Ch06 - The Behavior of Proteins: Enzymes Flashcards
How much faster is a reaction with the fastest enzyme than without a catalyst?
a. About 10 times faster.
b. About 100 times faster.
c. About 1,000 times faster.
d. About 10,000 times faster.
e. About 10^20 times faster.
e. About 10^20 times faster.
As catalysts, enzymes are
a. significantly less effective than nonenzymatic catalysts
b. slightly less effective than nonenzymatic catalysts
c. significantly more effective than nonenzymatic catalysts
d. slightly more effective than nonenzymatic catalysts
e. exactly the same as nonenzymatic catalysts
c. significantly more effective than nonenzymatic catalysts
The rate of a reaction depends on
a. the free energy change
b. the activation energy
c. the enthalpy change
d. the entropy change
b. the activation energy
Enzymatic activity has an optimum temperature because
a. the component amino acids have varying melting points
b. the rate of reactions is thermodynamically controlled
c. the side chains of essential residues are chemically degraded at higher temperatures
d. raising the temperature speeds up the reaction until protein denaturation sets in
e. the organism dies beyond a certain temperature
d. raising the temperature speeds up the reaction until protein denaturation sets in
The main difference between a catalyzed and an uncatalyzed reaction is that
a. the activation energy of the catalyzed reaction is lower.
b. the catalyzed reaction has a more favorable free energy change.
c. the catalyzed reaction has a more favorable enthalpy change.
d. the catalyzed reaction has a more favorable entropy change.
a. the activation energy of the catalyzed reaction is lower.
Which of the following is not true?
a. In thermodynamics, spontaneous does not mean instantaneous or even fast.
b. If a reaction is spontaneous then it has a negative ΔG.
c. Speed of a reaction is a kinetic parameter, not a thermodynamic one.
d. A reaction with a positive ΔG^0 can never happen
d. A reaction with a positive ΔG^0 can never happen
What effect does a catalyst have on the ΔG° of a reaction?
a. A catalyst lowers the ΔG°.
b. A catalyst raises the ΔG°.
c. A catalyst has no effect on the ΔG°.
d. It depend on the specific catalyst.
c. A catalyst has no effect on the ΔG°.
Which of the following is most directly related to the speed of a reaction?
a. The temperature
b. The ΔG^0 of the reaction
c. The ΔG of the reaction
d. The ΔG^0‡ of the reaction
e. None of these is correct.
d. The ΔG^0‡ of the reaction
A rate constant is
a. the rate of a reaction at standard temperature and pressure.
b. the rate of a reaction at equilibrium.
c. a proportionality constant relating the rate of a reaction to the concentration(s) of the reactant(s).
d. a kind of transition state
c. a proportionality constant relating the rate of a reaction to the concentration(s) of the reactant(s).
The rate of a reaction is always dependent on the concentration(s) of the reactant(s).
a. True
b. False
b. False
All catalysts work by lowering the activation energy for a reaction.
a. True
b. False
a. True
The amount of energy released during a reaction tells nothing about the rate at which that reaction will occur.
a. True
b. False
a. True
Thermodynamically favorable reactions all release energy.
a. True
b. False
a. True
The sign of Gibb’s Free Energy is positive (“+”) when energy is released.
a. True
b. False
b. False
Which of the following is true about lactate dehydrogenase (LDH)?
a. There are two types of subunits, H amd M, that combine to form 5 types of isozymes
b. An increase in H-based isozymes was once used to diagnose heart attacks
c. An increase in blood levels of LDH of any kind is indicative of some sort of problem
d. One type of LDH has only H subunits
e. All of the choices
e. All of the choices
The order of a reaction can be determined from the balanced equation for the reaction.
a. True
b. False
b. False
The kinetic order of a reaction
a. can be determined by inspection from the coefficients of the balanced equation
b. must be determined experimentally
c. always depends on the concentration of enzyme
d. never depends on concentrations of reactants
b. must be determined experimentally
Given the rate law, rate = k[A][B], the overall reaction order is
a. zero
b. one
c. two
d. cannot be determined
c. two
First order kinetics means:
a. The rate of a reaction is independent of the amount of reactant measured.
b. The rate of the reaction varies directly with the amount of reactant measured.
c. The rate of the reaction varies with the square of the amount of the reactant measured.
d. More information is needed to answer this question.
e. None of these is correct.
b. The rate of the reaction varies directly with the amount of reactant measured.
The active site of an enzyme
a. is frequently located in a cleft in the enzyme.
b. is the portion of the enzyme to which the substrate binds.
c. contains the reactive groups that catalyze the reaction.
d. all of these are correct
d. all of these are correct
The substrate will only bind to the enzyme when the shapes fit together rigidly.
a. True
b. False
b. False
In the induced-fit model of substrate binding to enzymes
a. the substrate changes its conformation to fit the active site
b. the active site changes its conformation to fit the substrate
c. there is a conformational change in the enzyme when the substrate binds
d. there is aggregation of several enzyme molecules when the substrate binds
c. there is a conformational change in the enzyme when the substrate binds
The E-S complex often shows as a slight depression in the energy profile for the reaction.
a. True
b. False
a. True
The active site of an enzyme is the place where the following happens:
a. The enzyme substrate complex forms here.
b. The catalytic reaction happens here.
c. Allosteric regulation of enzyme rate occurs here.
d. The enzyme-substrate complex forms and the reaction occurs at the active site.
d. The enzyme-substrate complex forms and the reaction occurs at the active site.
Which of the following is implied by induced fit between the enzyme’s active site and the substrate?
a. The enzyme is a flexible molecule.
b. An enzyme will work equally well with different substrates.
c. An active site can bind to different substrates.
d. The enzyme is a flexible molecule so different substrates can bind.
e. All of these are correct
e. All of these are correct
Which of the following is true?
a. The E-S complex often dissociates with no reaction taking place.
b. The E-S complex must form before a reaction can take place
c. Once the E-S complex forms, it can go on to form product or dissociate to E + S
d. All of these are correct
d. All of these are correct
Which of the following is true about the enzyme chymotrypsin?
a. The enzyme can cleave peptides.
b. The enzyme can cleave esters.
c. The enzyme only binds to aromatic substrates.
d. The enzyme can cleave substrates which are not naturally occurring.
e. All of these are correct
e. All of these are correct
The reaction catalyzed by aspartate transcarbamoylase is
a. the first step in the synthesis of amino acids.
b. the first step in the synthesis of fatty acids.
c. the first step in the synthesis of CTP and UTP.
d. is part of glycolysis.
c. the first step in the synthesis of CTP and UTP.
In the reaction catalyzed by chymotrypsin, a graph in which the rate is plotted against the concentration of substrate
a. is sigmoidal, characteristic of an allosteric enzyme
b. shows that cooperative kinetics are observed
c. shows that the reaction is zero order
d. is hyperbolic, characteristic of a nonallosteric enzyme
d. is hyperbolic, characteristic of a nonallosteric enzyme
In the reaction catalyzed by aspartate transcarbamoylase, a graph in which the rate is plotted against the concentration of substrate
a. is sigmoidal, characteristic of an allosteric enzyme
b. shows that noncooperative kinetics are observed
c. shows that the reaction is zero order
d. is hyperbolic, characteristic of a nonallosteric enzyme
a. is sigmoidal, characteristic of an allosteric enzyme
The Michaelis-Menten approach to describing the kinetics of an enzyme-catalyzed reaction makes which of the following assumptions about the conversion of product into substrate?
a. The product binds reversibly to the enzyme in order to be converted into the substrate.
b. The product is not converted to substrate to any appreciable extent.
c. The product is converted to substrate following simple first order kinetics.
d. The product is converted to substrate following simple second order kinetics
b. The product is not converted to substrate to any appreciable extent.
The initial rate of an enzymatic reaction is usually determined in order to assure that
a. the enzyme is active
b. there is no reverse reaction of product to the enzyme-substrate complex
c. the substrate is not used up
d. the experiment can be completed quickly
b. there is no reverse reaction of product to the enzyme-substrate complex
According to the steady-state assumption
a. the product concentration does not change significantly
b. the substrate concentration is large and does not change significantly
c. the concentration of enzyme-substrate complex remains constant with time
d. the free enzyme concentration is always in great excess to the concentration of enzyme-substrate complex
c. the concentration of enzyme-substrate complex remains constant with time
Most enzyme reactions display first order kinetics for the individual substrates when the substrate concentration is low.
a. True
b. False
a. True
When the substrate concentration is low, an enzyme reaction
a. will display zero-order kinetics.
b. will display first-order kinetics.
c. will display second-order kinetics.
d. will denature and cease to function.
b. will display first-order kinetics.
When an enzyme is saturated with substrates,
a. it will display zero-order kinetics.
b. it will display first-order kinetics.
c. it will display second-order kinetics.
d. it will denature and cease to function.
a. it will display zero-order kinetics.
The Michaelis constant is
a. related to the molecular weight of the enzyme
b. a measure of the resistance of the enzyme to denaturation
c. a reflection of the percentage of polar amino acids in the enzyme
d. a rough measure of the affinity of the enzyme for the substrate
d. a rough measure of the affinity of the enzyme for the substrate
The KM expression is equal to
a. (k1 + k2) / k−1
b. (k−1 + k2) / k1
c. (k1 + k−1) / k2
d. k−1 / k1
b. (k−1 + k2) / k1
Which of the following are related for a given enzyme?
a. Vmax, KM, and percentage of α-helix
b. Vmax, kcat, and percentage of β-sheet
c. Vmax, kcat, and turnover number
d. Vmax, KM, and molecular weight
e. None of these are related in any way
c. Vmax, kcat, and turnover number
The Michaelis constant is
a. the rate constant for the formation of the substrate-enzyme (E-S) complex.
b. the rate constant for the breakdown of the substrate-enzyme (E-S) complex to form free enzyme and substrate.
c. the rate constant for the breakdown of the substrate-enzyme (E-S) complex to form free enzyme and product.
d. a compilation of several rate constants for the reaction
d. a compilation of several rate constants for the reaction
The drug acetazolamide:
a. Is used to help fight altitude sickness
b. Was found to ruin the taste of carbonated beverages
c. Does not affect the taste of non-carbonated liquors
d. Causes its effect on taste by inhibiting carbonic anhydrase 4
e. All of these
e. All of these
The substrate-enzyme (E-S) complex
a. always proceeds to form the products rapidly.
b. always breaks down to form free enzyme and substrate.
c. always breaks down to form free enzyme and product.
d. may break down to form free enzyme and substrate, or free enzyme and product.
d. may break down to form free enzyme and substrate, or free enzyme and product.
When an enzyme-catalyzed reaction has two substrates and substrate A must bind before substrate B, the mechanism is called
a. a ping-pong mechanism
b. a random mechanism
c. an ordered mechanism
d. a suicide mechanism
e. none of these is true
c. an ordered mechanism
Which of the following is true concerning a ping-pong mechanism?
a. Either substrate can bind first
b. Either product can leave first
c. One product leaves before the second substrate binds
d. One product leaves before either substrate has bound
e. none of these are true
c. One product leaves before the second substrate binds
A Lineweaver-Burk plot is useful in the analysis of enzymatic reactions because
a. it is easier to see whether points deviate from a straight line than from a curve
b. it is not affected by the presence of inhibitors
c. it can be used whether or not the enzyme displays Michaelis-Menten kinetics
d. all of the above
a. it is easier to see whether points deviate from a straight line than from a curve
The steady state of an enzyme reaction is the following:
a. The rate observed just after mixing the enzyme and substrate.
b. The rate observed and Vmax.
c. The rate of product formation.
d. The state which exists when E-S complex is forming as fast as it is breaking down.
e. The state which exists when substrate concentration equals KM.
d. The state which exists when E-S complex is forming as fast as it is breaking down.
If the y-intercept of a Lineweaver-Burk plot = 1.91 (sec/millimole) and the slope = 75.3 L/sec, Vmax equals:
a. 0.0254 millimoles per second.
b. 0.523 millimoles per second.
c. 5.23 millimoles per second.
d. 39.4 millimoles per second.
e. 75.3 millimoles per second.
b. 0.523 millimoles per second.
If the y-intercept of a Lineweaver-Burk plot = 1.91 (sec/millimole) and the slope = 75.3 L/sec, KM equals:
a. 0.0254 millimolar (mM).
b. 0.523 millimolar (mM).
c. 5.23 millimolar (mM).
d. 39.4 millimolar (mM).
e. 75.3 millimolar (mM).
d. 39.4 millimolar (mM).
The Michaelis constant determines the Vmax of an enzymatic reaction
a. True
b. False
b. False
It is important that at physiological conditions, enzymes work at Vmax.
a. True
b. False
b. False
Refer to Exhibit 6A. What is the equilibrium constant for the uncatalyzed reaction?
a. 0.9
b. 1.1
c. 2.5
d. Cannot be determined from the information provided
a. 0.9
Refer to Exhibit 6A. What is the KM of the enzyme?
a. 10 nM
b. 0.1 µM
c. 1 µM
d. 10 µM
d. 10 µM
Refer to Exhibit 6A. What is the Vmax of the enzyme?
a. 90 nM/s
b. 4500 µM/s
c. 200 µM/s
d. 0.5 M/s
a. 90 nM/s
Refer to Exhibit 6A. What is the actual velocity of the forward reaction under physiologic conditions?
a. 2 nM/s
b. 45 nM/s
c. 500 nM/s
d. 30 nM/s
d. 30 nM/s
Refer to Exhibit 6A. What is the equilibrium constant for the enzyme-catalyzed reaction?
a. 0.9
b. 1.1
c. 2.5
d. Cannot be determined from the information provided
a. 0.9
Refer to Exhibit 6A. “Restrainin” is an inhibitor of triose phosphate isomerase. When it is added to cells at a concentration of 0.4 nM, the enzyme’s apparent KM for the substrate is altered to 100 µM, but the Vmax is unchanged.
a. This is a competitive inhibitor.
b. This is an uncompetitive inhibitor.
c. This is a noncompetitive inhibitor.
d. This is an irreversible inhibitor.
a. This is a competitive inhibitor.
Refer to Exhibit 6A. “Restrainin” is an inhibitor of triose phosphate isomerase. When it is added to cells at a concentration of 0.4 nM, the enzyme’s apparent KM for the substrate is altered to 100 µM, but the Vmax is unchanged.
In the following graph, which line best represents the Lineweaver-Burk plot obtained in the presence of restrainin?
[graph]
a. A
b. B
c. C
d. D
e. E
c. C
Refer to Exhibit 6A. “Hindrate” is an inhibitor of triose phosphate isomerase. When it is added to cells at a concentration of 0.1 nM, the enzyme’s KM for the substrate is unchanged, but the apparent Vmax is altered to 50 nM/sec.
a. This is a competitive inhibitor.
b. This is an uncompetitive inhibitor.
c. This is a noncompetitive inhibitor.
d. This is an irreversible inhibitor.
c. This is a noncompetitive inhibitor
Refer to Exhibit 6A. “Hindrate” is an inhibitor of triose phosphate isomerase. When it is added to cells at a concentration of 0.1 nM, the enzyme’s KM for the substrate is unchanged, but the apparent Vmax is altered to 50 nM/sec.
In the following graph, which line best represents the Lineweaver-Burk plot obtained in the presence of hindrate?
[graph]
a. A
b. B
c. C
d. D
e. E
a. A
Which of the following statements regarding the Michaelis constant is false?
a. It is similar to the affinity constant between the enzyme and substrate.
b. The dimension for the Michaelis constant is concentration, such as molarity.
c. The Michaelis constant determines the Vmax.
d. It is the substrate concentration necessary to reach 1/2 Vmax.
c. The Michaelis constant determines the Vmax.
To study the nature of an enzyme, Vmax is not as good a measurement as the catalytic rate constant kcat because:
a. The Vmax is not a true constant since it depends on the concentration of enzyme
b. The Vmax cannot be measured
c. The Vmax is only valid for allosteric enzymes
d. none of these
a. The Vmax is not a true constant since it depends on the concentration of enzyme
The KM of hexokinase for glucose = 0.15 mM and for fructose, KM = 1.5 mM. Which is the preferred substrate?
a. Glucose.
b. Fructose.
c. Neither substrate is preferred over the other.
d. You cannot tell from the data given.
e. None of these answers is correct.
a. Glucose.
Competitive inhibitors have this effect:
a. Modifying the KM value.
b. Changing the value for Vmax.
c. Interfering with substrate binding.
d. This type of inhibitor both changes the KM and interferes with substrate binding.
e. All of these are correct.
d. This type of inhibitor both changes the KM and interferes with substrate binding.
Which of the following inhibitors binds to the enzyme at a site other than the active site?
a. competitive inhibitor
b. noncompetitive inhibitor
c. irreversible inhibitor
d. all of these
e. none of these
b. noncompetitive inhibitor
Inhibitors can have the following effects on enzyme kinetics:
a. Modifying the KM value.
b. Changing the value for Vmax.
c. Interfering with substrate binding.
d. An inhibitor can change the KM and interfere with substrate binding.
e. All of these are correct.
e. All of these are correct.
The value of Vmax changes in
a. competitive inhibition
b. noncompetitive inhibition
c. both forms of inhibition
d. neither form of inhibition
b. noncompetitive inhibition
The fundamental difference between competitive and noncompetitive inhibition is
a. the degree of cooperativity of the reaction
b. the size of the active site of the enzyme
c. the manner of binding of substrate to the enzyme
d. the manner of binding of inhibitor to the enzyme
d. the manner of binding of inhibitor to the enzyme
Which of the following is more likely to inhibit regulatory subunits of an allosteric enzyme?
a. A competitive inhibitor
b. A non-competitive inhibitor
c. An irreversible inhibitor
d. All of these are equally likely to inhibit a regulatory subunit
b. A non-competitive inhibitor
For competitive inhibition
a. the value of KM decreases
b. the value of Vmax decreases
c. it is possible to overcome the effect of the inhibitor by increasing the concentration of substrate
d. none of the above
c. it is possible to overcome the effect of the inhibitor by increasing the concentration of substrate
Irreversible inhibitors of enzymatic reactions
a. bind to the enzyme only at low temperatures.
b. affect only serine side chains.
c. react with the enzyme to produce a protein that is not enzymatically active and from which the original enzyme cannot be regenerated.
d. are bound to the enzyme by the lock-and-key mechanism.
c. react with the enzyme to produce a protein that is not enzymatically active and from which the original enzyme cannot be regenerated.
A noncompetitive inhibitor
a. binds to the enzyme at a site other than the active site
b. is structurally related to the substrate
c. does not affect the value of Vmax
d. decreases the value of KM
a. binds to the enzyme at a site other than the active site
What effect is seen on a Lineweaver-Burk graph when a competitive inhibitor is added?
a. The y-intercept is changed, but not change the slope of the line.
b. The slope of the line is changed, but not the y-intercept.
c. Both the y-intercept and the slope of the line are changed.
d. Neither the y-intercept not the slope of the line is changed.
b. The slope of the line is changed, but not the y-intercept.
What effect is seen on a Lineweaver-Burk graph when a mixed-type inhibitor is added?
a. The y-intercept is changed, but not change the slope of the line.
b. The slope of the line is changed, but not the y-intercept.
c. Both the y-intercept and the slope of the line are changed.
d. Neither the y-intercept not the slope of the line is changed.
c. Both the y-intercept and the slope of the line are changed.
Generally speaking, a competitive inhibitor and the substrate cannot both bind to the enzyme at the same time.
a. True
b. False
a. True
What effect is seen on a Lineweaver-Burk graph when a non-competitive inhibitor is added?
a. The y-intercept is changed, but not change the slope of the line.
b. The slope of the line is changed, but not the y-intercept.
c. Both the y-intercept and the slope of the line are changed.
d. Neither the y-intercept not the slope of the line is changed.
a. The y-intercept is changed, but not change the slope of the line.
Non-competitive inhibitors have this effect:
a. Modifying the KM value.
b. Changing the value for Vmax.
c. Interfering with substrate binding.
d. This type of inhibitor both changes the Vmax and interferes with substrate binding.
e. All of these are correct.
b. Changing the value for Vmax.
If an inhibitor changes the slope of the Lineweaver-Burk graph, but not the y-intercept, it is this type of inhibition:
a. Competitive.
b. Non-competitive.
c. Mixed Inhibition (uncompetitive inhibition).
d. You cannot tell from the data given.
a. Competitive.
If an inhibitor changes the slope of the Lineweaver-Burk graph, but not the x-intercept, it is this type of inhibition:
a. Competitive.
b. Non-competitive.
c. Mixed Inhibition (uncompetitive inhibition).
d. You cannot tell from the data given.
e. More than one answer is correct.
b. Non-competitive.
Which of the following diseases has not been successfully treated using the principles of enzyme inhibition?
a. AIDS.
b. Lactose intolerance
c. Virus infection
d. Neither AIDS nor virus infection.
e. All of these have been successfully treated using enzyme inhibitors.
b. Lactose intolerance
Which of the following is true about a mixed type inhibition?
a. A Lineweaver-Burk plot will give parallel lines
b. The KM will change but not the Vmax
c. The lines of a Lineweaver-Burk graph will cross in the top left quadrant
d. None of these is true
c. The lines of a Lineweaver-Burk graph will cross in the top left quadrant
Pure noncompetitive inhibition is a limiting case of
a. Competitive inhibition
b. Uncompetitive inhibition
c. Irreversible inhibition
d. Mixed inhibition
e. None of these is true
d. Mixed inhibition
Which of the following is the most unique about an uncompetitive inhibitor?
a. It affects the KM of the enzyme
b. It affects the Vmax of the enzyme
c. It can be identified by a Lineweaver-Burk plot
d. The inhibitor can bind to ES but not to free E
e. None of these is particularly unique to an uncompetitive inhibitor
d. The inhibitor can bind to ES but not to free E
Which of the following is not related to the others?
a. Suicide Substrate
b. Irreversible Inhibitor
c. Trojan Horse substrate
d. Competitive Inhibitor
e. All of these are related
d. Competitive Inhibitor
Nonenzymatic catalysts enhance the rate of a reaction by factors of _____.
a. 10^5 to 10^9
b. 10^2 to 10^4
c. 10^15 to 10^20
d. 10^22 to 10^24
b. 10^2 to 10^4
Identify a true statement about the enzyme creatine kinase (CK).
a. Creatine kinase (CK) is found only in brain and skeletal muscles.
b. After a heart attack, creatine kinase (CK) shows up more rapidly in blood than lactate dehydrogenase (LDH).
c. Creatine kinase (CK) is an isozyme as it can exist in five different forms.
d. A high level of creatine kinase (CK) in brain indicates normal functioning of the brain.
b. After a heart attack, creatine kinase (CK) shows up more rapidly in blood than lactate dehydrogenase (LDH).
The rate of the reaction of glycogen with inorganic phosphate, Pi, to form glucose-1-phosphate and glycogenn-1 is _____.
a. rate = k[Glycogen]^1 [Pi]^1
b. rate = k[Glycogen]^0 [Pi]^1
c. rate = k[Glucose-1-phosphate]^1 [Pi]^1
d. rate = k[Glucose-1-phosphate]^0 [Pi]^1
a. rate = k[Glycogen]^1 [Pi]^1
The rate of a zero-order reaction depends on the _____.
a. total number of reactants
b. concentration of products
c. concentration of reactants
d. presence of catalysts
d. presence of catalysts
Explain the mechanism of the lock-and-key model of enzyme-substrate binding.
The lock-and-key model assumes a high degree of similarity between the shape of the substrate and the geometry of the binding site on the enzyme. The substrate binds to a site whose shape complements its own shape, like a key in a lock or the correct piece in a three-dimensional jigsaw puzzle.
