Deck 7: Enzyme Mechanisms

Question 1

Hexokinase catalyzes the phosphorylation of glucose to glucose 6-phosphate. Hexokinase belongs to which enzyme class?

A) transferase
B) ligase
C) oxidoreductase
D) hydrolase

Answer 1

transferase

Question 2

Enzymes usually bind substrates with __________ affinity and specificity.

A) high; high.
B) high; low
C) low; low
D) low; high

Answer 2

high; high.

Question 3

Binding of glucose to hexokinase causes a conformational change in the enzyme. This is an example of the __________ model of enzyme catalysis.

A) substrate-induced
B) lock and key
C) induced-fit
D) glove and hand

Answer 3

induced-fit

Question 4

Which answer correctly pairs the enzyme class with the type of reaction catalyzed?

A) lyase; formation of two products by hydrolyzing a substrate
B) transferase; transfer of H or O atoms
C) isomerase; intramolecular rearrangements
D) oxidoreductase; transfer of groups within molecules

Question 5

The dihydrolipoyl transacetylase enzyme contains a lipoyl group. The lipoyl group is a(n)

A) ion.
B) apoenzyme.
C) prosthetic group.
D) holo group.

Question 6

What is the rate enhancement as a result of the presence of an enzyme if the uncatalyzed rate of the reaction is 1.2 * 10² mmol/sec and the catalyzed rate is 2.4 * 10⁴ mmol/sec?

A) 0.005
B) 200
C) 2.88 * 10⁶
D) 2

Question 7

Below is a ribbon diagram of an enzyme. Four regions of the enzyme are indicated. Which is most likely the active site?

A) A
B) B
C) C
D) D

Question 8

Which of the following is true of the induced-fit model of enzyme catalysis but NOT of the lock and key model of enzyme catalysis?

A) It was proposed by Emil Fischer.
B) It involves weak interactions of a substrate with an enzyme.
C) It involves a conformational change of the enzyme.
D) It involves noncovalent interactions of the substrate with the enzyme.

Question 9

The transition state of a reaction is

A) higher in free energy than the product.
B) lower in free energy than the ground state of the substrate.
C) easily isolated.
D) equal to the $\Delta$ G^‡ of the uncatalyzed reaction minus the $\Delta$ G^‡ of the catalyzed reaction.

Question 10

Which of the following is a prosthetic group?

A) Mg²⁺
B) NADH
C) Zn²⁺
D) heme

Question 11

A mutation of Lys₂₂₉ in aldolase leads to a loss of enzyme activity. This is most likely because the

A) active site can no longer exclude water.
B) active site can no longer include water.
C) enzyme can no longer hold the intermediate in the correct orientation for catalysis.
D) product will remain bound to the enzyme active site.

Question 12

An enzyme that requires the coenzyme nicotinamide adenine dinucleotide belongs to which enzyme class?

A) transferases
B) isomerases
C) ligases
D) oxidoreductases

Question 13

Which of the following is a way that an enzyme can increase the rate of a reaction inside a cell?

A) increasing the temperature of the cell
B) increasing the pressure inside the cell
C) increasing the substrate concentration inside the cell
D) orienting substrates appropriately for the reaction to occur

Question 14

A reaction coordinate diagram comparing an uncatalyzed reaction with an enzyme-catalyzed reaction can directly illustrate that the enzyme , but will not directly illustrate that the enzyme _.

A) orients the substrates appropriately for the reaction to occur; provides an alternative path for product formation
B) provides an alternative path for product formation; stabilizes the transition state
C) stabilizes the transition state; orients the substrates appropriately for the reaction to occur
D) stabilizes the transition state; provides an alternative path for product formation

Question 15

Which of the following is true of a coenzyme but NOT true of a prosthetic group?

A) It contains an organic component.
B) It is loosely associated with the enzyme.
C) It is necessary for enzyme function.
D) It is present in a holoenzyme but not an apoenzyme.

Question 16

Nitrite reductase contains two histidine amino acids that coordinate a Cu²⁺ ion. When the ion is present in the enzyme, the ion is a __________ and the enzyme is a _.

A) cofactor; apoenzyme
B) cofactor; holoenzyme
C) coenzyme; apoenzyme
D) coenzyme; holoenzyme

Question 17

An enzyme can increase the rate of a reaction inside a cell by the energy of the __________.

A) lowering; transition state
B) increasing; product
C) lowering; substrate
D) increasing; transition state

Question 18

Which of the following is a holoenzyme?

A) pyruvate kinase with a bound K⁺
B) alcohol dehydrogenase
C) nitrite reductase
D) hexokinase

Question 19

Consider the reaction coordinate diagram shown below. X is ; Y is .

A) "transition state; activation energy"
B) " $\Delta$ G^‡ of catalyzed reaction; transition state"
C) "activation energy of catalyzed reaction; transition state"
D) " $\Delta$ G^‡ of uncatalyzed reaction; transition state"

Question 20

Enzyme active sites

A) are nonspecific.
B) are a pocket or cleft.
C) always exclude water.
D) can only bind a single substrate at a time.

Question 21

If an enzyme carries out acid-base catalysis, which of the following amino acids could act as general acid?

A) phenylalanine
B) glycine
C) histidine
D) alanine

Question 22

The side chains of the amino acids that make up the catalytic triad of chymotrypsin contain all EXCEPT

A) a hydroxyl group.
B) a carboxylic acid.
C) an aromatic group.
D) an imidazole.

Question 23

Which type of reaction does not change the molecular formula of the product compared with that of the substrate?

A) condensation
B) reduction
C) hydrolysis
D) isomerization

Question 24

The three general categories of enzyme-mediated reactions, which are determined on the basis of the work they accomplish, include all EXCEPT

A) coenzyme-dependent redox reactions.
B) reversible covalent modification.
C) hydrophobic collapse reactions.
D) metabolite transformation reactions.

Question 25

The conversion of L-proline to D-proline is shown below. Which of the following characteristics would most likely be found in a transition state analog that inhibits an enzyme that catalyzes the reaction?

A) planar
B) positively charged at the $\alpha$ -carbon
C) tetrahedral geometry at the $\alpha$ -carbon
D) double bonds within the ring

Question 26

Reaction coordinate diagrams clearly show that the energy of an enzyme bound to a transition state is higher than the energies of the E + S, E + P, and ES that occur along the same reaction coordinate. The energy of an enzyme bound to a transition state analog would lie in the diagram.

A) above the E + S but below the transition state
B) below the E + S
C) above the transition state
D) above E + S but below ES

Question 27

Enzymes from four different species catalyze the same reaction. Based on the reaction coordinate diagrams below, which species contains an enzyme that experiences more bonding interactions with the transition state of the reaction?

A) species A
B) species B
C) species C
D) species D

Question 28

Refer to the reaction coordinate diagram below. The change in the activation energy of the reaction because of the presence of an enzyme is illustrated by the energy of minus the energy of _.

A) B; A
B) B; C
C) B; D
D) C; D

Question 29

Which pair correctly matches the coenzymes most often used to mediate the described redox reactions?

A) NAD⁺/NADH; redox at C-O bonds
B) NADP⁺/NADPH; redox at C-C bonds
C) FAD/FADH₂; redox at C-O bonds
D) FMN/FMNH₂; redox at C-O bonds

Question 30

Which of the following is true of the tetrahedral intermediate in the chymotrypsin mechanism?

A) It is lower in free energy than the substrate.
B) It is partially positively charged.
C) All bonds are the same length.
D) It contains an oxyanion.

Question 31

Refer to the reaction coordinate diagram below. At what point is there a maximum number of interactions between the enzyme and the compound that it is binding?

A) A
B) B
C) C
D) D

Question 32

Which amino acid acts as a general acid and a general base in the mechanism of chymotrypsin?

A) Ser195
B) His57
C) Gly193
D) Asp102

Question 33

Both the substrate and the tetrahedral intermediate, when associated with chymotrypsin,

A) contain an oxyanion.
B) interact with the oxyanion hole.
C) undergo a nucleophilic attack.
D) hydrogen bond to Asp102.

Question 34

When a nucleophile present in the enzyme attacks an electrophilic substrate to form an enzyme-substrate intermediate, this is an example of catalysis.

A) covalent
B) acid-base
C) metal ion
D) hydrophobic

Question 35

The catalytic triad of chymotrypsin is composed of His57, Ser195, and

A) Gly193.
B) Glu103.
C) Asp120.
D) Asp102.

Question 36

The regulation of a biomolecule through the addition or removal of a molecular tag involves __________ reactions.

A) coenzyme-dependent redox
B) reversible covalent modification
C) metabolite transformation
D) isomerization

Question 37

All of the following are common catalytic reaction mechanisms in enzyme active sites EXCEPT catalysis.

A) acid-base
B) covalent
C) metal ion
D) van der Waals

Question 38

The conversion of 2-phosphoglycerate to phosphoenolpyruvate is an example of which type of reaction?

A) hydrolysis
B) dehydration
C) isomerization
D) condensation

Question 39

Many medicinal drugs are transition state analogs. They are good drugs because they can interact with the target enzyme active site and are

A) higher in energy than the transition state.
B) identical in structure to the transition state.
C) stable.
D) polar.

Question 40

In a reversible covalent modification reaction involving the phosphorylation of a target protein, which of the following amino acids is LEAST likely to be modified with a phosphate group?

A) Ser
B) Phe
C) Tyr
D) Thr

Question 41

Place the following HMG-CoA reductase steps in the correct order:
A. Reduction of aldehyde
B. Breakdown of hemithioacetal
C. Reduction of thioester
D. Cofactor exchange

A) A, D, C, B
B) C, D, B, A
C) A, B, D, C
D) C, B, D, A

Question 42

A mutation results in the change of Ser to Asp in the substrate binding pocket of chymotrypsin. Most likely, the mutant enzyme will

A) no longer catalyze the hydrolysis of a peptide bond because Asp cannot facilitate the nucleophilic attack.
B) no longer catalyze the hydrolysis of a peptide bond because Asp is unable to be deprotonated by His57.
C) preferentially hydrolyze substrates containing phenylalanine.
D) preferentially hydrolyze substrates containing lysine.

Question 43

K_M is equal to

A) "k₁ / (k _{- 1} + k₂)."
B) "(k _{- 1} + k₂) / k₁."
C) "¹⁄₂ v_max."
D) "v_max[S] / v₀."

Question 44

The substrate binding pocket of is best at accommodating substrates with small side chains.

A) elastase
B) chymotrypsin
C) enolase
D) trypsin

Question 45

The hemithioacetal intermediate formed during the action of HMG-CoA reductase is stabilized by

A) Lys267.
B) Asp283.
C) His381.
D) Glu83.

Question 46

The y-axis of a Lineweaver-Burk plot is

A) v₀.
B) 1 / v_o.
C) K_m / v_max.
D) 1 / [S].

Question 47

The substrate binding pocket of __________ contains a(n) __________, which facilitates substrate specificity.

A) trypsin; Ser
B) chymotrypsin; Asp
C) trypsin; Asp
D) elastase; Gly

Question 48

The role of Glu211 in the mechanism of enolase is to

A) facilitate the orientation of the phosphate group of the substrate.
B) act as a general base on the substrate.
C) act as a general acid on the intermediate.
D) make the proton at the C-2 position more acidic.

Question 49

The glutamate side chain in the active site of HMG-CoA reductase acts as a general base only after

A) the mevalonate binds to the active site.
B) the hydride from the second NADPH attacks the carbonyl center of the aldehyde.
C) a conformational change triggers the exchange of NADP⁺ for NADPH.
D) CoA is reduced to CoA-SH.

Question 50

Which of the following functions as a general base in the mechanism of enolase?

A) Lys345
B) His57
C) Mg²⁺
D) Glu211

Question 51

In the figure below, K_M is indicated at

A) A.
B) B.
C) C.
D) D.

Question 52

The mechanism of HMG-CoA reductase involves

A) two NADPH.
B) one NADPH.
C) two NADH.
D) one NADH.

Question 53

On a plot of [product] versus time for an enzyme-catalyzed reaction, the v₀ is equal to the

A) slope of the line / [S].
B) [P] at the lowest time point.
C) slope of the line.
D) y-intercept.

Question 54

Which of the following is an assumption made when using Michaelis-Menten kinetics?

A) The conversion of E + P $\rightarrow$ ES does not occur.
B) k₁ > k₂
C) v₀ = v_max at low [S]
D) The conversion of EP $\rightarrow$ E + P is rapid.

Question 55

Which of the following is NOT a function of the Mg²⁺ ions in the mechanism of enolase?

A) orientation of substrate in the active site
B) stabilizing the intermediate
C) making the proton at the C-2 position more acidic
D) electrophilic attack on the scissile bond

Question 56

In the steady-state condition assumed in Michaelis-Menten kinetics, is relatively constant.

A) [ES]
B) [S]
C) v₀
D) [ES] / [S]

Question 57

For a reaction of S $\rightarrow$ P, the rate constant of the reaction is equal to

A) "k."
B) "-k."
C) "1 / v."
D) "[S] / v."

Question 58

For a reaction of Q + R $\rightarrow$ P, the rate constant

A) is equal to [Q][R] / [P].
B) has units of s ^{- 1}.
C) is a second-order rate constant.
D) is equal to [P] / [Q][R].

Question 59

If the rate constant for a reaction is determined to be equal to v / [Q][R], the reaction is the conversion of

A) Q to R.
B) R to Q.
C) R plus Q to a product.
D) It is impossible to determine the reaction given the information provided.

Question 60

The initial velocity of an enzyme-catalyzed reaction is followed at various substrate concentrations. At very high substrate concentrations it is observed that the initial velocity no longer increases as more substrate is added. The velocity under these conditions is known as

A) "the ultimate velocity."
B) "the maximum velocity."
C) "^v_[S]."
D) "optimal velocity."

Question 61

A mixture of enzyme and inhibitor is run through a size-exclusion chromatography column. The activity of the enzyme is assessed before and after the chromatography. The enzyme has more activity after the chromatography step. Which of the following is true?

A) The enzyme was not eluted fully from the column.
B) The enzyme was denatured during chromatography.
C) The inhibitor is a reversible inhibitor.
D) The inhibitor is an irreversible inhibitor.

Question 62

Which of the following statements are true?

A) ATCase is regulated by feedback inhibition.
B) CTP is an allosteric activator of ATCase.
C) ATP is an allosteric inhibitor of ATCase.
D) GTP is an allosteric inhibitor of ATCase.

Question 63

Which answer correctly classifies the compound with its relationship to aspartate transcarbamoylase?

A) ATP; allosteric inhibitor
B) ATP; allosteric activator
C) CTP; allosteric activator
D) CTP; substrate

Question 64

Which of the following is NOT a primary mechanism that affects catalytic efficiency?

A) binding of regulatory molecules
B) covalent modification
C) proteolytic processing
D) cofactor degradation

Question 65

Below is a Lineweaver-Burk plot in which the axis labels have been removed. Interpret the plot to determine the v_max.

A) 0.2
B) 5
C) 3
D) 0.33

Question 66

An experiment is performed in which the kinetics of an enzyme-catalyzed reaction at different pHs is monitored. It is found that the K_m does not change but that the k_cat increases as the pH goes above 7. Which of the following is true?

A) A chemical group within the enzyme that has a pKa of around 7 is likely involved in the catalytic mechanism.
B) A chemical group with a pKa of around 7 must be deprotonated in order for substrate to bind.
C) A chemical group with a pKa of around 7 must be positively charged in order for the substrate to bind.
D) Protons are acting as positive heterotropic allosteric effectors of this enzyme.

Question 67

The specificity constant is equal to

A) [E_t][S] / v₀.
B) K_m / v₀.
C) k_cat / K_m.
D) k_cat / [E_t].

Question 68

A kinase adds a phosphate group to a target enzyme, altering the catalytic efficiency of the enzyme. This is an example of

A) covalent modification.
B) proteolytic processing.
C) binding of regulatory molecules.
D) feedback inhibition.

Question 69

Acetylcholinesterase is an important enzyme in the nervous system. Acetylcholinesterase activity is blocked by the nerve agent sarin gas, which forms a covalent bond with a Ser in the active site of the enzyme. Sarin gas is a(n)

A) allosteric effector.
B) competitive inhibitor.
C) reversible inhibitor.
D) irreversible inhibitor.

Question 70

An inhibitor that binds only to the ES complex and not free enzyme is known as a(n) __________ inhibitor.

A) irreversible
B) competitive
C) uncompetitive
D) mixed

Question 71

The Lineweaver-Burk plot shows data obtained for an enzyme in the absence and presence of a noncompetitive inhibitor. If the [I] is increased significantly in the experiment, the V_max^app would and the K ^app would _.

A) decrease; decrease
B) stay the same; decrease
C) decrease; stay the same
D) stay the same; stay the same

Question 72

A plot of 1 / v₀ versus 1/[S] is called a plot. Data in this plot have a slope equal to _.

A) Lineweaver-Burk; K_m/v_max
B) Lineweaver-Burk; -1/K_m
C) Michaelis-Menten; K_m/v_max
D) Michaelis-Menten; v_max

Question 73

The Lineweaver-Burk plot shows data obtained for an enzyme in the absence and presence of a reversible inhibitor. Which type of inhibitor was used in the experiment?

A) competitive
B) uncompetitive
C) mixed
D) noncompetitive

Question 74

A Lineweaver-Burk plot displays parallel lines for an enzyme in the absence and presence of increasing amounts of an inhibitor. The inhibitor in this experiment

A) binds both the free enzyme and the ES complex.
B) is competitive.
C) alters the K_m but not the v_max.
D) is uncompetitive.

Question 75

An enzyme undergoes a mutation that causes it to lose the ability to be regulated via phosphorylation. Which of the following mutations may lead to this loss of regulation? Assume that the overall structure is not altered by the mutation.

A) Ser $\rightarrow$ Thr
B) Thr $\rightarrow$ Ser
C) Tyr $\rightarrow$ Phe
D) Ser $\rightarrow$ Tyr

Question 76

Which of the following pairs correctly matches the type of interaction observed between an inhibitor and an enzyme with the type of inhibition?

A) ionic; irreversible
B) covalent; irreversible
C) hydrogen bonding; reversible
D) hydrophobic; irreversible

Question 77

A plot of v_o versus [S] for aspartyl transcarbamoylase displays three sigmoidal lines. If the line in the middle represents the enzyme activity in the absence of any allosteric effectors, then the line to the __________ represents the enzyme in the __________ when bound to __________.

A) right; R state; CTP
B) right; T state; CTP
C) left; R state; GTP
D) left; T state; GTP

Question 78

When compared with the T state of aspartate transcarbamoylase, the R state

A) has dissociated into two C₃R₃ complexes.
B) has greater separation of the catalytic subunits.
C) is bound to CTP.
D) has substrate bound in the ATP binding site.

Question 79

Which type of interaction is more likely to be found between an enzyme and an irreversible inhibitor?

A) covalent bond
B) hydrogen bond
C) ionic interaction
D) van der Waals interaction

Question 80

In the formation of an ESI complex, __________ inhibition can result.

A) mixed
B) competitive
C) covalent
D) anticompetitive