A reaction may not occur at a detectable rate even though it has a favorable equilibrium.
B) After a reaction, the enzyme involved becomes available to catalyze the reaction again.
C) For S → P, a catalyst shifts the reaction equilibrium to the right.
D) Lowering the temperature of a reaction will lower the reaction rate.
E) Substrate binds to an enzyme’s active site.

Answer 7

A) A reaction may not occur at a detectable rate even though it has a favorable equilibrium.
B) After a reaction, the enzyme involved becomes available to catalyze the reaction again.
C) For S → P, a catalyst shifts the reaction equilibrium to the right.
D) Lowering the temperature of a reaction will lower the reaction rate.
E) Substrate binds to an enzyme’s active site.

Question 8

Enzymes differ from other catalysts in that only enzymes:
A) are not consumed in the reaction.
B) display specificity toward a single reactant.
C) fail to influence the equilibrium point of the reaction.
D) form an activated complex with the reactants.
E) lower the activation energy of the reaction catalyzed.

Answer 8

display specificity toward a single reactant.

Question 9

Which of the following is true of the binding energy derived from enzyme-substrate interactions?
A) It cannot provide enough energy to explain the large rate accelerations brought about by enzymes.
B) It is sometimes used to hold two substrates in the optimal orientation for reaction.
C) It is the result of covalent bonds formed between enzyme and substrate.
D) Most of it is derived from covalent bonds between enzyme and substrate.
E) Most of it is used up simply binding the substrate to the enzyme.

Answer 9

It is sometimes used to hold two substrates in the optimal orientation for reaction

Question 10

The concept of “induced fit” refers to the fact that:
A) enzyme specificity is induced by enzyme-substrate binding.
B) enzyme-substrate binding induces an increase in the reaction entropy, thereby catalyzing the reaction.
C) enzyme-substrate binding induces movement along the reaction coordinate to the transition state.
D) substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.

Answer 10

substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.

Question 11

The benefit of measuring the initial rate of a reaction V0 is that at the beginning of a reaction:
A) [ES] can be measured accurately.
B) changes in [S] are negligible, so [S] can be treated as a constant.
C) changes in Km are negligible, so Km can be treated as a constant.
D) V0 = Vmax.
E) varying [S] has no effect on V0.

Answer 11

changes in [S] are negligible, so [S] can be treated as a constant.

Question 12

Which of the following statements about a plot of V0 vs. [S] for an enzyme that follows Michaelis- Menten kinetics is false?
A) As [S] increases, the initial velocity of reaction V0 also increases.
B) At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.
C) Km is the [S] at which V0 = 1/2 Vmax.
D) The shape of the curve is a hyperbola.
E) The y-axis is a rate term with units of μm/min.

Answer 12

A) As [S] increases, the initial velocity of reaction V0 also increases.
B) At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.
C) Km is the [S] at which V0 = 1/2 Vmax.
D) The shape of the curve is a hyperbola.
E) The y-axis is a rate term with units of μm/min.

Question 13

The steady state assumption, as applied to enzyme kinetics, implies:
A) Km=Ks.
B) the enzyme is regulated.
C) the ES complex is formed and broken down at equivalent rates.
D) the Km is equivalent to the cellular substrate concentration.
E) the maximum velocity occurs when the enzyme is saturated.

Answer 13

the ES complex is formed and broken down at equivalent rates.

Question 14

An enzyme-catalyzed reaction was carried out with the substrate concentration initially a thousand times greater than the Km for that substrate. After 9 minutes, 1% of the substrate had been converted to product, and the amount of product formed in the reaction mixture was 12 μmol. If, in a separate experiment, one-third as much enzyme and twice as much substrate had been combined, how long would it take for the same amount (12 μmol) of product to be formed?
A) 1.5 min
B) 13.5 min
C) 27 min
D) 3min
E) 6min

Answer 14

C) 27 min

Question 15

Which of these statements about enzyme-catalyzed reactions is false?
A) At saturating levels of substrate, the rate of an enzyme-catalyzed reaction is proportional to the enzyme concentration.
B) If enough substrate is added, the normal Vmax of a reaction can be attained even in the presence of a competitive inhibitor.
C) The rate of a reaction decreases steadily with time as substrate is depleted.
D) The activation energy for the catalyzed reaction is the same as for the uncatalyzed reaction, but
the equilibrium constant is more favorable in the enzyme-catalyzed reaction.
E) The Michaelis-Menten constant Km equals the [S] at which V = 1/2 Vmax.

Answer 15

The activation energy for the catalyzed reaction is the same as for the uncatalyzed reaction, but
the equilibrium constant is more favorable in the enzyme-catalyzed reaction.

Question 16

For enzymes in which the slowest (rate-limiting) step is the reaction
k2 ES → P
Km becomes equivalent to:
A) kcat.
B) the [S] where V0 = Vmax.
C) the dissociation constant, Kd, for the ES complex.
D) the maximal velocity.
E) the turnover number.

Answer 16

C) the dissociation constant, Kd, for the ES complex.

Question 17

The Lineweaver-Burk plot is used to:
A) determine the equilibrium constant for an enzymatic reaction.
B) extrapolate for the value of reaction rate at infinite enzyme concentration.
C) illustrate the effect of temperature on an enzymatic reaction.
D) solve, graphically, for the rate of an enzymatic reaction at infinite substrate concentration.
E) solve, graphically, for the ratio of products to reactants for any starting substrate concentration.

Answer 17

D) solve, graphically, for the rate of an enzymatic reaction at infinite substrate concentration.

Question 18

The double-reciprocal transformation of the Michaelis-Menten equation, also called the Lineweaver- Burk plot, is given by
1/V0 = Km /(Vmax[S]) + 1/Vmax.
To determine Km from a double-reciprocal plot, you would:
A) multiply the reciprocal of the x-axis intercept by −1.
B) multiply the reciprocal of the y-axis intercept by −1.
C) take the reciprocal of the x-axis intercept.
D) take the reciprocal of the y-axis intercept.
E) take the x-axis intercept where V0 = 1/2 Vmax.

Answer 18

multiply the reciprocal of the x-axis intercept by −1.

Question 19

To calculate the turnover number of an enzyme, you need to know:
A) the enzyme concentration.
B) the initial velocity of the catalyzed reaction at [S]&raquo_space; Km.
C) the initial velocity of the catalyzed reaction at low [S].
D) the Km for the substrate.
E) both A and B.

Answer 19

E) both A and B.

Question 20

The number of substrate molecules converted to product in a given unit of time by a single enzyme molecule at saturation is referred to as the:
A) dissociation constant.
B) half-saturation constant.
C) maximum velocity.
D) Michaelis-Menten number.
E) turnover number.

Answer 20

E) turnover number.

Question 21

In a plot of l/V against 1/[S] for an enzyme-catalyzed reaction, the presence of a competitive inhibitor will alter the:
A) curvature of the plot.
B) intercept on the l/[S] axis.
C) intercept on the l/V axis.
D) pK of the plot.
E) Vmax.

Answer 21

intercept on the l/[S] axis.

Question 22

In competitive inhibition, an inhibitor:
A) binds at several different sites on an enzyme.
B) binds covalently to the enzyme.
C) binds only to the ES complex.
D) binds reversibly at the active site.
E) lowers the characteristic Vmax of the enzyme.

Answer 22

) binds reversibly at the active site.

Question 23

Vmax for an enzyme-catalyzed reaction:
A) generally increases when pH increases.
B) increases in the presence of a competitive inhibitor.
C) is limited only by the amount of substrate supplied.
D) is twice the rate observed when the concentration of substrate is equal to the Km.
E) is unchanged in the presence of a uncompetitive inhibitor.

Answer 23

D) is twice the rate observed when the concentration of substrate is equal to the Km.

Question 24

Both water and glucose share an —OH that can serve as a substrate for a reaction with the terminal phosphate of ATP catalyzed by hexokinase. Glucose, however, is about a million times more reactive as a substrate than water. The best explanation is that:
A) glucose has more —OH groups per molecule than does water.
B) the larger glucose binds better to the enzyme; it induces a conformational change in hexokinase
that brings active-site amino acids into position for catalysis.
C) the —OH group of water is attached to an inhibitory H atom, while the glucose —OH group is
attached to C.
D) water and the second substrate, ATP, compete for the active site resulting in a competitive
inhibition of the enzyme.
E) water normally will not reach the active site because it is hydrophobic.

Answer 24

the larger glucose binds better to the enzyme; it induces a conformational change in hexokinase
that brings active-site amino acids into position for catalysis.

Question 25

A good transition-state analog:
A) binds covalently to the enzyme.
B) binds to the enzyme more tightly than the substrate.
C) binds very weakly to the enzyme.
D) is too unstable to isolate.
E) must be almost identical to the substrate.

Answer 25

binds to the enzyme more tightly than the substrate.

Question 26

A transition-state analog:
A) is less stable when binding to an enzyme than the normal substrate.
B) resembles the active site of general acid-base enzymes.
C) resembles the transition-state structure of the normal enzyme-substrate complex.
D) stabilizes the transition state for the normal enzyme-substrate complex.
E) typically reacts more rapidly with an enzyme than the normal substrate.

Answer 26

resembles the transition-state structure of the normal enzyme-substrate complex.

Question 27

The role of the metal ion (Mg2+) in catalysis by enolase is to
A) act as a general acid catalyst
B) act as a general base catalyst
C) facilitate general acid catalysis
D) facilitate general base catalysis
E) stabilize protein conformation

Answer 27

facilitate general base catalysis

Question 28

Which of the following statements about allosteric control of enzymatic activity is false?
A) Allosteric effectors give rise to sigmoidal V0 vs. [S] kinetic plots.
B) Allosteric proteins are generally composed of several subunits.
C) An effector may either inhibit or activate an enzyme.
D) Binding of the effector changes the conformation of the enzyme molecule.
E) Heterotropic allosteric effectors compete with substrate for binding sites.

Answer 28

Heterotropic allosteric effectors compete with substrate for binding sites.

Question 29

A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic site is termed a(n):
A) allosteric inhibitor.
B) alternative inhibitor.
C) competitive inhibitor.
D) stereospecific agent.
E) transition-state analog.

Answer 29

A) allosteric inhibitor.

Question 30

Allosteric enzymes:
A) are regulated primarily by covalent modification.
B) usually catalyze several different reactions within a metabolic pathway.
C) usually have more than one polypeptide chain.
D) usually have only one active site.
E) usually show strict Michaelis-Menten kinetics.

Answer 30

usually have more than one polypeptide chain.

Question 31

Which of the following has not been shown to play a role in determining the specificity of protein kinases?
A) Disulfide bonds near the phosphorylation site
B) Primary sequence at phosphorylation site
C) Protein quaternary structure
D) Protein tertiary structure
E) Residues near the phosphorylation site

Answer 31

) Disulfide bonds near the phosphorylation site

Question 32

How is trypsinogen converted to trypsin?
A) A protein kinase-catalyzed phosphorylation converts trypsinogen to trypsin.
B) An increase in Ca2+ concentration promotes the conversion.
C) Proteolysis of trypsinogen forms trypsin.
D) Trypsinogen dimers bind an allosteric modulator, cAMP, causing dissociation into active trypsin
monomers.
E) Two inactive trypsinogen dimers pair to form an active trypsin tetramer.

Answer 32

Proteolysis of trypsinogen forms trypsin.