BIO 360 EXAM 3

1. Allosteric inhibitors always bind at a site different from the active site. a) True b) False

true

17. The E-S complex often dissociates with no reaction taking place. a) True b) False

true

21. Enzymes which display cooperative, sigmoidal kinetics are always polymeric enzymes. a) True b) False

true

23. Most enzyme reactions display first order kinetics for the individual substrates. a) True b) False

true

24. When an enzyme is saturated with substrates, it will display zero-order kinetics. a) True b) False

true

32. The Michaelis-Menton constant is a compilation of several rate constants for the reaction. a) True b) False

true

33. The Michaelis-Menton equation applies to all enzymatic reactions. a) True b) False

true

34. One practical advantage of measuring Km, for neophyte biochemists, is that it can be measured independent of enzyme concentration or enzyme purity [assuming no interfering activity]. a) True b) False

true

40. Enzyme kinetics falls into two general categories, simple saturation and cooperative kinetics. a) True b) False

true

43. Allosteric enzymes are especially important in the regulation of metabolic pathways. a) True b) False

true

44. The term K0.5 is analogous to the KM a) True b) False

true

58. In the concerted model the most active enzyme form will be when all subunits are in the R state. a) True b) False

true

62. The sequential model differs from the concerted one, since it allows different subunits to be in different states at the same time. a) True b) False

true

64. Kinase reactions describe enzymes which adds phosphate groups to another molecule. a) True b) False

true

7. Almost all enzymes display kinetics which indicate that they function like surface catalysts. a) True b) False

true

8. The amount of energy released during a reaction tells nothing about the rate at which that reaction will occur. a) True b) False

true

9. Thermodynamically favorable reactions all release energy. a) True b) False

true

16. 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 the above

D) all of the above

6. All catalysts work by lowering the activation energy for a reaction. a) True b) False

True

50. For aspartate transcarbamylase (ATCase) a) ATP is an inhibitor, while CTP is an activator. b) ATP is an activator, while CTP is an inhibitor. c) both ATP and CTP are inhibitors. d) both ATP and CTP are activators.

a) ATP is an inhibitor, while CTP is an activator

61. The sequential model for allosteric enzymes was proposed by: a) Koshland b) Monod c) Wyman and Changeux d) Monod, Wyman and Changeux e) All of these

a) Koshland

48. ATP is a negative allosteric effector for glycogen phosporylase. This is an example of a) feedback inhibition. b) positive cooperativity. c) negative cooperativity. d) competitive inhibition.

a) feedback inhibition.

20. 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

30. 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

5. 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

63. The main distinguishing feature of the concerted model for the behavior of allosteric enzymes is that a) the conformation of all subunits changes simultaneously b) it applies only to dimeric enzymes c) it involves three possible conformations for all subunits d) the T and R conformations exist in roughly equal amounts

a) the conformation of all subunits changes simultaneously

49. Many metabolic pathways involve multistep reactions. Consider the following pathway. E1 E2 E3 E4 A→ B→ C→ D→ F (final product) Feedback inhibition is usually associated with a) the product of the final reaction, F, interacting with E1 b) F interacting with an allosteric site in E4 c) B interacting with an allosteric site in E1 d) all of the intermediates or products in the reaction interacting with the active site in E1

a) the product of the final reaction, F, interacting with E1

51. Allosteric effectors a) typically bind to the enzyme's active site. b) typically bind at a site unique from the active site. c) bind to the substrate rather than the enzyme. d) only bind to the enzyme-substrate complex.

a) typically bind to the enzyme's active site.

27. 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

73. The active site of chymotrypsin contains all of the following, except: a) Histidine residue b) A magnesium ion c) Hydrophobic pocket to bind the substrate d) Serine residue e) All of these are in the active site of chymotrypsin

b) A magnesium ion

45. In order to display cooperative kinetics, an enzyme must have all these characteristics, except: a) Multiple subunits. b) A value for the Michaelis constant, Km. c) Allosteric sites which affect the binding of substrate to the active site. d) Ability to display a Vmax. e) All of these are characteristic of cooperative enzymes.

b) A value for the Michaelis constant, Km.

72. The amino acids in the active site can be involved in all of these processes, except: a) Binding of the substrate b) Becoming part of the product of the reaction c) The actual chemical mechanism for the reaction d) Binding of some necessary cofactor for the reaction e) All of these can be functions of the amino acids in the active site

b) Becoming part of the product of the reaction

15. 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.

52. Allosteric enzymes must exhibit which of the following? a) feedback inhibition b) a phosphorylation site c) general acid-base catalysis d) a quaternary structure

b) a phosphorylation site

59. According to the concerted model of allosteric behavior, an allosteric activator a) favors the taut (tight) form of the enzyme. b) favors the relaxed form of the enzyme. c) can only bind to the enzyme if the substrate is already bound. d) can only bind to the enzyme if the substrate has not already bound.

b) favors the relaxed form of the enzyme.

13. 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

56. The sequential model for allosteric behavior a) cannot account for reactions that display negative cooperativity b) postulates binding of substrates and inhibitors by the induced-fit model c) requires that the conformation of all subunits change simultaneously d) is mathematically simpler than the concerted model

b) postulates binding of substrates and inhibitors by the induced-fit model

4. 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

2. 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 a million times faster.

e) About a million times faster.

22. 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

39. Enzyme activators are likely to include: The choices are: 1. The final product of a metabolic pathway. 2. Products of a second pathway which utilizes the product from the first pathway. 3. Substances which depend on having sufficient amounts of the product of the first pathway. The answer is: a) 1 b) 2 c) 3 d) Both 1 and 2 e) All of these are correct.

c) 3

35. The Michaelis-Menton constant has all of the following characteristics, except: a) It is similar to the affinity constant between the enzyme and substrate. b) The dimension for the Michaelis-Menton constant is concentration, such as molarity. c) The Michaelis-Menton constant determines the Vmax. d) It is the substrate concentration necessary to reach 1/2 Vmax. e) All of these describe the Michaelis-Menton constant.

c) The Michaelis-Menton constant determines the Vmax.

37. The Lineweaver-Burk graph is easier to use for determining enzyme parameters than a saturation curve for all the following reasons, except: a) It is linear. b) A computer-graphing program can easily provide the equation for a straight line. c) The slope of the graph = Vmax/Km. d) The x and y intercepts for the straight line provide easy calculation of Km and Vmax. e) All of these are correct.

c) The slope of the graph = Vmax/Km.

55. In the concerted model the binding of the first substrate molecule will achieve all except: a) To facilitate the binding of other substrate molecules. b) To facilitate the conversion of other subunits to the active state. c) To facilitate the binding of inhibitors to the enzyme. d) All of these are facilitated by the binding of the first substrate molecule. e) None of these answers is correct

c) To facilitate the binding of inhibitors to the enzyme.

28. Which of the following are related for a given enzyme? a) V max, KM, and percentage of α-helix b) V max, k cat, and percentage of β-sheet c) V max, k cat, and turnover number d) V max, KM, and molecular weight

c) V max, k cat, and turnover number

71. An important step in elucidating the behavior of an enzyme is a) obtaining a crystalline sample of the enzyme b) insuring that metal ions are always excluded from the enzyme sample c) determining the active site residues d) none of the above

c) determining the active site residues

66. Proteins that catalyze phosphorylation reactions are called a) dehydrogenases. b) phosphorylases. c) kinases. d) proteases.

c) kinases.

3. 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

c) significantly more effective than nonenzymatic catalysts

68. Phosphorylation of enzymes a) has no effect on their catalytic activity b) does not require ATP c) takes place on serine, threonine, and tyrosine residues d) is not easily characterized

c) takes place on serine, threonine, and tyrosine residues

25. 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

60. The concerted and sequential models for the behavior of allosteric enzymes differ in a) the conformational change in the enzyme in one model and not in the other b) the number of predicted binding sites on the enzyme c) the manner in which changes in quaternary structure take place d) the response of the enzyme to changes in temperature

c) the manner in which changes in quaternary structure take place

47. In reactions catalyzed by allosteric enzymes a) substrate, activators, and inhibitors all compete for the same binding site on the enzyme b) there is no distinction between catalytic and regulatory subunits c) the presence of an activator makes the plot of reaction rate against substrate concentration less cooperative d) the presence of an inhibitor makes the plot of reaction rate against substrate concentration less cooperative

c) the presence of an activator makes the plot of reaction rate against substrate concentration less cooperative

14. 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

38. Enzyme inhibitors are likely to include the following: The choices are: 1. The final product of a metabolic pathway 2. Products of a second pathway which utilizes the product from the first pathway 3. Substances which depend on having sufficient amounts of the product of the first pathway The answer is: a) 1 b) 2 c) 3 d) Both 1 and 2 e) All of these are correct

d) Both 1 and 2

54. The concerted model for allosteric behavior was first described by: a) Koshland b) Monod c) Wyman and Changeux d) Monod, Wyman and Changeux e) All of these

d) Monod, Wyman and Changeux

36. 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.

1. Enzyme catalysts are more effective than inorganic and other catalysts because: a) They lower the activation energy. b) They hold substrates in the proper position to enhance the reaction rate. c) They are specific for certain substrates. d) They are specific for certain substrates and hold the substrates in the proper position. e) All of these are correct.

d) They are specific for certain substrates and hold the substrates in the proper position.

26. 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 measure of how tightly the substrate is bound to the enzyme

d) a measure of how tightly the substrate is bound to the enzyme

67. Phosphorylation and allosteric control of enzymes a) are not involved in reactions of carbohydrates b) play an insignificant role in generating energy c) are important processes in prokaryotes, but not in eukaryotes d) can be combined to afford a high degree of control over enzymatic reactions

d) can be combined to afford a high degree of control over enzymatic reactions

53. The behavior of allosteric enzymes a) does not play any role in feedback inhibition in metabolic pathways b) is strongly dependent on the presence of metal ions c) is related to their ability to hydrolyze themselves d) depends on changes in their quaternary structure on binding of substrates or inhibitors

d) depends on changes in their quaternary structure on binding of substrates or inhibitors

19. 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

11. A catalyst a) lowers the free energy change for a reaction b) does not affect the transition state of a reaction c) does not change the mechanism of the reaction d) speeds up the forward and reverse reaction to the same extent

d) speeds up the forward and reverse reaction to the same extent

46. A comparison of reactions catalyzed by allosteric enzymes with those catalyzed by non-allosteric enzymes always indicates that a) allosteric enzymes are less efficient as catalysts than non-allosteric enzymes b) allosteric enzymes do not display cooperative effects, whereas non-allosteric enzymes do so c) the same control mechanisms apply to both d) the plot of reaction rate against substrate concentration differs

d) the plot of reaction rate against substrate concentration differs

18. Which of the following is not 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 true for chymotrypsin.

e) All of these are true for chymotrypsin.

69. Zymogens are particularly important in which of these processes? a) Blood clotting. b) Activation of digestive enzymes. c) Activation of proteins which are hormones. d) Both blood clotting and activation of digestive enzymes. e) All of these processes.

e) All of these processes.

10. The sign of Gibb's Free Energy is positive ("+") when energy is released. a) True b) False

false

12. The order of a reaction can be determined from the balanced equation for the reaction. a) True b) False

false

29. The substrate-enzyme (E-S) complex always proceeds to form the products rapidly. a) True b) False

false

31. The Michaelis-Menton constant determines the Vmax of an enzymatic reaction. a) True b) False

false

41. The Michaelis-Menton equation is useful when studying allosteric enzymes. a) True b) False

false

57. In the concerted model the relaxed (R) state binds very loosely to the substrate. a) True b) False

false

65. Generally speaking, enzymes involved in pathways which generate ATP will be activated by addition of phosphate groups to the enzyme. a) True b) False

false

70. Amino acids active in enzyme catalysis are often have hydrocarbon side-chains. a) True b) False

false