Deck 10: Enzymes: Classification, Kinetics, and Control

Question 1

Why is the imidazole R-group of histidine a particularly versatile amino acid side chain in terms of its involvement in enzymatic reaction mechanisms?

A) It is strongly involved in specific acid-base catalysis.
B) It acts as a strong electrophile.
C) It disrupts H-bonds and stabilizes exposed peptide residues.
D) It acts as a strong nucleophile as well as a general acid or base.
E) It contains a high energy phosphate bond.

Answer 1

It acts as a strong nucleophile as well as a general acid or base.

Question 2

Coenzymes are derived from many vitamins. Which of the following is derived from vitamin B₆ (niacin)

A) tetrahydrofolic acid
B) nicotinamide adenine dinucleotide (NAD)
C) pyridoxal phosphate
D) flavin adenine dinucleotide (FAD)
E) coenzyme A (CoASH)

Answer 2

nicotinamide adenine dinucleotide (NAD)

Question 3

Which one of the following statements about the maximum velocity (Vmax) of an enzyme catalyzed reaction is true?

A) Vmax is unaltered in the presence of a noncompetitive inhibitor.
B) Vmax is directly proportional to enzyme concentration.
C) Vmax is directly proportional to substrate concentration.
D) Vmax is directly proportional to the measured rate (v).
E) Vmax is directly proportional to Km, being higher for enzymes with higher Km values for their substrates.

Answer 3

Vmax is directly proportional to enzyme concentration.

Question 4

Which of the following represents a protein modification which changes enzyme activity in a non-reversible manner?

A) Competitive inhibition
B) Zymogen activation
C) Phosphorylation of a serine residue of the protein
D) coenzyme activation
E) allosteric activation

Question 5

In the case of an enzyme displaying a sigmoid relationship between velocity and substrate concentration, it can be concluded that

A) the binding of one substrate molecule hinders the binding of others.
B) the enzyme contains only one binding site for substrate.
C) substrate binds more easily at low concentrations than at high concentrations.
D) the enzyme must contain at least four subunits.
E) the enzyme displays cooperativity in terms of substrate binding.

Question 6

The influence of pH on the rate of enzymatic reactions may be a result of

A) a requirement that the substrate be in a definite ionic form.
B) a requirement for specific ionic groups on the enzyme protein for enzyme activity.
C) a requirement for specific groups which do not dissociate at any pH.
D) all of the above.
E) A and B only.

Question 7

Activation of zymogens is often accomplished by

A) aggregation of monomers to form the active oligomer.
B) post-synthetic modification.
C) formation of covalent crosslinks.
D) hydrolysis of specific peptide bonds.
E) addition of a cofactor.

Question 8

When the substrate concentration is equal to Km for a given enzyme and substrate you would expect which of the following?

A) Vo = Vmax
B) Vo = Vmax/2
C) Vo = Vmax/3
D) Vo = Vmax/4
E) Vo = Vmax/10

Question 9

For the reaction A---> B, catalyst I has an activation energy of 1400 calories per mol. Catalyst II has an activation energy of 2800 calories per mol.

A) catalyst II is more efficient.
B) catalyst I is more efficient.
C) both catalysts should have about the same catalytic efficiency.
D) one cannot determine which is more efficient without information about Delta Ho for the reaction.

Question 10

Which of the following is NOT a property of catalysts?

A) are unchanged by the net reaction
B) facilitate both the forward and reverse reactions
C) increase the equilibrium constant to favor products
D) enhance the rate of the reactions
E) are required in only trace amounts compared to the reactants

Question 11

Enzymes that contain aspartate or glutamate at the active site would most likely be involved in what type of catalysis?

A) Induced fit
B) Destabilization
C) Acid Base Catalysis
D) Covalent catalysis
E) none of the above

Question 12

From the diagram above which amino acid is necessary for catalysis of this reaction?

A) Protonated asp
B) Protonated lys
C) Unprotonated arg
D) Unprotonated his
E) Unprotonated tyr

Question 13

A competitive inhibitor

A) forms an irreversible complex with the active site of an enzyme
B) forms an irreversible complex with a site on an enzyme other than the active site.
C) decreases the maximal velocity of the reaction catalyzed by an enzyme.
D) competes with the substrate for the active site of an enzyme.
E) lowers the Km for the substrate.

Question 14

An enzyme transition state inhibitor

A) would probably bind much more tightly than a normal substrate.
B) probably would be a noncompetitive inhibitor of an enzyme.
C) would probably bind much less tightly than a normal substrate.
D) would have a Ki about equal to the Km for the substrate.

Question 15

An enzyme affects the rate of a chemical reaction by

A) decreasing the free energy of the reaction.
B) increasing the free energy of the reaction.
C) lowering the energy of activation of the reaction.
D) raising the energy of activation of the reaction.
E) displacing the equilibrium constant.

Question 16

All of the following statements about allosteric enzymes are correct EXCEPT which one?

A) They are often involved in major mechanisms of metabolic control.
B) They are generally oligomeric proteins.
C) They do not follow typical Michaelis-Menten kinetics.
D) Some of them contain two kinds of polypeptide chains - one kind binding substrate: the other binding an inhibitor.
E) Positive allosteric effectors are frequently products of a reaction sequence for which an allosteric enzyme is the catalyst of the first committed reaction.

Question 17

An enzyme which removes groups (other than hydrogens) and leaves double bonds is called

A) an isomerase.
B) a lyase.
C) a ligase.
D) a transferase.
E) none of the above

Question 18

How many binding constants would there be for a tetrameric enzyme where each monomer can bind substrate in the Monod Wyman Changeux model of allosterism?

A) 1
B) 2
C) 3
D) 4
E) 5

Question 19

For Michaelis-Menten kinetics, Vmax is

A) proportional to k₁.
B) given in units of moles/liter.
C) the velocity which is observed when the substrate concentration is one half of Km.
D) proportional to k₂.
E) none of the above.

Question 20

For an enzyme reaction where velocity (V) is plotted against substrate concentration (S), the portion of the curve where V is proportional to S is considered to be

A) the Vmax.
B) zero order.
C) first order.
D) enzyme limiting.
E) useless for studying enzyme reactions.

Question 21

Select the statement which most correctly describes biotin.

A) It is a coenzyme for certain carboxylation reactions.
B) It is a cofactor for kinases.
C) It is involved in methyl group transfer.
D) It is a cofactor for the malate shuttle.
E) It is involved in phosphoryl transfer.

Question 22

A derivative of thiamine participates as a coenzyme in

A) the conversion of pyruvate to oxaloacetate.
B) reactions involving the decarboxylation of alpha-keto acids.
C) the conversions of glucose to glucuronic acid.
D) the transamination of alpha-amino acids.
E) the conversion of methylmalonyl-CoA to succinyl-CoA.

Question 23

Allosteric control of enzyme action requires

A) a cofactor derived from a vitamin.
B) a metal ion.
C) the conversion of one form of the enzyme to another by the action of proteolytic enzymes.
D) a conformational change in an enzyme that changes its catalytic activity.
E) an effector that is the product of the enzymatic reaction and which competes for the active site of the enzyme.

Question 24

In the case of noncompetitive inhibition

A) the inhibitor generally resembles the substrate.
B) the inhibition can be overcome by raising the substrate concentration.
C) Ki is the concentration of inhibitor required for half maximal inhibition.
D) a high Ki means that the inhibitor has a high affinity for the enzyme.
E) the inhibitor is commonly the product of the enzymatic reaction.

Question 25

A substance binds to an enzyme to decrease the apparent Km for the substrate. This substance will

A) be an inhibitor at low substrate concentration.
B) be an inhibitor at any substrate concentration.
C) be an activator at substrate concentrations below saturation.
D) be an activator only when the substrate is present at a saturating concentration.
E) interfere with substrate binding.

Question 26

When a reaction reaches equilibrium,

A) delta Go is equal to zero.
B) delta G is equal to zero.
C) no substrate is converted to product.
D) no product can be converted back to substrate.
E) the reaction becomes temperature independent.

Question 27

The effect of pH on enzyme-catalyzed reactions can be related to all of the following EXCEPT

A) the influence of pH on the equilibrium.
B) the need for certain forms of ionizable groups on the substrate.
C) the need for certain forms of ionizable groups in the enzyme.
D) the general effect of pH on protein structure.
E) the influence of pH on the interaction of hydrophobic residues in the protein.

Question 28

A Ramachandran plot is useful in

A) understanding spatial relationships in secondary protein structure.
B) making predictions of tertiary and quaternary protein structure.
C) identifying which residues are likely to form disulfide bonds.
D) determining the best strategy to purify a protein from a heterogeneous mixture.
E) comparing conserved amino acids in related primary protein structures.

Question 29

In the presence of a typical noncompetitive inhibitor of the activity of a particular enzyme toward a given substrate

A) the enzyme is irreversibly inactivated.
B) the potential maximum velocity will remain unchanged from that in the absence of the inhibitor.
C) the interaction of the inhibitor with the substrate is a major factor leading to the inhibition observed.
D) sigmoidal kinetics would be expected if there is no change in protein conformation on the binding of either S or I.
E) the apparent Michaelis constant for the substrate will be the same with or without inhibitor present.

Question 30

Isoenzymes are

A) proteolytic enzymes which are activated by the hydrolysis of a part of the peptide chain.
B) proteins without enzymatic activity which are necessary to activate enzyme proteins.
C) enzymes containing identical amino acid compositions and sequences but which catalyzed different reaction.
D) different enzymes which catalyze the same reaction.
E) enzymes which catalyze several different reactions.

Question 31

What is the condition under which an enzyme is defined as having a "ping-pong" mechanism?

A) Enzyme (or enzyme/prosthetic group) is modified by substrate
B) The enzyme has a dissociable coenzyme
C) The first substrate on and the last product off show competitive binding
D) The reaction is bimolecular
E) The two substrates bind in a random order

Question 32

An inhibitor that must be metabolized by the enzyme before it becomes covalently attached to the enzyme is called a mechanism-based inhibitor. What is another term that has been used for such an inhibitor?

A) Noncompetitive
B) Suicide
C) Terminal
D) Transition State
E) Turnover

Question 33

Which of the following is NOT CHARACTERISTIC of catalysts?

A) they are unchanged by the reaction.
B) they alter the standard free energy change (Delta Go') in a chemical reaction.
C) they accelerate the rate of a chemical reaction in both directions.
D) they exhibit specificity in the reactions that they catalyze.

Question 34

When enzyme rate, the constraint on the ratio of k cat/Km must be kept in mind. Which values of the two constants, respectively, will give the greatest enzyme rate at a given substrate concentration?

A) 1 s -1 / 10-6 M
B) 102 s -1 / 10-4 M
C) 103 s -1 / 10-3 M
D) 104 s -1 / 10-2 M
E) 105 s -1 / 10-1 M

Question 35

In considering the 'active site' of enzymes, a fair generalization is that

A) nearly all the amino acid side-chains of the protein must directly participate in catalysis.
B) all the amino acid residues which are involved in the active site are adjacent because they are located on the same short stretch of the polypeptide backbone.
C) the conformation of the active site region does not exist at all until the substrate is bound.
D) the amino acid residues which form the active site are in close proximity because of the specific three-dimensional conformation the protein has adopted.
E) only one amino acid residue is involved in the active site.

Question 36

A measurement of cooperativity is the

A) energy of activation.
B) maximum velocity.
C) inhibition constant.
D) Hill coefficient.
E) Arrhenius coefficient.

Question 37

If an inorganic ion such as Mg²⁺ is required for enzyme activity, it is usually described as a

A) coenzyme
B) prosthetic group
C) holoenzyme
D) cofactor
E) apoenzyme

Question 38

The enzyme hexokinase, which can utilize ATP and glucose to form glucose-6-phosphate and ADP, would be classified as a(an):

A) Transferase
B) Hydrolase
C) Ligase
D) Lyase
E) Oxido-reductase

Question 39

In a Lineweaver-Burke plot, 1 / V vs 1 / [S], the slope of the plot is equal to

A) Km / Vmax.
B) Km.
C) 1/Vmax.
D) 1/Km.
E) Vmax.

Question 40

In a reaction catalyzed by a dehydrogenase, it was observed that NAD⁺ was required for binding of the second substrate. The reduced form of NAD was found to be a competitive inhibitor against NAD⁺. Which of the mechanisms listed below describes the action of this enzyme?

A) Random
B) Ordered sequential
C) Ping Pong
D) Concerted
E) None of the above

Question 41

The oligomeric structure of all lactate dehydrogenase isozymes is a

A) monomer.
B) dimer.
C) trimer.
D) tetramer.
E) pentamer.

Question 42

All of the following statements about the Michaelis constant are correct EXCEPT which one?

A) For many enzymes, Km approximates the equilibrium constant for the conversion of ES to E and P.
B) A low value for Km is generally associated with a high affinity of an E for an S.
C) Km is calculated from the intersection, in a Lineweaver-Burk plot, of the experimentally determined line with the horizontal axis.
D) Km may be defined as the substrate concentration which gives half-maximal velocity.
E) At a substrate concentration equal to Km, half of the total number of enzyme molecules may be present as an ES complex.

Question 43

Which is LEAST likely to be characteristic of a regulated enzyme in a metabolic pathway?

A) catalyzes the slowest step of the pathway
B) has a short half-life in vivo
C) concentration is hormonally regulated
D) activity under allosteric control
E) catalyzes a physiologically reversible reaction

Question 44

Stereospecificity may be explained by

A) the catalytic constant.
B) reversibility of formation of the enzyme-substrate complex.
C) three point attachment.
D) competitive inhibitors.
E) saturation.

Question 45

For an enzyme-substrate pair, a graph of reaction rate versus substrate concentration gives an S-shaped curve. From these data which of the following is the most likely conclusion?

A) The enzyme conforms closely to Michaelis-Menten kinetics.
B) The enzyme contains more than one kind of polypeptide chain.
C) At high concentrations, the substrate is an inhibitor of the reaction.
D) More than one binding site for the substrate is present in each enzyme subunit.
E) The enzyme is subject to product inhibition.

Question 46

Considering the active site of serine protease enzymes, a fair generalization is that:

A) Nearly all of the amino acid R-groups of the protein must directly participate in catalysis.
B) All of the amino acid residues which are involved in the active site are adjacent because they are located on the same short stretch of the polypeptide chain.
C) The conformation of the active site region does not exist at all until the substrate is bound to the enzyme.
D) The amino acid residues which form the active site are in close proximity because of the specific three-dimensional shape the protein adopts in its native state.
E) Only one amino acid residue is involved in the active site.

Question 47

What happens when substrate concentration is much higher than Km?

A) Equilibrium is reached
B) Km is increased
C) The rate is equal to Kcat [E]
D) The rate is linear
E) The reaction cannot continues

Question 48

For an enzyme-substrate pair, a graph of reaction rate versus substrate concentration gives an S-shaped curve. From these data which of the following is the most likely conclusion?

A) The enzyme conforms closely to the Michaelis equation.
B) The enzyme contains more than one kind of polypeptide chain.
C) In high concentrations, the substrate is an inhibitor.
D) More than one binding site for the substrate is present in each enzyme unit.
E) The enzyme is subject to product inhibition.

Question 49

The movement of a tyrosyl residue in carboxypeptidase A when its substrate binds is an example of

A) substrate strain.
B) induced fit.
C) acid-base catalysis.
D) covalent catalysis.
E) stabilization of the transition state.

Question 50

Enzymes which are in an inactive form after the removal of a cofactor or prosthetic group are termed:

A) isozymes
B) holoenzymes
C) zymogens
D) apoenzymes
E) proenzymes

Question 51

Which of the following is NOT an assumption in the Michaelis Menten model for enzyme kinetic behavior?

A) A complex is formed between enzyme and substrate.
B) All substrates for the same enzyme will be used at the same rate.
C) The concentration of substrate is much larger than that of the enzyme .
D) Only initial velocities are measured
E) A steady state concentration of enzyme-substrate complex is rapidly established and maintained during measurement.

Question 52

All reactions are technically reversible. Why are some considered de facto irreversible?

A) At equilibrium substrate concentration is below the level of detection
B) Equilibrium can never be achieved
C) Keq for the reaction is so low that it cannot be calculated
D) Product inhibits the reverse reaction
E) The Keq is 1

Question 53

All of the following effects may occur in enzyme catalysis EXCEPT which one?

A) Stabilization of transition states occurring between substrates and products.
B) General acid and base catalysis.
C) Lowering of the free energy of products, making a forward reaction thermodynamically more favorable.
D) Nucleophilic covalent catalysis, creating intermediates not possible in an uncatalyzed reaction.
E) Local concentration effects the orienting of substrate in configurations favorable to catalysis by reactive groups on an enzyme.

Question 54

Which of the following statements relative to the maximum velocity (Vmax) of an enzyme catalyzed reaction (assayed under conditions of excess substrate) is correct?

A) Vmax is equal to twice the Km.
B) Vmax is directly proportional to the concentration of the substrate.
C) Vmax is reached when the concentration of the enzyme-substrate complex is equal to the original concentration of the enzyme.
D) Vmax is reached if an enzyme concentration is used that is at least ten times higher than the Km.
E) Vmax is independent of the enzyme concentration.

Question 55

The turnover number of an enzyme catalyzed reaction is:

A) used as a measure of enzyme purity.
B) related to the equilibrium constant of the reaction.
C) changes when the concentration of the enzyme is altered.
D) is a measure of the efficiency of the enzyme in catalyzing the reaction.

Question 56

The fact that an enzyme-catalyzed reaction is first order with respect to substrate at low substrate concentration but becomes zero order with respect to substrate at high substrate concentration is explained by

A) the catalytic constant.
B) reversibility of formation of the enzyme-substrate complex.
C) three point attachment.
D) specificity.
E) saturation.

Question 57

Induced fit

A) is a conformational change brought about by the binding of substrate to an allosteric site.
B) is also known as cooperative binding and requires enzymes with more than one subunit.
C) occurs when cholesterol becomes embedded in the lipid bilayer of the plasma membrane.
D) is the change in the absorption spectrum of an enzyme upon formation of ES complex.
E) is a conformational change evoked by binding of substrate to the active site of an enzyme.

Question 58

In the spectrophotometric assay for lactate dehydrogenase (LDH) activity which of the following reaction components is monitored at a wavelength of nanometers?

A) lactate
B) pyruvate
C) NADH
D) lactate dehydrogenase

Question 59

In the presence of effector A an enzyme displays sigmoidal behavior when initial velocity is plotted against substrate concentration. With increasing concentrations of A the curve is displaced to the left (i.e., towards the ordinate). Effector A is thus

A) a positive allosteric effector.
B) a competitive inhibitor.
C) a negative allosteric effector.
D) a non-competitive inhibitor.
E) an uncompetitive inhibitor.

Question 60

Standard free energy of a reaction can be used for all of the following EXCEPT

A) predict the direction in which a reaction will proceed under standard conditions of temperature, concentrations and pH.
B) predict free energy changes for reactions given the concentrations of reactants and products that are unequal to 1 M.
C) predict the rate of a chemical reaction.
D) predict the standard free energy change of the reaction at pH 7.
E) predict the equilibrium constant for a reaction.

Question 61

Which of the following methods are used to regulate enzyme activity?

A) covalent modification
B) competitive inhibition
C) allosteric regulation
D) synthesis and degradation of the enzyme
E) all of the above are used to regulate enzyme activity

Question 62

Which of the following could be a positive allosteric effector of an enzyme?

A) a competitive inhibitor
B) a noncompetitive inhibitor
C) a compound, other than the substrate, which binds to the active site
D) a compound, other than the substrate, which binds to the enzyme but not to the active site
E) a coenzyme which is essential for the activity of the enzyme

Question 63

In the regulation of an overall metabolic sequence, which of the following reactions is most likely to be modulated by allosteric effectors?

A) Any reaction of the sequence.
B) the last reaction of the sequence.
C) The last reaction of the sequence.
D) The fastest reaction of the sequence.
E) The slowest reaction of the sequence.

Question 64

Which of the following is the most distinctive property of a competitive inhibitor?

A) binding to the enzyme-substrate complex
B) binding to both the enzyme-substrate complex and free enzyme
C) no effect on Vmax
D) a covalent bond formed between the inhibitor and the enzyme

Question 65

The amino acid containing a group that can readily act as a general acid-base catalyst at pH7 is

A) proline.
B) lysine.
C) arginine.
D) asparagine.
E) histidine.

Question 66

The vitamin which functions as the coenzyme (or part of the coenzyme) for a variety of reactions of the type, RH + carbon dioxide --> R-COOH, is

A) biotin.
B) pyridoxine.
C) pantothenate.
D) B₁₂.
E) folate.

Question 67

An example of reversible covalent modification of an enzyme is

A) ADP-ribosylation.
B) glycosylation.
C) phosphorylation.
D) allosteric modification.
E) interaction with cAMP.

Question 68

Which of the following reactions is catalyzed by a kinase?

A) glycerol + ATP <===> glycerol phosphate + ADP
B) lactate + NAD⁺ <===> pyruvate + NADH + H⁺
C) glucose 6-phosphate <===> fructose 6-phosphate
D) UDP-galactose + glucose 1-phosphate <===> UDP-glucose + galactose 1-phosphate
E) fructose 1,6-bisphosphate + H₂O <===> fructose 6-phosphate + Pi

Question 69

In a two substrate-two product enzymatic reaction, the apparent Km for one of the substrates is

A) independent of the concentration of the other substrate.
B) dependent upon the concentration of the products.
C) dependent upon the concentration of the other substrate.
D) dependent upon the concentration of enzyme.
E) dependent upon the maximum velocity.

Question 70

Koshland has proposed the induced fit theory of enzyme action. This concept implies that:

A) the template or lock-and-key theory adequately explains the mechanism of enzyme catalysis.
B) the active site is flexible; the catalytic group(s) of the enzyme is/are brought into proper alignment by the substrate.
C) the conformation of the substrate changes before binding to the enzyme.
D) the formation of the enzyme-substrate complex is always the fastest step in an enzyme catalyzed reaction.

Question 71

The lock and key model of the active site:

A) is a conformational change brought about by the binding of inhibitor to an allosteric site.
B) is also known as cooperative binding and requires enzymes with more than one subunit.
C) does not require a conformational change for the substrate to bind to the active site.
D) is the change in the isoelectric point of an enzyme upon formation of ES complex.
E) is a conformational change evoked by binding of substrate to the active site of an enzyme.

Question 72

Which of the amino acids below is unlikely to be involved in the reaction mechanism at the active site of an enzyme?

A) alanine
B) serine
C) cysteine
D) histidine
E) aspartic acid

Question 73

The Hill relationship can be used to determine the

A) reversibility of a reaction.
B) number of independent binding sites on a protein for a ligand.
C) the number of cooperative binding sites in a protein for a single ligand.
D) whether an effector is homotropic or heterotropic.
E) oxygen binding capacity of a protein.

Question 74

A proteolytic enzyme such as trypsin is an example of

A) an oxidoreductase.
B) a hydrolase.
C) a transferase.
D) an isomerase.
E) a ligase.

Question 75

Which of the following factors can be significant in enzyme catalysis?

A) induced fit
B) distortion or strain in the substrate
C) general acid-base catalysis
D) covalent catalysis
E) all of the above are correct

Question 76

The Km

A) is always a function of rate constants.
B) can always be interpreted as a dissociation constant for the ES complex.
C) cannot be calculated from experimental data.
D) changes in response to noncompetitive inhibitors.
E) changes whenever Vmax is changed experimentally.

Question 77

Chymotrypsin is classified as a(n)

A) oxidoreductase.
B) transferase.
C) hydrolase.
D) lyase.
E) ligase.

Question 78

The degree of inhibition caused by a competitive inhibitor can be decreased by

A) adding water, thus diluting out the EI complex.
B) increasing the enzyme concentration.
C) increasing the substrate concentration.
D) increasing both the enzyme and inhibitor concentration.
E) none of the above.

Question 79

What is an allosteric effector of an enzymic reaction?

A) A competitive inhibitor.
B) An uncompetitive inhibitor.
C) A substance which affects enzymic activity by binding to a site other than the substrate binding site.
D) A denaturant of the enzyme.
E) A substance which binds to the substrate and thus makes it unavailable to the enzyme.

Question 80

Enzyme cofactors

A) can be only inorganic ions or compounds.
B) combine with the protein moiety of the enzyme to form an apoenzyme.
C) are not know to combine simultaneously with both enzyme and substrate.
D) can act only by helping to maintain the native conformation of the enzyme protein.
E) may participate in the catalytic mechanism of an enzyme, providing a reactive group not found in the side chains of the common amino acids.