The Behavior of Proteins: Enzymes, Mechanisms, and Control

Question 1

Which of the following best describes negative cooperativity?
a. Binding of one substrate molecule prevents the enzyme from working at all.
b. Binding of one substrate molecule inhibits the binding of a second substrate.
c. Binding of one substrate molecule enhances the binding of a second substrate.
d. Binding of one substrate molecule inhibits the binding of other effectors

Answer 1

b

Question 2

The saturation curve for aspartyl transcarbamylase has a similar shape to the curve for:
a. Myoglobin
b. Hemoglobin
c. Chymotrypsin
d. Both hemoglobin and chymotrypsin.
e. All of these

Answer 2

e

Question 3

CTP is a known inhibitor of ATCase, the enzyme that catalyzes the first reaction in the pathway for the synthesis of this
compound. This is an example of
a. irreversible inhibition
b. feedback inhibition
c. zymogenic inhibition
d. negative cooperativity

Answer 3

b

Question 4

Homotrophic effects for allosteric enzymes involve
a. the same molecule binding to different sites in the enzyme.
b. different molecules binding to the same site in an enzyme.
c. different molecules binding to different sites in the same enzyme.
d. All of these are homotrophic effects.

Answer 4

a

Question 5

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

Answer 5

a

Question 6

Which of the following is a mechanism of regulating enzyme activity?
a. Feedback inhibition by product.
b. Addition or removal of phosphate groups from of the enzyme.
c. Presence of activators.
d. Activation of zymogens.
e. All of these regulate enzyme activity.

Answer 6

e

Question 7

Which of the following is true?
a. Allosteric enzymes are rarely important in the regulation of metabolic pathways.
b. Michaelis-Menten kinetics describe the reactions of allosteric enzymes
c. Allosteric enzymes have a hyperbolic plot of reaction rate vs. substrate concentration
d. none of these is true

Answer 7

d

Question 8

Which of the following is NOT required in order for an enzyme to display cooperative kinetics?
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.

Answer 8

b

Question 9

In a comparison of allosteric and non-allosteric enzymes
a. it is always possible to define a KM
b. it is always possible to define a Vmax
c. competitive inhibition is always a possibility
d. much of the terminology is completely unchanged

Answer 9

b

Question 10

Is the Michaelis-Menten equation useful when studying allosteric enzymes?
a. Yes
b. No
c. Only if the enzyme displays positive cooperativity.
d. Only if the enzyme displays negative cooperativity.

Answer 10

b

Question 11

Where do allosteric inhibitors bind on an enzyme?
a. They always bind at a site different from the active site.
b. They always bind at the active site.
c. They can bind at either active site or another site.
d. They always bind directly to the substrate
e. none of these

Answer 11

a

Question 12

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

Answer 12

a

Question 13

How do each of these compounds affect the function of ATCase?
a. ATP inhibits and CTP activates
b. ATP activates and CTP inhibits
c. Both ATP and CTP inhibit
d. Both ATP and CTP activate

Answer 13

b

Question 14

How do each of these compounds affect the function of ATCase?
a. ATP is a K effector and CTP is a V effector.
b. ATP is a V effector and CTP is a K effector.
c. Both ATP and CTP are K effectors.
d. Both ATP and CTP are V effectors.
e. none of these

Answer 14

c

Question 15

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.

Answer 15

c

Question 16

A velocity curve (V vs. [S]) for a typical allosteric enzyme will be
a. a rectangular hyperbola.
b. a sigmoid curve.
c. a straight line.
d. a parabola.
e. none of these

Answer 16

b

Question 17

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.

Answer 17

a

Question 18

Allosteric enzymes must exhibit which of the following?
a. feedback inhibition
b. a phosphorylation site
c. general acid-base catalysis
d. a quaternary structure
e. none of these must be exhibited

Answer 18

d

Question 19

What happens when a K-acting inhibitor is added to an allosteric enzyme system?
a. The apparent KM for the substrate increases.
b. The apparent KM for the substrate decreases.
c. The apparent Vmax for the substrate increases.
d. The apparent Vmax for the substrate decreases.

Answer 19

a

Question 20

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

Answer 20

d

Question 21

The binding of aspartate to different subunits of ATCase is an example of
a. enzyme inhibition
b. a heterotropic effect
c. a homotropic effect
d. negative cooperativity

Answer 21

c

Question 22

The concerted model for allosteric behavior was proposed by:
a. Koshland
b. Pauling
c. Pasteur
d. Monod, Wyman and Changeux
e. All of these

Answer 22

d

Question 23

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

Answer 23

a

Question 24

Which of the following does not apply to the concerted model for subunit behavior:
a. Each subunit can exist in a relaxed (R) and taut (T) conformation.
b. All subunits will be in either the R or the T conformation at the same time.
c. Some subunits can be in the R state while others are in the T state.
d. The presence of inhibitors will lead to more of the enzyme being in the T form
e. the presence of activators will lead to more of the enzyme being in the R form

Answer 24

c

Question 25

In the concerted model for allosteric enzymes
a. the relative affinities of substrate for the T and R conformations plays an important role in the cooperativity of
the reaction.
b. the equilibrium between the T and R conformations plays a minor role.
c. the enzymatic activity of the T conformation is considerably higher than that of the R form.
d. it is possible to describe the reactions of all allosteric enzymes accurately.

Answer 25

a

Question 26

In the concerted model, which state binds the substrate more tightly?
a. the relaxed (R) state
b. the taut (T) state
c. Both states bind equally well.

Answer 26

a

Question 27

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.

Answer 27

b

Question 28

In the concerted model the binding of the first substrate molecule will achieve all except
a. facilitation of the binding of other substrate molecules.
b. facilitation of the conversion of other subunits to the active state.
c. facilitation of 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

Answer 28

c

Question 29

In the concerted model the most active enzyme form will be when
a. all subunits are in the R state.
b. all subunits are in the T state.
c. there is a 50:50 mix of R & T states.
d. none of these

Answer 29

a

Question 30

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.

Answer 30

b

Question 31

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.

Answer 31

c

Question 32

Which of the following is not a difference between the concerted model and the sequential model of allosteric
enzymes?
a. The sequential model allows for different subunits to be in different conformations while the concerted model
does not
b. Negative cooperativity can be explained by the sequential model but not by the concerted model
c. Positive cooperativity can be explained by the sequential model but not by the concerted model
d. The sequential model is explained better by considering the induced-fit model of substrate binding, whereas
the concerted model focuses on perturbing the equilibrium between the T and R forms.

Answer 32

c

Question 33

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.

Answer 33

a

Question 34

In the concerted model for allosteric enzymes, if the taut form of the enzyme cannot bind substrate then
a. KT < KR
b. L = R/T
c. c = 0
d. an inhibitor must be present
e. none of these

Answer 34

c

Question 35

Which of the following is an advantage of an allosteric drug compared to an orthosteric one?
a. they can modulate their effect in a more subtle way
b. an allosteric drug may be more specific for a specific reaction than one that binds to the receptor itself
c. they may be safer as they have no effect at all unless the natural substrate is present
d. all of these are advantages
e. none of these is an advantage

Answer 35

d

Question 36

Kinase reactions describe enzymes which
a. add phosphate groups to another molecule.
b. oxidize alcohols to aldehydes.
c. use NAD+
/NADH in their reactions.
d. transfer groups from one part of a molecule to another.
e. add or remove double bonds in molecules

Answer 36

a

Question 37

Kinases usually transfer phosphates from
a. ATP.
b. inorganic phosphate.
c. NADP+
/NADPH.
d. amino acids.

Answer 37

a

Question 38

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

Answer 38

b

Question 39

Which of the following is true?
a. Phosphorylation always increases enzyme activity
b. Kinases often use AMP as a co-substrate in their phosphorylation reactions
c. Some enzymes are activated by phosphorylation while others are inhibited
d. ADP is the most common substrate for a kinase reaction

Answer 39

c

Question 40

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.
e. none of these

Answer 40

d

Question 41

Phosphorylation of enzymes
a. has no effect on their catalytic activity.
b. does not require ATP.
c. usually takes place on serine, threonine, and tyrosine residues.
d. always increases the activity of the enzyme

Answer 41

c

Question 42

Which of the following is true about salicylate?
a. it is a natural compound from the bark of a tree
b. it is structurally similar to aspirin
c. it stimulates the protein kinase, AMPK
d. it has therapeutic effects in lowering the risk of heart disease
e. all of these

Answer 42

e

Question 43

Zymogens are
a. inactive precursors of enzymes which can be activated by the irreversible cleavage of covalent bonds.
b. inactive forms of enzymes which require phosphorylation by a kinase to become active.
c. allosteric enzymes that are always in the R state.
d. allosteric enzymes that are always in the T state.

Answer 43

a

Question 44

In zymogen activation
a. only digestive enzymes are involved.
b. a conformational change takes place with no alteration of primary structure.
c. an inactive protein is converted to an active one by bond cleavage.
d. there is aggregation of several enzyme molecules when the substrate binds.

Answer 44

c

Question 45

Which of the following is true about caspases?
a. they are a class of proteases
b. they are involved in apoptosis
c. they are initially produced as inactive procaspases
d. once activated, they attach specific targets leading to cell death
e. all of these

Answer 45

e

Question 46

Which of the following types of amino acids in an active site is least likely to be involved in enzyme catalysis?
a. Those with hydrophilic, neutral side-chains.
b. Those with negatively charged side-chains.
c. Those with positively charge side-chains.
d. Those with hydrocarbon side-chains

Answer 46

d

Question 47

Inhibitors which bind covalently to specific amino acids are useful in determining which amino acids are in the active
site of an enzyme.
a. True
b. False

Answer 47

a

Question 48

Which of the following enzymes is not a serine protease?
a. trypsin
b. chymotrypsin
c. thrombin
d. aspartyl transcarbamylase (ATCase)

Answer 48

d

Question 49

The initial bond formation in the covalent intermediate in the reaction catalyzed by chymotrypsin is between
a. serine and the carbonyl carbon in the peptide backbone
b. serine and the nitrogen in the peptide backbone
c. histidine and the carbonyl carbon in the peptide backbone
d. histidine and the nitrogen in the peptide backbone

Answer 49

a

Question 50

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 these

Answer 50

c

Question 51

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

Answer 51

b

Question 52

The critical serine residue in the active site of chymotrypsin functions as
a. a nucleophile.
b. an electrophile.
c. a base.
d. a methyl donor.
e. none of these

Answer 52

a

Question 53

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.

Answer 53

b

Question 54

The pH profile of an enzyme can help identify specific amino acids in the active site because:
a. all enzymes have a pH optimum
b. only the active site amino acids can detect changes in pH
c. acidic and basic amino acids are often involved in the active site and pH changes can change their ability to
catalyze a reaction
d. the pH optimum is always the pI of the most critical amino acid in the active site

Answer 54

c

Question 55

Which groups of amino acids are most likely to be found in the active site of an enzyme?
a. leucine, lysine, alanine.
b. cysteine, isoleucine, phenylalanine.
c. tyrosine, threonine, leucine.
d. serine, histidine, aspartate.

Answer 55

d

Question 56

Labeling the amino acid residues in the active site of an enzyme requires
a. a reagent structurally similar to the substrate.
b. a highly polar reagent.
c. a reagent that contains an aromatic group.
d. a reagent that contains a halogen atom.

Answer 56

a

Question 57

Which of the following is unlikely to occur in binding of a substrate to an enzyme?
a. stereospecific interactions.
b. hydrogen bonding.
c. adsorption to surfaces of metallic catalysts.
d. interactions with metal-ions

Answer 57

c

Question 58

Which of the following amino acid side chains would best serve as a general acid, assuming the protein functions at a
pH of 7?
a. alanine
b. aspartic acid
c. lysine
d. asparagine

Answer 58

c

Question 59

Metal ions play an important role in reaction mechanisms because
a. they block the active site of enzymes so that inhibitors cannot bind.
b. they can act as Lewis acids.
c. water is excluded from the active site when metal ions are bound.
d. they prevent protein aggregation.

Answer 59

b

Question 60

The serine in the active site of chymotrypsin functions as
a. a Lewis acid.
b. a metal ion.
c. an electrophile.
d. a nucleophile.

Answer 60

d

Question 61

. Important mechanisms of enzymatic catalysis include
a. nucleophilic reactions
b. general acid-base catalysis
c. Lewis acid-base catalysis
d. all of these

Answer 61

d

Question 62

Enzymes that catalyze similar functions will invariably have
a. similar overall structures.
b. serine in their active sites.
c. histidine in their active sites.
d. active sites that can catalyze the reactions in question

Answer 62

d

Question 63

A transition-state analog is likely to bind to an enzyme
a. more tightly than the substrate.
b. less tightly than the substrate.
c. about as tightly as the substrate.
d. at a site other than the catalytic site.

Answer 63

a

Question 64

Abzymes
a. invariably bind to pyridoxal phosphate.
b. are antibodies with catalytic activity.
c. differ markedly from transition states in enzymatic reactions.
d. have proline as part of their structure.

Answer 64

b

Question 65

Which of the following is true about cocaine addiction
a. it is easy to treat by using a drug that blocks a neurotransmitter receptor
b. it is based on release of the neurotransmitter GABA
c. it can be treated with catalytic antibodies
d. catalytic antibodies against coacaine are designed to mimic the cocaine molecule itself
e. none of these

Answer 65

c

Question 66

How are cofactors bound to their enzymes?
a. always covalently
b. always non-covalently
c. either covalently or non-covalently
d. via linking with a metal ion
e. none of these

Answer 66

c

Question 67

Which of the following is not true about B vitamins?
a. They are usually fully active in the form we eat them
b. They are usually water soluble
c. Niacin and riboflavin are examples
d. They are important in many metabolic reactions

Answer 67

a

Question 68

Cofactors are
a. non-protein in chemical nature.
b. always small proteins.
c. modified amino acids.
d. never required for enzymatic activity.

Answer 68

a

Question 69

Nicotinamide adenine dinucleotide is
a. an enzyme inhibitor used in smoking cessation programs.
b. an inhibitor of ATP production.
c. a coenzyme in reactions that transfer acyl groups.
d. a coenzyme in oxidation-reduction reactions.

Answer 69

d

Question 70

Which of the following statements about coenzymes is true?
a. They are commonly derived from vitamins.
b. They bind to the active site region on specific types of enzymes.
c. They can be metal ions, such as Zn(II).
d. NAD+, FAD and biotin are all examples of coenzymes.
e. All of these statements are true.

Answer 70

e

Question 71

The vitamin biotin is involved in this type of reaction:
a. Carboxylation reactions
b. Decarboxylation reactions
c. Redox reactions
d. Acyl transfer reactions
e. Transamination reactions

Answer 71

a

Question 72

Redox reactions often use this cofactor:
a. Riboflavin
b. Lipoic acid
c. Pyridoxal
d. Thiamine
e. Biotin

Answer 72

a

Question 73

Which of the following can function as coenzymes?
a. lead ion, biotin, and lipoic acid.
b. copper ion, p-hydroxymercuribenzoate, diisopropylphophofluoridate.
c. zinc ion, pyridoxal phosphate, and nicotinamide adenine nucleotides.
d. lead ion, p-hydroxymercuribenzoate, diisopropylphophofluoridate.

Answer 73

c

Question 74

The vitamin pantothenic acid is involved in this type of reaction:
a. Carboxylation reactions
b. Decarboxylation reactions
c. Redox reactions
d. Acyl transfer reactions
e. Transamination reactions

Answer 74

a

Question 75

Pyridoxal phosphate is required for transfer of
a. one-carbon groups
b. amino groups
c. acyl groups
d. aldehyde groups

Answer 75

b

Question 76

B vitamins are often stored in the body.
a. True
b. False

Answer 76

b

Question 77

Acyl transfer reactions often use this cofactor:
a. Riboflavin
b. Lipoic acid
c. Pyridoxal
d. Thiamine
e. Biotin

Answer 77

b

Question 78

The process by which the final product of a series of reactions inhibits the first reaction in the series is called _____.
a. competitive inhibition
b. feedback inhibition
c. mixed inhibition
d. slow-tight inhibition

Answer 78

b

Question 79

How do K systems and V systems of enzymes differ from each other?

Answer 79

There are two types of enzyme systems, called K systems and V systems. K systems include
combinations of allosteric enzymes and inhibitors or activators in which the presence of the
inhibitor/activator changes the substrate concentration that yields one-half Vmax. On the other hand,
V systems include combinations of allosteric enzymes and inhibitors or activators in which the
presence of the inhibitor/activator changes the maximal velocity of the enzyme but not the substrate
level that yields one-half Vmax.

Question 80

Which of the following substances can modify the quaternary structure and the behavior of a protein such as aspartate
transcarbamoylase (ATCase) by binding to it?
a. Apoplastic effectors
b. Bacterial effectors
c. Allosteric effectors
d. Cytoplasmic effectors

Answer 80

c

Question 81

What is the advantage of the sequential model over the concerted model for allosteric enzymes?

Answer 81

The concerted model is simple and describes the behavior of enzyme systems very well. However,
the sequential model, though less simple, gives a more realistic picture of the structure and
behavior of proteins. It also deals very well with the behavior of some enzyme systems.

Question 82

Explain how an active site affects the specificity of enzymes.

Answer 82

The nature of the active site plays an important role in the specificity of enzymes. An enzyme that
displays absolute specificity, catalyzing the reaction of one, and only one, substrate to a particular
product, is likely to have a rigid active site that is best described by the lock-and-key model of
substrate binding. The enzymes that display relative specificity, catalyzing the reactions of
structurally related substrates to related products, apparently have more flexibility in their active
sites and are better characterized by the induced-fit model of enzyme–substrate binding. Also, there
are stereospecific enzymes with specificity in which optical activity plays a role. The binding site
itself must be asymmetric in this situation.