Deck 6: Enzymes

Question 1

The double-reciprocal transformation of the Michaelis-Menten equation,also called the Lineweaver-Burk plot,is given by
1/V₀ = K_m /(V_max[S])+ 1/V_max
To determine K_m from a double-reciprocal plot,you would:

A)multiply the reciprocal of the x-axis intercept by -1.
B)multiply the reciprocal of the y-axis intercept by -1.
C)take the reciprocal of the x-axis intercept.
D)take the reciprocal of the y-axis intercept.
E)take the x-axis intercept,where V₀ = 1/2 V_max.

Answer 1

multiply the reciprocal of the x-axis intercept by -1.

Question 2

The Lineweaver-Burk plot is used to:

A)determine the equilibrium constant for an enzymatic reaction.
B)extrapolate for the value of reaction rate at infinite enzyme concentration.
C)illustrate the effect of temperature on an enzymatic reaction.
D)solve,graphically,for the rate of an enzymatic reaction at infinite substrate concentration.
E)solve,graphically,for the ratio of products to reactants for any starting substrate concentration.

Answer 2

solve,graphically,for the rate of an enzymatic reaction at infinite substrate concentration.

Question 3

Which of the following statements is false?

A)A reaction may not occur at a detectable rate even though it has a favorable equilibrium.
B)After a reaction,the enzyme involved becomes available to catalyze the reaction again.
C)For S $\rightarrow$ P,a catalyst shifts the reaction equilibrium to the right.
D)Lowering the temperature of a reaction will lower the reaction rate.
E)Substrate binds to an enzyme's active site.

Answer 3

For S $\rightarrow$ P,a catalyst shifts the reaction equilibrium to the right.

Question 4

The steady state assumption,as applied to enzyme kinetics,implies:

A)K_m = K_s.
B)the enzyme is regulated.
C)the ES complex is formed and broken down at equivalent rates.
D)the K_m is equivalent to the cellular substrate concentration.
E)the maximum velocity occurs when the enzyme is saturated.

Question 5

Which of these statements about enzyme-catalyzed reactions is false?

A)At saturating levels of substrate,the rate of an enzyme-catalyzed reaction is proportional to the enzyme concentration.
B)If enough substrate is added,the normal V_max of a reaction can be attained even in the presence of a competitive inhibitor.
C)The rate of a reaction decreases steadily with time as substrate is depleted.
D)The activation energy for the catalyzed reaction is the same as for the uncatalyzed reaction,but the equilibrium constant is more favorable in the enzyme-catalyzed reaction.
E)The Michaelis-Menten constant K_m equals the [S] at which V = 1/2 V_max.

Question 6

Which of the following statements about a plot of V₀ vs.[S] for an enzyme that follows Michaelis-Menten kinetics is false?

A)As [S] increases,the initial velocity of reaction V₀ also increases.
B)At very high [S],the velocity curve becomes a horizontal line that intersects the y-axis at K_m.
C)K_m is the [S] at which V₀ = 1/2 V_max.
D)The shape of the curve is a hyperbola.
E)The y-axis is a rate term with units of $\mu$ m/min.

Question 7

For enzymes in which the slowest (rate-limiting)step is the reaction


K_m becomes equivalent to:

A)k_cat.
B)the [S],where V₀ = V_max.
C)the dissociation constant,K_d,for the ES complex.
D)the maximal velocity.
E)the turnover number.

Question 8

Enzymes are potent catalysts because they:

A)are consumed in the reactions they catalyze.
B)are very specific and can prevent the conversion of products back to substrates.
C)drive reactions to completion while other catalysts drive reactions to equilibrium.
D)increase the equilibrium constants for the reactions they catalyze.
E)lower the activation energy for the reactions they catalyze.

Question 9

In the following diagram of the first step in the reaction catalyzed by the protease chymotrypsin,the process of general base catalysis is illustrated by the number ________,and the process of covalent catalysis is illustrated by the number _________.

A)1;2
B)1;3
C)2;3
D)2 ;3
E)3;2

Question 10

The role of an enzyme in an enzyme-catalyzed reaction is to:

A)bind a transition state intermediate,such that it cannot be converted back to substrate.
B)ensure that all of the substrate is converted to product.
C)ensure that the product is more stable than the substrate.
D)increase the rate at which substrate is converted into product.
E)make the free-energy change for the reaction more favorable.

Question 11

Compare the two reaction coordinate diagrams below and select the answer that correctly describes their relationship.In each case,the single intermediate is the ES complex.

A)(a)describes a strict "lock and key" model,whereas (b)describes a transition-state complementarity model.
B)The activation energy for the catalyzed reaction is 5 in (a)and is 7 in (b).
C)The activation energy for the uncatalyzed reaction is given by 5 + 6 in (a)and by 7 + 4 in (b).
D)The contribution of binding energy is given by 5 in (a)and by 7 in (b).
E)The ES complex is given by 2 in (a)and 3 in (b).

Question 12

Which one of the following statements is true of enzyme catalysts?

A)They bind to substrates but are never covalently attached to substrate or product.
B)They increase the equilibrium constant for a reaction,thus favoring product formation.
C)They increase the stability of the product of a desired reaction by allowing ionizations,resonance,and isomerizations not normally available to substrates.
D)They lower the activation energy for the conversion of substrate to product.
E)To be effective,they must be present at the same concentration as their substrates.

Question 13

One of the enzymes involved in glycolysis,aldolase,requires Zn²⁺ for catalysis.Under conditions of zinc deficiency,when the enzyme may lack zinc,it would be referred to as the:

A)apoenzyme.
B)coenzyme.
C)holoenzyme.
D)prosthetic group.
E)substrate.

Question 14

Which of the following is true of the binding energy derived from enzyme-substrate interactions?

A)It cannot provide enough energy to explain the large rate accelerations brought about by enzymes.
B)It is sometimes used to hold two substrates in the optimal orientation for reaction.
C)It is the result of covalent bonds formed between enzyme and substrate.
D)Most of it is derived from covalent bonds between enzyme and substrate.
E)Most of it is used up simply binding the substrate to the enzyme.

Question 15

The concept of "induced fit" refers to the fact that:

A)enzyme specificity is induced by enzyme-substrate binding.
B)enzyme-substrate binding induces an increase in the reaction entropy,thereby catalyzing the reaction.
C)enzyme-substrate binding induces movement along the reaction coordinate to the transition state.
D)substrate binding may induce a conformational change in the enzyme,which then brings catalytic groups into proper orientation.
E)when a substrate binds to an enzyme,the enzyme induces a loss of water (desolvation)from the substrate.

Question 16

Which one of the following statements is true of enzyme catalysts?

A)Their catalytic activity is independent of pH.
B)They are generally equally active on D and L isomers of a given substrate.
C)They can increase the equilibrium constant for a given reaction by a thousand fold or more.
D)They can increase the reaction rate for a given reaction by a thousand-fold or more.
E)To be effective,they must be present at the same concentration as their substrate.

Question 17

Enzymes differ from other catalysts in that only enzymes:

A)are not consumed in the reaction.
B)display specificity toward a single reactant.
C)fail to influence the equilibrium point of the reaction.
D)form an activated complex with the reactants.
E)lower the activation energy of the reaction catalyzed.

Question 18

The benefit of measuring the initial rate of a reaction V₀ is that at the beginning of a reaction:

A)[ES] can be measured accurately.
B)changes in [S] are negligible,so [S] can be treated as a constant.
C)changes in K_m are negligible,so K_m can be treated as a constant.
D)V₀ = V_max.
E)varying [S] has no effect on V₀.

Question 19

An enzyme-catalyzed reaction was carried out with the substrate concentration initially a thousand times greater than the K_m for that substrate.After 9 minutes,1% of the substrate had been converted to product,and the amount of product formed in the reaction mixture was 12 $\mu$ mol.If,in a separate experiment,one-third as much enzyme and twice as much substrate had been combined,how long would it take for the same amount (12 $\mu$ mol)of product to be formed?

A)1)5 min
B)13.5 min
C)27 min
D)3 min
E)6 min

Question 20

Which one of the following is not among the six internationally accepted classes of enzymes?

A)Hydrolases
B)Ligases
C)Oxidoreductases
D)Polymerases
E)Transferases

Question 21

Allosteric enzymes:

A)are regulated primarily by covalent modification.
B)usually catalyze several different reactions within a metabolic pathway.
C)usually have more than one polypeptide chain.
D)usually have only one active site.
E)usually show strict Michaelis-Menten kinetics.

Question 22

The role of the metal ion (Mg²⁺)in catalysis by enolase is to:

A)act as a general acid catalyst.
B)act as a general base catalyst.
C)facilitate general acid catalysis.
D)facilitate general base catalysis.
E)stabilize protein conformation.

Question 23

In a plot of l/V against 1/[S] for an enzyme-catalyzed reaction,the presence of a competitive inhibitor will alter the:

A)curvature of the plot.
B)intercept on the l/[S] axis.
C)intercept on the l/V axis.
D)pK of the plot.
E)V_max.

Question 24

The number of substrate molecules converted to product in a given unit of time by a single enzyme molecule at saturation is referred to as the:

A)dissociation constant.
B)half-saturation constant.
C)maximum velocity.
D)Michaelis-Menten number.
E)turnover number.

Question 25

A good transition-state analog:

A)binds covalently to the enzyme.
B)binds to the enzyme more tightly than the substrate.
C)binds very weakly to the enzyme.
D)is too unstable to isolate.
E)must be almost identical to the substrate.

Question 26

Phenyl-methane-sulfonyl-fluoride (PMSF)inactivates serine proteases by binding covalently to the catalytic serine residue at the active site;this enzyme-inhibitor bond is not cleaved by the enzyme.This is an example of what kind of inhibition?

A)Irreversible
B)Competitive
C)Non-competitive
D)Mixed
E)pH inhibition

Question 27

Enzyme X exhibits maximum activity at pH = 6.9.X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4.One likely interpretation of this pH activity is that:

A)a Glu residue on the enzyme is involved in the reaction.
B)a His residue on the enzyme is involved in the reaction.
C)the enzyme has a metallic cofactor.
D)the enzyme is found in gastric secretions.
E)the reaction relies on specific acid-base catalysis.

Question 28

The allosteric enzyme ATCase is regulated by CTP,which binds to the T-state of ATCase.CTP is a:

A)positive regulator.
B)negative regulator.
C)co-factor.
D)competitive inhibitor.
E)coenzyme.

Question 29

In competitive inhibition,an inhibitor:

A)binds at several different sites on an enzyme.
B)binds covalently to the enzyme.
C)binds only to the ES complex.
D)binds reversibly at the active site.
E)lowers the characteristic V_max of the enzyme.

Question 30

To calculate the turnover number of an enzyme,you need to know:

A)the enzyme concentration.
B)the initial velocity of the catalyzed reaction at [S] >> K_m.
C)the initial velocity of the catalyzed reaction at low [S].
D)the K_m for the substrate.
E)both A and B.

Question 31

Blood coagulation involves:

A)a kinase cascade.
B)zymogen activation.
C)serine proteases.
D)A and B.
E)B and C.

Question 32

A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic site is termed a(n):

A)allosteric inhibitor.
B)alternative inhibitor.
C)competitive inhibitor.
D)stereospecific agent.
E)transition-state analog.

Question 33

V_max for an enzyme-catalyzed reaction:

A)generally increases when pH increases.
B)increases in the presence of a competitive inhibitor.
C)is limited only by the amount of substrate supplied.
D)is twice the rate observed when the concentration of substrate is equal to the K_m.
E)is unchanged in the presence of a uncompetitive inhibitor.

Question 34

Which of the following statements about allosteric control of enzymatic activity is false?

A)Allosteric effectors give rise to sigmoidal V₀ vs.[S] kinetic plots.
B)Allosteric proteins are generally composed of several subunits.
C)An effector may either inhibit or activate an enzyme.
D)Binding of the effector changes the conformation of the enzyme molecule.
E)Heterotropic allosteric effectors compete with substrate for binding sites.

Question 35

Which of the following has not been shown to play a role in determining the specificity of protein kinases?

A)Disulfide bonds near the phosphorylation site
B)Primary sequence at phosphorylation site
C)Protein quaternary structure
D)Protein tertiary structure
E)Residues near the phosphorylation site

Question 36

Penicillin and related drugs inhibit the enzyme ;this enzyme is produced by .

A)( $\beta$ -lacamase;bacteria)
B)transpeptidase;human cells
C)transpeptidase;bacteria
D)lysozyme;human cells
E)aldolase;bacteria

Question 37

A transition-state analog:

A)is less stable when binding to an enzyme than the normal substrate.
B)resembles the active site of general acid-base enzymes.
C)resembles the transition-state structure of the normal enzyme-substrate complex.
D)stabilizes the transition state for the normal enzyme-substrate complex.
E)typically reacts more rapidly with an enzyme than the normal substrate.

Question 38

How is trypsinogen converted to trypsin?

A)A protein kinase-catalyzed phosphorylation converts trypsinogen to trypsin.
B)An increase in Ca²⁺ concentration promotes the conversion.
C)Proteolysis of trypsinogen forms trypsin.
D)Trypsinogen dimers bind an allosteric modulator,cAMP,causing dissociation into active trypsin monomers.
E)Two inactive trypsinogen dimers pair to form an active trypsin tetramer.

Question 39

Both water and glucose share an -OH that can serve as a substrate for a reaction with the terminal phosphate of ATP catalyzed by hexokinase.Glucose,however,is about a million times more reactive as a substrate than water.The best explanation is that:

A)glucose has more -OH groups per molecule than does water.
B)the larger glucose binds better to the enzyme;it induces a conformational change in hexokinase that brings active-site amino acids into position for catalysis.
C)the -OH group of water is attached to an inhibitory H atom,while the glucose -OH group is attached to C.
D)water and the second substrate,ATP,compete for the active site resulting in a competitive inhibition of the enzyme.
E)water normally will not reach the active site because it is hydrophobic.

Question 40

Define the terms "cofactor" and "coenzyme."

Question 41

The difference in (standard)free energy content, $\Delta$ G'°,between substrate S and product P may vary considerably among different reactions.What is the significance of these differences?

Question 42

Two different enzymes are able to catalyze the same reaction,A $\rightarrow$ B.They both have the same V_max,but differ their K_m the substrate A.For enzyme 1,the K_m is 1.0 mM;for enzyme 2,the K_m is 10 mM.When enzyme 1 was incubated with 0.1 mM A,it was observed that B was produced at a rate of 0.0020 mmoles/minute.
a)What is the value of the V_max of the enzymes?
b)What will be the rate of production of B when enzyme 2 is incubated with 0.1 mM A?
c)What will be the rate of production of B when enzyme 1 is incubated with 1 M (i.e. ,1000 mM)A?

Question 43

Cells can develop a resistance to drugs by increasing the cellular concentration of the enzyme that that drug inhibits.If a cell increases its concentration of a given enzyme to 10 times the normal amount,which parameters listed below will be increased ten-fold?
K_m
K_I
[S]
V_max
v₀ when [S] = K_m
k_cat
Catalytic efficiency

Question 44

Fifteen $\mu$ g of an enzyme of M_r 30,000 working at V_max catalyzes the conversion of 60 $\mu$ mol of substrate into product in 3 min.What is the enzyme's turnover number?

Question 45

Methanol (wood alcohol)is highly toxic because it is converted to formaldehyde in a reaction catalyzed by the enzyme alcohol dehydrogenase:
NAD⁺ + methanol $\rightarrow$ NADH + H⁺ + formaldehyde
Part of the medical treatment for methanol poisoning is to administer ethanol (ethyl alcohol)in amounts large enough to cause intoxication under normal circumstances.Explain this in terms of what you know about examples of enzymatic reactions.

Question 46

The enzymatic activity of lysozyme is optimal at pH 5.2 and decreases above and below this pH value.Lysozyme contains two amino acid residues in the active site essential for catalysis: Glu³⁵ and Asp⁵².The pK value for the carboxyl side chains of these two residues are 5.9 and 4.5,respectively.What is the ionization state of each residue at the pH optimum of lysozyme? How can the ionization states of these two amino acid residues explain the pH-activity profile of lysozyme?

Question 47

Why does pH affect the activity of an enzyme?

Question 48

What is the difference between general acid-base catalysis and specific acid-base catalysis? (Assume that the solvent is water. )

Question 49

For the reaction E + S $\rightarrow$ ES $\rightarrow$ P the Michaelis-Menten constant,K_m,is actually a summary of three terms.What are they? How is K_m determined graphically?

Question 50

For a reaction that can take place with or without catalysis by an enzyme,what would be the effect of the enzyme on the:
(a)standard free energy change of the reaction?
(b)activation energy of the reaction?
(c)initial velocity of the reaction?
(d)equilibrium constant of the reaction?

Question 51

Write an equilibrium expression for the reaction S $\rightarrow$ P and briefly explain the relationship between the value of the equilibrium constant and free energy.

Question 52

When 10 $\mu$ g of an enzyme of M_r 50,000 is added to a solution containing its substrate at a concentration one hundred times the K_m,it catalyzes the conversion of 75 $\mu$ mol of substrate into product in 3 min.What is the enzyme's turnover number?

Question 53

Why is the Lineweaver-Burk (double reciprocal)plot (see Box 6,p.206)more useful than the standard V vs.[S] plot in determining kinetic constants for an enzyme? (Your answer should probably show typical plots. )

Question 54

How does the total enzyme concentration affect turnover number and V_max?

Question 55

Sometimes the difference in (standard)free-energy content, $\Delta$ G'°,between a substrate S and a product P is very large,yet the rate of chemical conversion,S $\rightarrow$ P,is quite slow.Why?

Question 56

Define the term "suicide inhibitor."

Question 57

Michaelis-Menten kinetics is sometimes referred to as "saturation" kinetics.Why?

Question 58

An enzyme follows Michaelis-Menten kinetics.Indicate (with an "x")which of the kinetic parameters at the left would be altered by the following factors.Give only one answer for each.

Question 59

Give the Michaelis-Menten equation and define each term in it.Does this equation apply to all enzymes? If not,to which kind does it not apply?

Question 60

An enzyme catalyzes a reaction at a velocity of 20 $\mu$ mol/min when the concentration of substrate (S)is 0.01 M.The K_m for this substrate is 1 * 10^-5 M.Assuming that Michaelis-Menten kinetics are followed,what will the reaction velocity be when the concentration of S is (a)1 * 10^-5 M and (b)1 * 10^-6 M?

Question 61

For serine to work effectively as a nucleophile in covalent catalysis in chymotrypsin a nearby amino acid,histidine,must serve as general base catalyst.Briefly describe,in words,how these two amino acids work together.

Question 62

Penicillin and related antibiotics contain a 4-membered $\beta$ -lactam ring.Explain why this feature is important to the mechanism of action of these drugs.

Question 63

Why is a transition-state analog not necessarily the same as a competitive inhibitor?

Question 64

Chymotrypsin belongs to a group of proteolytic enzymes called the "serine proteases," many of which have an Asp,His,and Ser residue that are crucial to the catalytic mechanism.The serine hydroxyl functions as a nucleophile.What do the other two amino acids do to support this nucleophilic reaction?

Question 65

On the enzyme hexokinase,ATP reacts with glucose to produce glucose 6-phosphate and ADP five orders of magnitude faster than ATP reacts with H₂O to form phosphate and ADP.The intrinsic chemical reactivity of the -OH group in water is about the same as that of the glucose molecule,and water can certainly fit into the active site.Explain this rate differential in two sentences or less.

Question 66

Explain how a biochemist might discover that a certain enzyme is allosterically regulated.

Question 67

What is a zymogen (proenzyme)? Explain briefly with an example.