799 - 835 Bioenergetics

Question 1

The term specific activity differs from the term activity in that specific activity:

Select one:
a. is the activity (enzyme units) of a specific protein.
b. is the activity (enzyme units) in a milligram of protein.
c. is measured only under optimal conditions.
d. refers to proteins other than enzymes.
e. refers only to a purified protein.

Answer 1

b. is the activity (enzyme units) in a milligram of protein.

Question 2

Enzymes are potent catalysts. They:

Select one:
a. lower the activation energy for the reactions they catalyze.
b. are consumed in the reactions they catalyze.
c. are very specific and can prevent the conversion of products back to substrates.
d. increase the equilibrium constants for the reactions they catalyze.
e. drive reactions to completion while other catalysts drive reactions to equilibrium.

Answer 2

a. lower the activation energy for the reactions they catalyze.

Question 3

Which of the following statements is true of enzyme catalysts?

Select one:
a. They are generally equally active on D and L isomers of a given substrate.
b. They can increase the equilibrium constant for a given reaction by a thousand-fold or more.
c. To be effective, they must be present at the same concentration as their substrate.
d. Their catalytic activity is independent of pH.
e. They lower the activation energy for conversion of substrate to product.

Answer 3

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

Question 4

The role of an enzyme in an enzyme-catalyzed reaction is to:

Select one:
a. ensure that the product is more stable than the substrate.
b. make the Gibbs’ energy change for the reaction more favorable.
c. increase the rate at which substrate is converted into product.
d. ensure that all the substrate is converted to product.
e. forms a transition state that is less stable than the transition state of the substrate alone.

Answer 4

c. increase the rate at which substrate is converted into product.

Question 5

Which of the following statements is true of enzyme catalysts?

Select one:
a. They lower the activation energy for the conversion of substrate to product.
b. To be effective they must be present at the same concentration as their substrates.
c. They increase the equilibrium constant for a reaction, thus favoring product formation.
d. All enzymes are proteins.
e. They bind to substrates, but are never covalently attached to substrate or product.

Answer 5

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

Question 6

Which of the following statements is false?

Select one:
a. For S⇔P, a catalyst shifts the reaction equilibrium to the right.
b. At the end of an enzyme-catalyzed reaction, the functional enzyme becomes available to catalyze the reaction again.
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 below the optimal value will lower the reaction rate.

Answer 6

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

Question 7

Enzymes differ from other catalysts in that enzymes:
Select one:

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

Answer 7

c. usually display specificity toward a single reactant.

Question 8

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

Select one:
a. substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.
b. when a substrate binds to an enzyme, it induces enzyme activity to fit the metabolic requirements of the organism.
c. enzyme-substrate binding induces an increase in the reaction entropy, thereby catalyzing the reaction.
d. enzyme specificity is induced by enzyme-substrate binding.
e. enzyme-substrate binding induces movement along the reaction coordinate to the transition state.

Answer 8

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

Question 9

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

Select one:
a. changes in [S] are negligible, so [S] can be treated as a constant
b. [ES] can be measured accurately.
c. [ES] is negligible compared to [E]0
d. initial rate equals Vmax.
e. changes in Km are negligible, so Km can be treated as a constant.
f. varying [S] has no effect on V.

Answer 9

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

Question 10

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

Answer 10

b. At very high [S]≫Km, the velocity curve becomes a horizontal line that intersects the y-axis at Km.

Question 11

The double-reciprocal transformation of the Michaelis-Menten equation, also called the Lineweaver-Burk plot, is given by 1/v=(Km/Vmax) *(1/[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=1/2Vmax
d. take the reciprocal of the y-axis intercept.
e. multiply the reciprocal of the y-axis intercept by -1

Answer 11

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

Question 12

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]≫Km.
c. Km for the substrate.
d. enzyme concentration.

Answer 12

b. initial velocity of the catalyzed reaction at [S]≫Km.
d. enzyme concentration.

Question 13

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:

Select one:
a. Michaelis-Menten number.
b. half-saturation constant.
c. dissociation constant.
d. turnover number.
e. maximum velocity.

Answer 13

d. turnover number.

Question 14

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

Select one:
a. intercept on the 1/v axis.
b. intercept on the 1/[S] axis.
c. Vmax
d. curvature of the plot.
e. pK of the plot.

Answer 14

b. intercept on the 1/[S] axis.

Question 15

What is characteristic of a competitive inhibitor:

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

Answer 15

c. binds reversibly to the free enzyme.

Question 16

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=1/2Vmax
d. If enough substrate is added, the normal Vmax of a reaction can be attained even in the presence of a competitive inhibitor.
e. The rate of a reaction decreases steadily with time as substrate is depleted.

Answer 16

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 17

Vmax for an enzyme-catalyzed reaction:

Select one:
a. monotonously increases when pH increases.
b. increases in the presence of a competitive inhibitor.
c. is unchanged in the presence of an uncompetitive inhibitor.
d. is twice the rate observed when the concentration of substrate is equal to the Km.
e. is limited only by the amount of substrate supplied.

Answer 17

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

Question 18

Which of the following statements about allosteric control of enzymatic activity is false?
Select one:
a. Homotropic allosteric effectors does not have a separate binding site.
b. Allosteric proteins are generally composed of several subunits.
c. Binding of the effector changes the conformation of the enzyme molecule.
d. An effector may either inhibit or activate an enzyme.
e. Heterotropic allosteric effectors compete with substrate for binding sites.

Answer 18

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

(A homotropic allosteric effector is a substrate for the enzyme, as well as a regulatory molecule – the prefix ‘homo’ refers to them being the same. They are usually activators of the enzyme.

A heterotropic allosteric effector is a regulatory molecule which is not also the substrate for the enzyme. It can either activate or inhibit the enzyme it binds to. )

Question 19

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

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

Answer 19

c. allosteric inhibitor.

Question 20

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

Answer 20

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

Question 21

At which points of the G-curve a tetrahedral structure is formed in serine- proteases?

Select one:
a. 1,2
b. 2,4
c. 1,3
d. 2,3
e. 1,4

Answer 21

c. 1,3

Question 22

What is true about a cofactor?

Select one:
a. It is a catalyst on its own.
b. It is a part of the apoenzyme.
c. It is present in some, but not all isoenzymes of the same enzyme
d. Before and after the reaction it is in the same form.
e. It helps the regulation, but not the catalytic action of the enzyme.

Answer 22

d. Before and after the reaction it is in the same form.

Question 23

Which feature is common for all isoenzymes of the same enzyme?

Select one:
a. the catalysed reaction
b. the biological function
c. the regulation
d. the tissue localization
e. the intracellular localization

Answer 23

a. the catalysed reaction

Question 24

What is true about serine proteases?

Select one:
a. They cleave peptide bonds of serine.
b. Serine is their cofactor.
c. Serine is found in the catalytic site.
d. Serine is found in the substrate binding site.
e. Serine is released in the catalysed reaction.

Answer 24

c. Serine is found in the catalytic site.

Question 25

Which parameter characterizes the specificity of the enzyme?

Select one:
a. kcat
b. Km
c. kcat/Km
d. kcat*Km
e. Hill-coefficient

Answer 25

c. kcat/Km

Question 26

Which statement is NOT valid concerning the Km?

Select one:
a. It characterizes the enzyme affinity for the substrate.
b. It characterizes the enzyme specificity for the substrate
c. It characterizes the equilibrium E+S⇔ES
d. It expresses the concentration of S at half-maximal reaction rate
e. Doubling the reaction rate at [S]=Km, one gets the Vmax of the enzyme

Answer 26

b. It characterizes the enzyme specificity for the substrate

Question 27

Which statement is NOT valid concerning the Km? Select one:

a. It characterizes the enzyme affinity for the substrate.
b. The reaction rate at [S]=2Km equals Vmax
c. It characterizes the equilibrium E+S⇔ES
d. It expresses the concentration of S at half-maximal reaction rate
e. Doubling the reaction rate at [S]=Km, one gets the Vmax of the enzyme

Answer 27

b. The reaction rate at [S]=2Km equals Vmax

Question 28

Which statement is NOT valid concerning the kcat?

Select one:
a. It characterizes the catalytic efficiency at saturating S concentration
b. It expresses the maximal number of catalytic cycles in unit time
c. It depends on the temperature
d. It depends on the enzyme concentration
e. It indicates the number of substrate molecules used by one molecule of enzyme per unit time at saturating substrate concentrations

Answer 28

d. It depends on the enzyme concentration

Question 29

Which one is the most efficient isoenzyme of the same enzyme at [S]≤Km?

Select one:
a. the one with the highest kcat value
b. the one with the highest Km value
c. the one with the highest specificity constant
d. by definition there is no difference in the efficiency of isoenzymes
e. the one with the lowest Km

Answer 29

c. the one with the highest specificity constant

Question 30

Which one is the most efficient isoenzyme of the same enzyme at [S]≫Km?

Select one:
a. the one with the highest kcat value
b. the one with the highest Km value
c. the one with the highest specificity constant
d. by definition there is no difference in the efficiency of isoenzymes
e. the one with the lowest Km
e. the one with the lowest Km

Answer 30

a. the one with the highest kcat value

Question 31

Which of the following statements is true for the action of serpins?

Select one:
a. The protease is cleaved irreversibly.
b. The inhibitor is cleaved irreversibly.
c. A tetrahedral intermediate is conserved permanently.
d. The active site serine forms an amide bond with the serpin

Answer 31

b. The inhibitor is cleaved irreversibly.

Question 32

What is true about the effect of regulator R on the enzyme with a sensitivity index S=81?

a. The dose-response of enzyme activity to R is linear.
b. The dose-response of enzyme activity is exponential (~R3).
c. The R-enzyme interaction follows a simple binding scheme E+R⇔ ER.
d. The R and enzyme binding is cooperative.
e. The enzyme with sensitivity index S=81 is more sensitive to the regulator that the enzyme with S=3.

Answer 32

c. The R-enzyme interaction follows a simple binding scheme E+R⇔ ER.

Question 33

What kind of reactions are the best targets for metabolic control?

Select one:
a. reversible and saturated with substrates
b. irreversible and saturated with substrates
c. near-equilibrium and saturated with substrates
d. reversible and Vmax significantly below the metabolic flux
e. near-equilibrium at in vivo substrate [Km]

Answer 33

b. irreversible and saturated with substrates

Question 34

What is true about the flux control coefficient?

Select one:
a. it is always a positive number
b. it is a constant characteristic of the enzyme
c. the sum of the control coefficients in a pathway is always 1
d. the sum of the control coefficients in a pathway is always 0
e. the enzyme with the lowest control coefficient in a pathway is called rate- limiting

Answer 34

c. the sum of the control coefficients in a pathway is always 1

Question 35

How is catalytic amplification achieved in the regulation of metabolic pathways?

Select one:
a. through cooperative ligand binding
b. through allosteric regulation
c. through a series of enzyme-catalyzed modifications of enzymes
d. through substrate cycling
e. through negative feedback

Answer 35

c. through a series of enzyme-catalyzed modifications of enzymes

Question 36

How is the most efficient signal amplification achieved in the regulation of metabolic pathways?

Select one:
a. through cooperative ligand binding
b. through allosteric regulation
c. through a series of enzyme-catalysed modifications of enzymes
d. through substrate cycling
e. through negative feedback

Answer 36

d. through substrate cycling