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9. Enzyme Kinetics

Biochemistry 233 (Curtin University)

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Enzyme Kinetics

Questions and Answers

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Table of Contents
Multiple Choice........................................................................................................................................................1
Question 1............................................................................................................................................................1
Question 2............................................................................................................................................................1
Question 3............................................................................................................................................................1
Question 4............................................................................................................................................................1
Question 5............................................................................................................................................................1
Question 6............................................................................................................................................................1
Question 7............................................................................................................................................................1
Question 8............................................................................................................................................................1
Question 9............................................................................................................................................................1
Question 10..........................................................................................................................................................1
Tue or False...............................................................................................................................................................2
Question 1............................................................................................................................................................2
Question 2............................................................................................................................................................2
Question 3............................................................................................................................................................2
Question 4............................................................................................................................................................2
Question 5............................................................................................................................................................2
Question 6............................................................................................................................................................2
Question 7............................................................................................................................................................2
Question 8............................................................................................................................................................2
Question 9............................................................................................................................................................2
Question 10..........................................................................................................................................................2
Fill in the Blanks........................................................................................................................................................3
Question 1............................................................................................................................................................3
Question 2............................................................................................................................................................3
Question 3............................................................................................................................................................3
Question 4............................................................................................................................................................3
Question 5............................................................................................................................................................3
Question 6............................................................................................................................................................3
Question 7............................................................................................................................................................3
Question 8............................................................................................................................................................3
Question 9............................................................................................................................................................3
Question 10..........................................................................................................................................................3
Answers....................................................................................................................................................................3
Multiple Choice Answers.....................................................................................................................................3
True or False Answers..........................................................................................................................................4

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Fill in the Blanks...................................................................................................................................................4

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Multiple Choice

Question 1
The benefit of measuring the initial rate of a reaction, Vo, is that at the beginning of a reaction from substrate
[S] to product [P]:
A. changes in [S] and [P] are negligible.
B. [ES] can be measured accurately.
C. Vo = Vmax.
D. changes in Km are negligible, so Km can be treated as a constant.
E. varying [S] has no effect on Vo.

Question 2
To calculate the turnover number of an enzyme you need to know the:
A. initial velocity of the catalyzed reaction at low [S].
B. initial velocity of the catalyzed reaction at [S] >> Km.
C. Km for the substrate.
D. enzyme concentration.
E. both B and D.

Question 3
A biotechnology company has cloned four different forms of the enzyme money synthetase which catalyzes the
reaction:
Garbage + ATP = Money + ADP + Phosphate + H+
The Km and Vmax values of these enzymes for the substrate garbage are as follows:
Enzyme 1; Km = 0.1mmol Vmax = 5.0 mmol/min
Enzyme 2; Km = 0.3 mmol Vmax = 2.0 mmol/min
Enzyme 3; Km = 1.0 mmol Vmax = 5.0 mmol/min
Enzyme 4; Km = 3.0 mmol Vmax = 20.0 mmol/min
Which of the four enzymes is fastest at a saturating ATP concentration and a garbage concentration of 10.0
mmol/L?
A. Enzyme 1
B. Enzyme 2
C. Enzyme 3
D. Enzyme 4
E. none of the above

Question 4
The Lineweaver-Burk plot is used to:
A. solve, graphically, for the ratio of products to reactants for any starting substrate concentration.
B. illustrate the effect of temperature on an enzymatic reaction.
C. determine the equilibrium constant for an enzymatic reaction.
D. solve, graphically, the rate of an enzymatic reaction at infinite substrate concentration.
E. extrapolate for the value of reaction rate at infinite enzyme concentration.

Question 5
The initial velocity of an enzyme reaction (vo) describes

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A. the concentration of the enzyme at maximal velocity.

B. the concentration of substrate at maximal velocity.
C. the concentration of enzyme and substrate at the start of the reaction.
D. the rate of the reaction at when the substrate and enzyme are first mixed

Question 6
In an enzyme reaction involving one enzyme and one substrate, the rate of the reaction depends on
A. substrate concentration.
B. enzyme concentration.
C. both substrate and enzyme concentrations.
D. the enzyme concentration at first and the substrate concentration later on.

Question 7
For the following enzme catalysed reaction:
E + S = ES = E + P
At the beginning of an enzyme-catalyzed reaction the ________ is negligible.
A. formation of ES
B. formation of E + P
C. conversion of ES to E + S
D. disappearance of ES

Question 8
The Michaelis constant, Km, is equal to the ________.
A. maximum velocity that any given enzyme reaction can achieve
B. substrate concentration which gives the best enzyme assay for an enzyme reaction
C. substrate concentration when the rate is equal to half its maximal value
D. maximum velocity divided by two

Question 9
Alchohol dehydrogenase (ADH) requires NAD+ for catalytic activity. In the reation catalysed by ADH an alcohol
is oxidized to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. The NAD+ is
functioning as:
A. apoenzyme
B. heterotropic effector
C. coenzyme and cosubstrate
D. coenzyme and prosthetic group
E. cofactor

Question 10
Allosteric modulators seldom resemble the substrate or product of the enzyme. What does this observation
show?
A. Modulators likely bind at a site other than the active site.
B. Modulators always act as activators.
C. Modulators bind non-covalently to the enzyme.
D. The enzyme catalyzes more than one reaction

Question 11
An inhibitor binds to a site other than the active site of the enzyme. Which statement below correlates with
this observation?
A. It must be a competitive inhibitor.

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B. The inhibition must be irreversible.

C. It could be noncompetitive or uncompetitive inhibition.
D. It could be irreversible, competitive, noncompetitive or uncompetitive.

Question 12
The enzyme fumarase catalyzes the reversible hydration of fumaric acid to L-malate; however, the enzyme will
not catalyze the hydration of maleic acid, the cis isomer of fumaric acid. This is an example of:
A. chiral activity.
B. stereospecificity.
C. racemization.
D. stereoisomerization.
E. biological activity.

Question 13
Which of the following parameters remains the same for S → P, regardless of whether the reaction is enzyme-
catalyzed or uncatalyzed?
A. rate constant k
B. velocity V
C. Initial velocity V0
D. Equilibrium constant Keq'
E. Michaelis constant Km

Question 14
Why is it important that the enzymes in lysosomes are more active at acidic pH than at neutral pH?
A. Since lysosomes are primarily found in the stomach acid of mammals, their pH dependence allows for
maximum efficiency for the digestion of foodstuffs
B. It prevents their diffusion out of the lysosomes.
C. It maximizes the interaction with their substrates which are always bases.
D. It prevents them from accidentally degrading the macromolecules in the cytosol.
E. It allows for regulation of their uptake by the mitochondria.

Question 15
Which of the following statements about a plot of V0 vs. [S] for an enzyme that follows Michaelis-Menten
kinetics is false?
A. Km is the [S] at which V0 = 1/2 Vmax.
B. The shape of the curve is a hyperbola.
C. The y-axis is a rate term and the units could be something like μmol/min.
D. As [S] increases, the initial velocity of reaction, V0, also increases.
E. At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.

Question 16
The dependence of the initial rate of an enzyme-catalysed reaction (vo) on the initial substrate concentration
([S]) was studied by adding the same amount of enzyme to a series of reaction mixtures containing different
initial concentrations of substrate. Values of Vo were determined by measuring the amount of substrate
converted to product per min in each reaction mixture. The volumes of all reaction mixtures were the same
and each mixture contained a total enzyme concentration of 10-9M. Data for the experiment were obtained as
shown in the following table.

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Choose the most appropriate answer to the question:

What is the estimate of Vmax if the total enzyme concentration in each reaction mixture is doubled?
A. 1.00 micromol per minute
B. Zero
C. 0.50 micromole per minute
D. 2.50 x 10-2M
E. 2.50 x 10-5M

Question 17
Which of the following is NOT true of enzyme-catalysed reactions?
A. At high substrate concentrations, relative to the Km, the initial rate is independent of the substrate
concentration
B. At low substrate concentrations, relative to the Km, the initial rate is proportional to the substrate
concentration
C. The maximum rate of reaction is proportional to the concentration of the substrate
D. The substrate concentration which gives a half-maximum rate is sometimes a good measure of the
binding affinity of the enzyme for the substrate
E. At high substrate concentration, relative to the Km, the initial rate is proportional to the concentration
of the enzyme

Question 18
The release of free energy as a result of the interaction between enzyme and substrate is sometimes called the
binding energy. Which of the following is true of the binding energy derived from enzyme-substrate
interactions?
A. Most of it is used for enzyme stability
B. Most of it is derived from covalent bonds between enzyme and substrate.
C. It is sometimes used to hold two substrates in the optimal orientation for reaction.
D. It is the difference in initial and final free energy between reactants and products.
E. It is used to provide more kinetic energy to the reactants

Question 19
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. maximum velocity.
B. half-saturation constant.
C. dissociation constant.
D. Michaelis-Menten number.
E. turnover number.

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Question 20
What are two essential things to consider when you want to determine the total amount of enzyme in a
sample (given that the temperature, ionicstrength, and pH optimum have been determined)?
A. Max velocity is measured and [S]>>Km
B. Maximum velocity is measured and [S]<
C. Initial velocity is measured and [S] << Km
D. Initial velocity is measured and [S] >>Km
E. None of the above

Question 21
Allosteric enzymes:
A. depend mainly upon covalent modification of the enzyme
B. bind ligands reversibly
C. have a Km that is 1/3 Vmax
D. usually show strict Michaelis-Menten kinetics.
E. have a Vmax that depends only on substrate concentration

Question 22
In a first order chemical reaction, the velocity of the reaction is proportional to the ________, while in a zero
order reaction, the velocity of the reaction is proportional to the ________.
A. amount of enzyme; concentration of substrate
B. concentration of substrate; amount of enzyme
C. concentration of substrate; speed of the reaction
D. speed of the reaction; concentration of substrate

Question 23
The Km values for enzyme reactions such as A + B → C + D
A. cannot be determined using the Lineweaver-Burk plot analysis.
B. can be determined by holding one (A or B) at high concentration, while varying the concentration of
the other substrate.
C. can be determined for one substrate and not the other.
D. do not indicate the efficiency of the enzyme.

Question 24
The following data were obtained in a study of an enzyme known to follow Michaelis-Menten kinetics:

The Km for this enzyme is approximately:

A. 488 μmol/min
B. 1 mM.
C. 325 μmol/min
D. 2 mM.

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E. 6 mM.

Question 25
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. The Michaelis-Menten constant Km equals the [S] at which V = 1/2 Vmax.
C. If enough substrate is added, the normal Vmax of a reaction can be attained even in the presence of a
competitive inhibitor.
D. The rate of a reaction decreases steadily with time as substrate is depleted.
E. 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.

Question 26
Enzymes differ from other catalysts in that enzymes:
A. lower the activation energy of the reaction catalyzed.
B. fail to influence the equilibrium point of the reaction.
C. form an activated complex with the reactants.
D. usually display specificity toward a single reactant.
E. are not consumed in the reaction.

Question 27
An enzyme that catalyzes conversions of L-sugars to D-sugars is called a/an ________.
A. lyase
B. hydrolase
C. synthetase
D. synthase
E. isomerase

Question 28
The Lineweaver-Burk plot and other linear transformation of the Michaelis-Menten curve of kinetics are
valuable for:
A. determination of Km.
B. determination of Vmax.
C. determination of kcat.
D. determination of types of enzyme inhibition.
E. All of the above

Question 29
It is difficult to determine either Km or Vmax from a graph of velocity verses substrate concentration because:
A. too much substrate is required to determine them.
B. the graph is sigmoid.
C. an asymptotic value must be determined from the graph.
D. the points on the graph are often not spread out on the hyperbola.

Question 30
The steady state assumption, as applied to enzyme kinetics, implies:
A. Km = Ks.
B. the maximum velocity occurs when the enzyme is saturated.
C. the ES complex is formed and broken down at equivalent rates.
D. the Km is equivalent to the cellular substrate concentration.

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E. the enzyme is regulated.

Question 31
The time that is required for an enzyme to convert one substrate molecule into one product molecule is
A. Km
B. Kcat
C. 1/Km
D. 1/kcat

Question 32
The Michaelis-Menton model of enzyme action
A. explains the mechanism of allosteric behaviour of certain regulatory enzymes
B. explains the mechanism for the saturation effect observed at high substrate concentrations
C. explains the mechanism of stereospecificity of enzyme reactions
D. assumes that the mechanism for the formation of an enzyme and substrate complex is a covalent
bond
E. explains the mechanism for the catalytic efficiency of enzymes

Question 33
Which of the following statements does not apply to the effect of temperature on an enyme-catalysed
reaction?
A. At high temperatures the enzyme may be denatured
B. The temperature at which the maximum occurs on the activity versus temperature graph is called the
optimum temperature
C. At low temperatures the rate doubles (approximately) for each 10°C rise
D. Denatured enzymes always regain their lost activity on cooling
E. Enzymes are inactivated both by being held for long periods of time at moderate temperatures and by
being held for short periods of time at high temperatures

Question 34
In the Lineweaver-Burk plot of an enzyme reaction, the Km is given by the ________.
A. x-intercept
B. y-intercept
C. negative reciprocal of the x-intercept
D. reciprocal of the y-intercept

Question 35
Chemical reactions in the absence of a catalyst may be distinguished from enzyme-catalysed reactions because
A. chemical reactions are saturable with substrate
B. enzyme-catalysed reactions produce a variety of products from a single substrate
C. chemical reactions proceed at lower temperatures
D. the free energy of activation is lower for an enzyme-catalysed reaction
E. enzyme-catalysed reactions are not first-order with respect to substrate concentration below the Km
value

Question 36
Which of the following statements is true of enzyme catalysts?
A. They increase the stability of the product of a desired reaction by allowing ionizations, resonance, and
isomerizations not normally available to substrates.
B. They lower the free energy of the reaction
C. They increase the equilibrium constant for a reaction, thus favoring product formation.

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D. They lower the free energy of the transition state

E. They specifically bind to substrates, but are never covalently attached to substrate or product.

Question 37
Phosphorylation that changes an enzymeʹs activity is an example of ________.
A. covalent modification
B. allosteric regulation
C. sequential modification
D. site-directed mutagenesis

Question 38
When varying the substrate concentration at a fixed concentration of enzyme it is observed that at low
concentrations of substrate the reaction is ________,while at high concentrations of substrate the reaction is
________.
A. maximal; initial
B. initial; maximal
C. second order; first order
D. first order; second order
E. first order; zero order

Question 39
Consider the following information:
Acid-base catalysis is the general term given to an enzyme-catalysed reaction that uses ionized groups in the
enzyme active site to act as proton donors or acceptors. The ionizable groups that participate in this reaction
must be in the correct protonation state for catalysis to occur. For example, if glutamate (about pKa 4) is to be a
proton acceptor (base) and histidine (about pKa 6.0) is to be a proton donor (acid) then acid-bases catalysis
can best occur over a pH range of 4.0 to 6.0.
Some pKa's for amino acid side chains are as follows:
E = 4.3
D = 3.9
C = 8.3
Y = 10.1
K = 10.8
R = 12.5
H = 6.0
Now answer this question:
An enzymatic reaction works best at pH 6 to 8. This is compatible with the assumption that the reaction
mechanism requires two ionizable amino acid side chains in the active site of the enzyme possibly:
A. protonated glutamate and deprotonated aspartate
B. protonated histidine and deprotonated lysine
C. protonated glutamate and deprotonated histidine
D. protonated arginine and deprotonated lysine
E. protonated cysteine and deprotonated histidine

Question 40
Vmax for an enzyme-catalyzed reaction:
A. generally increases when pH increases.
B. increases in the presence of a competitive inhibitor.
C. is unchanged in the presence of a uncompetitive inhibitor.

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D. is about twice the rate observed when the concentration of substrate is equal to the Km.
E. is limited only by the amount of substrate supplied.

True or False

Question 1
Enzymes are not as efficient as most catalysts used in organic chemistry, since they must function at body
temperature.
_________

Answers

Multiple Choice Answers

Answers Response Feedback

1 A If changes in [S] and [P] are negligible then it means the reaction will not be affected
either by depletion of substrate or the increased concentration of product. Depletion
of substrate would affect the rate of the reaction and increased product could affect
the rate of reaction by feedback inhibition.
2 E Turnover number is the number of times an enzyme molecule
transfroms a substrate molecule in a given time. It is calculated from
the equation: kcat = Vmax/[E]t. Remember, that Vmax is determined
at [S] >>Km
3 D An enzyme with a low Km, compared with an enzyme with a high Km,
would reach its 1/2Vmax sooner when incubated with a substrate
concentration that is less then its Km. The velocity can be calculated
from the Michaelis-Menton equation since this is the equation that
predicts the change in velocity with a change in substrate
concentration. The question stipulates that ATP is at saturating
concentration which means that it has no effect on velocity (it is at
the zero order part of the Michaelis-Menton curve), so that the only
change that would affect velocity would be the change in garbage
concentration.

v = (Vmax x [S])/Km + [S]

Enzyme 1
v = 5 x 10/0.1 + 10
v = 50/10.1 = 4.95 mmol/min

Enzyme 2
v = 2 x 10/0.3 + 10
v = 20/10.3 = 1.94 mmol/min

Enzyme 3
v = 5 x 10/1.0 +10
v = 50/11 = 4.54 mmol/min

Enzyme 4
v = 20 x 10/3.0 + 10

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v = 200/13 = 15.4 mmol/min

4 D The Lineweaver-Burk Plot is a linear transformation of the Michaelis-Menton plot. It

enables estimates to be made of Vmax (which approaches a limiting value at infinite
substrate concentration) and Km
5 D
6 C
7 B
8 C
9 C Coenzymes and cosubstrates are small organic molecules that associate transiently
with an enzyme and leave the enzyme in a changed form. Coenzyme-prosthetic
groups are small organic molecules that associate permanently with an enzyme and
are returned to their original form by the enzyme. Cofactors are metal ions.
Heterotropic effectors are not substrates.
10 A
11 C
12 B Enzymes are stereospecific. That is, the ability of the enzyme to specifically recognise
its particular substrate in 3D space.
13 D Enzymes do not affect the equilibrium constant of a reaction and so this parameter is
the same for particular conditions regardless of the reaction being catalysed or
uncatalysed.
14 D
15 E At very high [S], the velocity curve becomes an asymptote that approaches a limiting
value of Vmax. It does not intersect the y-axis
16 A Vmax is proportional to the enzyme concentration. Doubling the enzyme
concentration will produce a new Vmax of 2 x 0.50 micromol per minute = 1.00
micromole per minute.
17 C The initial velocity of an enzyme-catalysed reaction approaches a limiting value, Vmax,
as the substrate concentration is increased without limit. That is, the Vmax is
independent of the substrate concentration it is a zero order part of the Michaelis-
Menton curve.
18 C Many weak interactions can be formed between enzyme and substrate and contribute
to this binding energy. This binding energy is responsible for lowering the activation
energy of the reaction by increasing the binding to the transition state. This introduces
strain into the system and allows favourable interactions between the enzyme's active
site (its catalytic residues) and the reactants. This energy is potential rather than
kinetic energy.
19 E Turnover number is otherwise known as kcat = Vmax/[Et]. Where [Et] = total enzyme
concentration
20 D Vmax is proportional to the total amount of enzyme but must not be affected by
substrate depletion or product inhibition. Consequently, you need to choose
conditions that will give you a good estimate of Vmax and these are initial velocity (to
avoid substrate depletion and product inhibition) and excess substrate concentration
(so that velocity approaches the limiting value of Vmax)
21 B Allosteric enzymes usually usually have more than one polypeptide chain although
there are some instances where an allosteric enzyme is believed to have only one
polypeptide chain. Ligands (a metabolite or other chemical group) exert their effect by
binding non-covalently, and reversibly, to a site on the enzyme that is distinct from the
active site. This usually results in the sigmoid (S-shape) of the velocity-substrate curve.
Subtrate itself can be the cooperative ligand where the binding of substrate on one
subunit changes the conformation and affects the conformation of the other subunits
making it easier to bind more substrate. The idea here is that the subunits interact
with each other at their respective allosteric sites.
22 B
23 B

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24 D Km is defined as the substrate concentration that is half maximum velocity. This data
shows very high substrate concentration and a rate of 647 μmol/min this is probably
the close to the limiting value of Vmax. Hence the approximate Km would be the
substrate concentration at half of this value = 2mM
25 E The activation energy for a chemical reaction is decreased by an enzyme. An enzyme
does not affect the equilibrium of a reaction.
26 D The principal distinction for enzyme catalysts compared with non-enzyme catalysts is
that they are very specific for the reactions that they catalyse. This specificity is one of
the reasons for enzymes considerable efficacy compared with non-enzyme catalysts.
27 E
28 E
29 C
30 C Steady-state assumes that the enzyme substrate complex does not change appreciably
during the reaction. This assumption is important because it allows us to explain the
velocity-substrate curve, derive the Michaelis-Menton equation, and the parameters
Vmax and Km
31 D
32 B Michaelis-Menton (M-M) model of enzyme action assumes that the enzyme and
substrate bind to form an enzyme-substrate complex. Thus, there is a limited amount
of enzyme available to bind to substrate and this explains the mechanism for the
saturation effect at high substrate concentrations. Interactions between enzyme and
substrate can be either covalent or non-covalent but most tend to be non-covalent
interactions. The Michaelis-Menton model cannot directly explain the *mechanism*
of allosteric behaviour, stereospecificity, or catalytic efficiency of enzymes. However,
the M-M model does suggest that these properties are a result of a specific
interaction of enzyme and substrate.
33 D In general denaturation is an irreversible process
34 C
35 D Enzyme catalysed reactions are more rapid than the equivalent chemical reaction in
aqueous solution. Thus the activation energy must be lower in the presence of an
enzyme
36 D Enzymes lower the free energy of activation of the transition state between substrates
and products. They can do this by stabilizing the transition state through things such
as: affecting entropy between the substrates such that they combine in the correct
geometric orientation, and binding strongly to the transition state through particular
non-covalent interactions.
37 A
38 E
39 E The effective range is pH 6 to 8 hence you should look for the pK's of side chains that
are within that range. The only pair to fit this range is cysteine and histidine. Cysteine
would be mostly protonated within the range because it's pKa is 8.3 and any pH below
the pKa means that it will be mostly in the acid form. Histidine would be mostly in the
deprotonated form at any pH above the pKa of 6.0.
40 D Km is defined as the substrate concentration at which 1/2Vmax is reached. Vmax is
unaffected by competitive inhibitors but is affected by uncompeptitive inhibitors.
Vmax is affected by the amount of enzyme, pH, temperature and substrate
concentration.

True or False Answers

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