Enzymes

Question 1

Which of the following statements is true of enzyme catalysts?

Select one:

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

Answer 1

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

Question 2

Which of the following statements is false?

Select one:

a. At the end of an enzyme-catalyzed reaction, the functional enzyme becomes available to catalyze the reaction again.
b. For S P, a catalyst shifts the reaction equilibrium to the right.
c. Substrate binds to an enzyme’s active site.
d. A reaction may not occur at a detectable rate even though it has a favorable equilibrium
e. Lowering the temperature of a reaction will lower the reaction rate.

Answer 2

For S P, a catalyst shifts the reaction equilibrium to the right.

Question 3

Enzymes differ from other catalysts in that enzymes:

Select one:

a. usually display specificity toward a single reactant.
b. fail to influence the equilibrium point of the reaction.
c. form an activated complex with the reactants.
d. lower the activation energy of the reaction catalyzed.
e. are not consumed in the reaction.

Answer 3

a. usually display specificity toward a single reactant.

Question 4

The concept of “induced fit” refers to the fact that:

Select one:
a. enzyme-substrate binding induces an increase in the reaction entropy, thereby catalyzing the reaction.
b. when a substrate binds to an enzyme, the enzyme induces a loss of water (desolvation) from the substrate.
c. substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.
d. enzyme specificity is induced by enzyme-substrate binding.
e. enzyme-substrate binding induces movement along the reaction coordinate to the transition state.
Clear my choice

Answer 4

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

Question 5

The benefit of measuring the initial rate of a reaction, (V), is that at the beginning of a reaction:

Select one:

a. changes in KM are negligible, so KM can be treated as a constant.
b. [ES] can be measured accurately.
c. V
d. Vmax.
e. changes in [S] are negligible, so [S] can be treated as a constant.
f. varying [S] has no effect on V.

Answer 5

d. Vmax.

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

Question 6

Which of the following statements about a plot of V vs. [S] for an enzyme that follows Michaelis-Menten kinetics is false?

Select one:

a. The y-axis is a rate term with units of mmol/min.
b. The shape of the curve is a hyperbola.
c. At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at KM.
d. As [S] increases, the initial velocity of reaction(V) also increases.
e. KM is the [S] at which V
f. 1/2 Vmax.

Answer 6

At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at KM., 1/2 Vmax.

Question 7

The Lineweaver-Burk plot is used to:

Select one:

a. illustrate the effect of temperature on an enzymatic reaction.
b. solve, graphically, for the rate of an enzymatic reaction at infinite substrate concentration.
c. determine the equilibrium constant for an enzymatic reaction.
d. solve, graphically, for the ratio of products to reactants for any starting substrate concentration.
e. extrapolate for the value of reaction rate at infinite enzyme concentration.

Answer 7

d. solve, graphically, for the ratio of products to reactants for any starting substrate concentration.

Question 8

The double-reciprocal transformation of the Michaelis-Menten equation, also called the Lineweaver-Burk plot, is given by 1/V = KM /(Vmax[S]) + 1/Vmax. To determine KM from a double-reciprocal plot, you would:

Select one:

a. multiply the reciprocal of the x-axis intercept by -1.
b. take the reciprocal of the x-axis intercept.
c. take the x-axis intercept where V
d. 1/2 Vmax.
e. take the reciprocal of the y-axis intercept.
f. multiply the reciprocal of the y-axis intercept by -1

Answer 8

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

d. 1/2 Vmax.

Question 9

For enzymes in which the slowest (rate-limiting) step is the reaction k2 ES ® PKM becomes equivalent to:

Select one:

a. kcat.
b. the turnover number.
c. the dissociation constant(Kd) for the ES complex.
d. the [S] where V
e. Vmax.
f. the maximal velocity.

Answer 9

c. the dissociation constant(Kd) for the ES complex.

e. Vmax.

Question 10

To calculate the turnover number of an enzyme you need to know the:

Select one or more:

a. initial velocity of the catalyzed reaction at low [S].
b. initial velocity of the catalyzed reaction at [S]&raquo_space; KM.
c. KM for the substrate.
d. enzyme concentration.

Answer 10

b. initial velocity of the catalyzed reaction at [S]&raquo_space; KM.

d. enzyme concentration.

Question 11

In a plot of l/V against 1/[S] for an enzyme-catalyzed reaction, the presence of a competitive inhibitor will alter the:

Select one:

a. curvature of the plot.
b. intercept on the l/V axis.
c. Vmax.
d. intercept on the l/[S] axis.
e. pK of the plot.

Answer 11

d. intercept on the l/[S] axis.

Question 12

In competitive inhibition, an inhibitor:

Select one:

a. binds only to the ES complex.
b. binds at several different sites on an enzyme.
c. binds reversibly at the active site.
d. binds covalently to the enzyme.
e. lowers the characteristic Vmax of the enzyme.

Answer 12

c. binds reversibly at the active site.

Question 13

Which of these statements about enzyme-catalyzed reactions is false?

Select one:

a. 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.
b. At saturating levels of substrate, the rate of an enzyme-catalyzed reaction is proportional to the enzyme concentration.
c. The Michaelis-Menten constant (Km) equals the [S] at which V
d. 1/2 Vmax.
e. If enough substrate is added, the normal Vmax of a reaction can be attained even in the presence of a competitive inhibitor.
f. The rate of a reaction decreases steadily with time as substrate is depleted.

Answer 13

a. 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 14

Vmax for an enzyme-catalyzed reaction:

Select one:

a. is unchanged in the presence of a uncompetitive inhibitor.
b. increases in the presence of a competitive inhibitor.
c. is twice the rate observed when the concentration of substrate is equal to the KM
d. generally increases when pH increases.
e. is limited only by the amount of substrate supplied.

Answer 14

c. is twice the rate observed when the concentration of substrate is equal to the KM

Question 15

Enzyme X exhibits maximum activity at pH = 6.9. X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4. One likely interpretation of this pH activity is that:

Select one:

a. the enzyme is found in gastric secretions
b. a Lysine residue on the enzyme is involved in the reaction.
c. a Glutamate residue on the enzyme is involved in the reaction.
d. a Histidine residue on the enzyme is involved in the reaction.
e. the enzyme has a metallic cofactor.

Answer 15

d. a Histidine residue on the enzyme is involved in the reaction.

Question 16

A good transition-state analog:

Select one:

a. binds to the enzyme more tightly than the substrate.
b. binds covalently to the enzyme.
c. is too unstable to isolate.
d. binds very weakly to the enzyme.
e. does not react with the native enzyme

Answer 16

a. binds to the enzyme more tightly than the substrate.

Question 17

A transition-state analog:

Select one:

a. resembles the active site of general acid-base enzymes.
b. typically reacts more rapidly with an enzyme than the normal substrate.
c. is less stable when binding to an enzyme than the normal substrate.
d. stabilizes the transition state for the normal enzyme-substrate complex.
e. resembles the transition-state structure of the normal enzyme-substrate complex

Answer 17

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

Question 18

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:

Select one:

a. 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.
b. glucose has more –OH groups per molecule than does water.
c. water normally will not reach the active site because it is hydrophobic.
d. water and ATP compete for the active site resulting in a competitive inhibition of the enzyme.
e. the –OH group of water is attached to an inhibitory H atom while the glucose –OH group is attached to C.

Answer 18

a. 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 19

Which of the following statements about allosteric control of enzymatic activity is false?

Select one:

a. Allosteric proteins are generally composed of several subunits.
b. Heterotropic allosteric effectors compete with substrate for binding sites.
c. Binding of the effector changes the conformation of the enzyme molecule.
d. An effector may either inhibit or activate an enzyme.
e. Homotropic allosteric effectors do not have a separate binding site.

Answer 19

b. Heterotropic allosteric effectors compete with substrate for binding sites.

Question 20

A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic site is termed a:

Select one:

a. stereospecific agent.
b. alternative inhibitor.
c. allosteric inhibitor.
d. competitive inhibitor.
e. transition-state analog.

Answer 20

c. allosteric inhibitor.

Question 21

A metabolic pathway proceeds according to the scheme, R ®–S ® T ® U ® V ® W.A regulatory enzyme, X, catalyzes the first reaction in the pathway. Which of the following is most likely correct for this pathway?

Select one:
a. The first product S, is probably the primary negative modulator of X, leading to feedback inhibition
b. The last reaction will be catalyzed by a second regulatory enzyme.
c. Either metabolite U or V is likely to be a positive modulator, increasing the activity of X.
d. The last product, W, is likely to be a negative modulator of X, leading to feedback inhibition.
e. The last product, W, is likely to be a positive modulator, increasing the activity of X.
Clear my choice

Answer 21

d. The last product, W, is likely to be a negative modulator of X, leading to feedback inhibition.

Question 22

How is trypsinogen converted to trypsin?

Select one:

a. Proteolysis of trypsinogen forms trypsin.
b. Two inactive trypsinogen dimers pair to form an active trypsin tetramer.
c. An increase in Ca2+ concentration promotes the conversion.
d. Trypsinogen dimers bind an allosteric modulator, cAMP, causing dissociation into active trypsin monomers.
e. A protein kinase-catalyzed phosphorylation converts trypsinogen to trypsin.

Answer 22

a. Proteolysis of trypsinogen forms trypsin.

Question 23

Which of the following statements are true for the enzyme at the rate-limiting step of a metabolic pathway?

Select one or more:

a. Concentration of its substrate is higher than its Km value
b. The measured reaction rate is close to its Vmax
c. Its Vmax is higher than that of other enzymes in the pathway
d. Its Km value is higher than that of other enzymes in the pathway
e. Its Km value is lower than that of other enzymes in the pathway

Answer 23

a. Concentration of its substrate is higher than its Km value
b. The measured reaction rate is close to its Vmax

Question 24

Which input path of a branched metabolic pathway is more relevant physiologically?

Select one:

a. The one that has the highest metabolic flux in vivo
b. The one that has the enzyme with the highest Vmax
c. The one that has the enzyme with the lowest Km
d. The one that has lower number of enzymatic steps involved
e. The one that has the highest in vivo substrate concentration at the first enzymatic reaction

Answer 24

a. The one that has the highest metabolic flux in vivo

Question 25

Which of the following statements is true when the rate-limiting step of a reaction pathway is inhibited?

Select one:

a. Concentration of the substrate at the inhibited enzyme drops
b. Rate of the inhibited reaction is lower than that of the others in the pathway
c. Concentration of the product at the inhibited enzyme drops
d. Concentrations of the intermediers in the pathway do not change
e. Steady-state flux of the pathway is unaltered

Answer 25

c. Concentration of the product at the inhibited enzyme drops

Question 26

Which of the following statements is true for a reaction pathway under conditions of steady-state?

Select one:

a. Concentrations of intermediers are unaltered
b. Vmax values of enzymes involved are equal
c. Reaction rate at the rate-limiting step is the lowest
d. Concentrations of intermediers are equal
e. The system is in thermodynamic equilibrium

Answer 26

a. Concentrations of intermediers are unaltered

Question 27

Which conditions are necessary for two reactions to be enzymatically coupled? (Eact: activation energy)

Select one or more:

a. Both reactions include phosphoryl transfer.
b. Eact of both reactions is high.
c. At least one of the reactions is oxidoreduction.
d. ΔG of the two reactions has different signs (one negative and one positive).
e. Eact of both reactions is low.

Answer 27

b. Eact of both reactions is high.

d. ΔG of the two reactions has different signs (one negative and one positive).

Question 28

Which conditions rule out the enzymatic coupling of two reactions? (Eact: activation energy)

Select one or more:

a. One of the reactions has low Eact.
b. Eact of both reactions is high.
c. ΔG of both reactions is negative.
d. ΔG of the two reactions has different signs (one negative and one positive).
e. One of the reactions is oxidoreduction.

Answer 28

a. One of the reactions has low Eact.

c. ΔG of both reactions is negative.

Question 29

Which statement is true regarding the group transfer potential (gtp)?

Select one or more:

a. Gtp is the absolute value of the ΔG of hydrolysis (when the group is released).
b. Functional groups have a natural tendency to move from the lower toward the higher gtp.
c. Functional groups have a natural tendency to move from the higher toward the lower gtp.
d. ΔG of a group transfer equals the gtp of the group acceptor plus the gtp of the group donor.
e. ΔG of a group transfer equals the gtp of the group acceptor minus the gtp of the group donor.

Answer 29

a. Gtp is the absolute value of the ΔG of hydrolysis (when the group is released).
c. Functional groups have a natural tendency to move from the higher toward the lower gtp.
e. ΔG of a group transfer equals the gtp of the group acceptor minus the gtp of the group donor.

Question 30

Which statement is false regarding the group transfer potential (gtp)?

Select one or more:

a. The molecule of the lowest gtp is suitable as a group carrier.
b. Functional groups have a natural tendency to move from the lower toward the higher gtp.
c. ΔG of a group transfer equals the gtp of the group donor minus the gtp of the group acceptor.
d. Low gtp means that the group is attached to the molecule with a weak bond.
e. High gtp indicates a great tendency of the molecule to accept the group.

Answer 30

a. The molecule of the lowest gtp is suitable as a group carrier.
b. Functional groups have a natural tendency to move from the lower toward the higher gtp.
c. ΔG of a group transfer equals the gtp of the group donor minus the gtp of the group acceptor.
d. Low gtp means that the group is attached to the molecule with a weak bond.
e. High gtp indicates a great tendency of the molecule to accept the group.

Question 31

Which statement is false regarding the group transfer potential (gtp)?

Select one or more:

a. ΔG of a group transfer equals the gtp of the group acceptor plus the gtp of the group donor.
b. Functional groups have a natural tendency to move from the higher toward the lower gtp.
c. Low gtp means that the group is attached to the molecule with a weak bond.
d. High gtp indicates a great tendency of the molecule to accept the group.
e. Gtp is the absolute value of the ΔG of hydrolysis (when the group is released).

Answer 31

a. ΔG of a group transfer equals the gtp of the group acceptor plus the gtp of the group donor.
c. Low gtp means that the group is attached to the molecule with a weak bond.
d. High gtp indicates a great tendency of the molecule to accept the group.

Question 32

Which reactions can be enzymatically coupled to each other? (Eact: activation energy)

Select one:

a. one having high Eact and negative ΔG and one having high Eact and positive ΔG.
b. one having low Eact and negative ΔG and one having low Eact and positive ΔG.
c. one having high Eact and negative ΔG and one having low Eact and positive ΔG.
d. two reactions having low Eact and negative ΔG.
e. two reactions having high Eact and positive ΔG.

Answer 32

c. one having high Eact and negative ΔG and one having high Eact and positive ΔG.