Deck 5: Properties of Enzymes

Question 1

The Michaelis constant, K_m, 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

Answer 1

substrate concentration when the rate is equal to half its maximal value

Question 2

The initial velocity of an enzyme reaction v₀) describes

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

Answer 2

the rate of the reaction at when the substrate and enzyme are first mixed.

Question 3

Oxidases, peroxidases, oxygenases or reductases are all

A) lyases.
B) synthases.
C) synthetases.
D) oxidoreductases.
E) hydrolases.

Answer 3

oxidoreductases.

Question 4

When two different substrates react to form two different products, the rate constants for each separate substrate can be determined by

A) varying one substrate at a time, keeping the other in excess.
B) varying both substrates and measuring the appearance of the two products.
C) limiting one substrate and varying the other.
D) keeping both substrate concentrations high and detecting one product at a time.

Question 5

When there are two substrates in a reaction, the reaction is said to be second order if .

A) the reaction is first order with respect to each substrate concentration
B) the maximum rate is independent of either substrateʹs concentration
C) the substrate concentrations are equal
D) it proceeds at twice the rate upon double the concentrations of both substrates

Question 6

The assumptions made in calculating the Michaelis-Menten Equation include

A) that the formation and decomposition of ES is the same for a period of time.
B) that the concentration of the substrate is much greater than the concentration of E.
C) that the value of k_-2 can be ignored.
D) A, B and C
E) A and B only

Question 7

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 8

Which enzyme below is fastest?

A) kinase, k_cat = 10³
B) papain, k_cat = 10
C) carboxypeptidase, k_cat = 10²
D) catalase, k_cat = 10⁷

Question 9

Which of the following statements is FALSE?

A) Enzymes make reactions 10³ to 10²⁰ times faster.
B) Enzymes lower the amount of energy needed for a reaction.
C) Enzymes are chemically unchanged during the actual catalytic process.
D) Enzymes speed up the attainment of a reaction equilibrium.
E) Enzymes are proteins.

Question 10

The main difference between chemical and enzyme kinetics is that

A) enzyme reactions are altered by pH.
B) enzyme reactions depend on the concentration of the substrate.
C) enzyme reactions depend on the concentration of the enzyme and its recycling.
D) the rate constant for the formation of products is k₂.

Question 11

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 12

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 13

Unlike typical catalyzed reactions in organic chemistry enzyme reactions are

A) usually stereospecific.
B) reaction specific.
C) essentially 100% efficient.
D) modulated to change activity levels.
E) All of the above

Question 14

Most of the known enzymes are .

A) oxidoreductases
B) transferases
C) hydrolases
D) lyases
E) isomerases

Question 15

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 16

The concentrations of enzymes are determined by using pseudo first-order conditions in which

A) S concentration is kept the same and the rate is measured.
B) E concentration is kept the same and the rate is measured.
C) S concentration is constant, E concentration varied and the rate is measured.
D) E concentration is constant, S concentration varied and the rate is measured.
E) All of the above

Question 17

The non-enzymatic hydrolysis of sucrose into glucose and fructose is an example of a pseudo first-order reaction because

A) no enzymes are involved.
B) water has no role in the reaction.
C) water is of such high concentration as to be considered constant.
D) water is present at concentrations proportional to the sucrose.

Question 18

The hydrophobic cleft in globular proteins which bind substrate molecules is called the .

A) substrate pocket
B) modulator site
C) active site
D) activity site
E) oligomeric site

Question 19

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 20

Which of the following statements is FALSE?

A) The word enzyme is from a Greek word meaning ʺin yeast.ʺ
B) Enzymes are always made of protein.
C) The first enzyme was crystallized in 1926.
D) Enzymes can couple two different reactions.

Question 21

Which might be a method used for distinguishing among several mechanistic possibilities in a multi-substrate reaction?

A) Measure K_m in the presence of one substrate at a time.
B) Measure the rate with respect to one substrate while varying the concentration of another substrate.
C) Measure the rate while adding inactivators for all but one substrate.
D) Any of the above

Question 22

The catalytic proficiency of an enzyme is the .

A) turnover number
B) K_m/[E]
C) rate constant with the enzyme divided by the rate constant without the enzyme
D) rate constant with the enzyme times the rate constant without the enzyme

Question 23

In a certain enzyme-catalyzed reaction the following steps occur: 1. A phosphate group on substrate A is transferred to a side chain of an active site residue of the enzyme.
2. The dephosphorylated form of substrate A dissociates from the enzyme.
3. Substrate B enters the active site and is phosphorylated with simultaneous regeneration of the enzyme in its original form.
What kind of kinetic mechanism is described?

A) random
B) sequential
C) ordered
D) ping-pong

Question 24

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.
B) The inhibition must be irreversible.
C) It could be noncompetitive or uncompetitive inhibition.
D) It could be irreversible, competitive, noncompetitive or uncompetitive. The data do not relate to the type of inhibition.

Question 25

The enymatic rate constant k_cat/K_m) of orotidine 5ʹ-phosphate decarboxylase is 6 x 10⁷ M^-1s^-1 and the nonenzymatic rate constant k_n) is 3 x 10^-16 s^-1. What is the value of the enzymeʹs catalytic proficiency?

A) 2 x 10²³ M^-1
B) 5 x 10^-24 M
C) 12 x 10^-9 M^-1s^-2
D) 8.3 x 10⁷ Ms²
E) Cannot calculate the proficiency without knowing the value of V_max.

Question 26

Which equilibrium below applies to noncompetitive inhibition?

A) I
B) II
C) III
D) IV

Question 27

What distinguishes reversible inhibitors from irreversible inhibitors?

A) Reversible inhibitors are not covalently bound to enzymes but irreversible inhibitors are.
B) There is an equilibrium between bound and unbound reversible inhibitor. There usually is little back reaction for the binding of an irreversible inhibitor.
C) Reversible inhibitors are easier to purify from solutions of enzymes than irreversible inhibitors.
D) All of the above
E) A and B only

Question 28

In a ping-pong reaction which does not occur?

A) One product is released before a second substrate is bound.
B) The enzyme covalently binds a portion of the first substrate.
C) The enzyme is permanently converted to an altered form by the first substrate.
D) A group is transferred from one substrate to another.

Question 29

Nonclassical competitive inhibition involves .

A) binding of the substrate at both the active site and at the inhibitor site
B) binding of either the substrate to the active site or the inhibitor to its own binding site thus preventing the other from binding
C) binding of the inhibitor to the substrate followed by binding of this complex to the active site
D) chemical removal of the substrate from the active site by reaction with the inhibitor

Question 30

The ratio of k_cat/K_m for each of two different substrates present in equal concentrations in an enzyme reaction will measure enzyme affinity because

A) the rates of P formation are given by the values found for each substrate.
B) the rates of P formation are the same for each substrate.
C) it is an indication of the formation of ES for each substrate.
D) All of the above

Question 31

The expression: V_max = k₂[E]_total applies to

A) zero order kinetics of an enzyme-catalyzed reaction.
B) first order kinetics of an enzyme-catalyzed reaction.
C) second order kinetics of an enzyme-catalyzed reaction.
D) only the initial part near time = zero) of an enzyme-catalyzed reaction.

Question 32

The lower, higher) the value of K_m, the less, more) tightly the enzyme is bound to the substrate.

A) lower, less
B) lower, more
C) higher, less
D) higher, more
E) B and C

Question 33

The Lineweaver-Burk plot and other linear transformation of the Michaelis-Menten curve of kinetics are valuable for

A) determination of K_m.
B) determination of V_max.
C) determination of k_cat.
D) determination of types of enzyme inhibition.
E) All of the above

Question 34

Two substances are used to produce a certain biological product in an enzyme-catalyzed reaction. It is found that both substrates must bind to the enzyme, first one, then the other before the product is produced. This is an example of a/an .

A) linear reaction
B) ordered sequential reaction
C) random sequential reaction
D) ping-pong reaction

Question 35

The time that is required for an enzyme to convert one substrate molecule into one product molecule is

A) K_m.
B) k_cat.
C) 1/K_m.
D) 1/k_cat.

Question 36

The K_m 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 37

In the Lineweaver-Burk plot of an enzyme reaction, the K_m is given by the .

A) x-intercept
B) y-intercept
C) negative reciprocal of the x-intercept
D) reciprocal of the y-intercept

Question 38

It is difficult to determine either K_m or V_max from a graph of velocity vs. substrate concentration because

A) too much substrate is required to determine them.
B) the graph is sigmoidal.
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 39

Which statement is true about the removal of reversible inhibitors from enzyme solutions?

A) PAGE is the primary technique used for this purpose.
B) Dialysis and gel filtration are often used.
C) These inhibitors cannot be separated from the enzyme without treatment with diisopropylfluorophosphate.
D) Removal of reversible inhibitors is extremely difficult and can be achieved only after many purification steps.

Question 40

The reason to rewrite the Michaelis-Menten equation such as the Lineweaver-Burk plot) is to

A) visualize reactions better.
B) form enzyme kinetic data as a hyperbolic curve.
C) calculate catalytic proficiency.
D) calculate V_max and K_m.

Question 41

The theory explains cooperative binding by suggesting all subunits of a given protein have the same conformation, either all R or all T.

A) concerted
B) entropy-driven
C) sequential
D) simultaneous

Question 42

An enzyme is irreversibly inhibited by diisopropylfluorophosphate DFP). What does this show?

A) The enzyme has been denatured by DFP.
B) Serine is likely an important residue in the active site.
C) DFP is an allosteric modulator of the enzyme.
D) DFP is an analog of the enzymeʹs substrate.

Question 43

Protein A has four identical subunits, each of which binds one molecule of ligand Y. The binding of one molecule of Y to one of the subunits induces a conformational change in neighboring subunits that enhances the binding of additional units of Y. This is an example of .

A) negative cooperativity
B) competitive activation
C) symmetry-driven binding
D) the sequential theory

Question 44

The following data were obtained in the presence and absence of inhibitor. What type of inhibition is shown? $\begin{array}{|l|l|l|}\hline\begin{array}{l}\text { Substrateconcentration } \\\text { millimolar) }\end{array} & \begin{array}{l}\text { Rate withoutinhibitor } \\\mathrm{mmol} / \mathrm{min})\end{array} & \begin{array}{l}\text { Rate with inhibitor } \\\mathrm{mmol} / \mathrm{min})\end{array} \\\hline 0.100 & 2.5 & 1.6 \\\hline 0.200 & 4.2 & 2.9 \\\hline 0.500 & 6.6 & 5.1 \\\hline 0.750 & 7.4 & 6.2 \\\hline 1.000 & 9.9 & 9.8 \\\hline 2.000 & 10.0 & 10.1\\\hline\end{array}$

A) competitive
B) uncompetitive
C) noncompetitive
D) irreversible
E) cannot tell inhibition type from the information given

Question 45

Which statement is false about regulatory enzymes that are controlled allosterically?

A) They are always less active when a modulator is bound to them.
B) They are often larger than other enzymes.
C) They have more than one binding site.
D) They often catalyze the first step in a reaction pathway.

Question 46

Which statement is false about covalent modification?

A) It is reversible.
B) It is slightly slower than allosteric regulation.
C) It usually uses the same enzyme for activation and inactivation.
D) All of the above

Question 47

In E. coli is an activator and is a negative allosteric modulator of the enzyme phosphofructokinase-1.

A) DFP; phosphoenolpyruvate
B) phosphoenolpyruvate; ATP
C) ADP; phosphoenolpyruvate
D) fructose-6-phosphate; ADP

Question 48

In a multienzyme complex the process of directly transferring a product of one reaction to the next active site without allowing it to enter the bulk solvent is termed .

A) a ping-pong reaction
B) metabolite channeling
C) the activity pathway
D) the sequential mode

Question 49

Phosphorylation that changes an enzymeʹs activity is an example of .

A) covalent modification
B) allosteric regulation
C) sequential modification
D) site-directed mutagenesis

Question 50

Which statement is false about allosteric regulation?

A) It is usually the mode of regulation for the last step in reaction pathways since this step produces the final product.
B) Cellular response is faster with allosteric control than by controlling enzyme concentration in the cell.
C) The regulation usually is important to the conservation of energy and materials in cells.
D) Allosteric modulators bind non-covalently at sites other than the active site and induce conformational changes in the enzyme.

Question 51

Interconvertible enzymes .

A) are those controlled by covalent modification
B) follow a concerted mechanism to go back and forth between the T and R states
C) catalyze reactions to covalently modify another enzyme
D) are allosteric modulators

Question 52

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 53

Which is not a reason for metabolite channeling?

A) Protection of intermediates from degradation.
B) Increasing the overall rate of a reaction.
C) Producing locally high concentrations of intermediates.
D) Ensuring the enzyme is properly regulated.

Question 54

Two curves showing the rate versus substrate concentration are shown below for an enzyme-catalyzed reaction. One curve is for the reaction in the presence of substance X. The other curve is for data in the absence of substance X. Examine the curves and tell which statement below is false.

A) X is an activator of the enzyme.
B) The enzyme exhibits non-Michaelis-Menten kinetics.
C) X is likely an allosteric modulator.
D) X is a competitive inhibitor.

Question 55

Which statement is false about the sequential theory?

A) It treats the concerted theory as a limiting simple case.
B) It allows for a distribution of high and low affinity subunits in the same protein.
C) It assumes more than one conformation of a particular subunit can have high affinity for the ligand.
D) It is also called the ligand-induced theory and is more general than the concerted theory.

Question 56

The ʺTʺ state refers to the .

A) inactive conformation of the enzyme
B) transition state of the product
C) form of the enzyme without the modulator bound at the regulatory site
D) denatured form of the enzyme

Question 57

What type of inhibition is indicated by the data graphed below?

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

Question 58

Ethanol CH₃CH₂OH) is the alcohol found in beverages. It is oxidized in the body to acetaldehyde by the enzyme alcohol dehydrogenase. Methanol CH₃OH), also known as wood alcohol, is converted to formaldehyde by the same enzyme. Acetaldehyde is toxic, but formaldehyde is far more toxic to humans, which is why the ingestion of relatively small amounts of methanol can cause blindness or death. One treatment for mild methanol poisoning is the administration of ethanol. Why might a doctor choose this treatment?

A) Ethanol must act as a competitive inhibitor for the alcohol dehydrogenase and therefore slows the formation of formaldehyde.
B) Ethanol likely irreversibly binds to alcohol dehydrogenase which prevents the formation of formaldehyde.
C) The ethanol is likely an uncompetitive inhibitor and binds to a site other than the active site of the enzyme.
D) The doctor has given up on the patient and administers ethanol for sedation.

Question 59

Which is not an explanation of why site-directed mutagenesis is a more powerful tool than many other techniques for determining important residues in an enzyme?

A) It is a more specific technique than irreversible inhibition experiments.
B) It can be used on enzymes for which no specific irreversible inhibitor is known.
C) It allows for testing of the specific function of amino acid side chains in an enzyme.
D) It is especially useful for enzymes whose sequence is not known.

Question 60

The quaternary structure of hemoglobin changes from the T state to the R state .

A) only after four molecules of O₂ are bound
B) after the binding of one molecule of O₂ causes a change in the primary structure
C) when hemoglobin is completely deoxygenated
D) when at least one subunit on each dimeric unit αβ dimer) is oxygenated

Question 61

A noncompetitive inhibitor can bind to either the enzyme or the enzyme-substrate complex.

Question 62

Rational drug design involves the study of enzyme structure and the use of computers to generate structures of possible enzyme inhibitors.

Question 63

Both reversible and irreversible inhibitors can be separated from solutions of enzymes by dialysis.

Question 64

The regulatory and active sites on an oligomeric enzyme can be on different subunits.

Question 65

At sufficiently high substrate concentration all types of reversible inhibition can be overwhelmed. At high substrate concentration the enzyme will be saturated and the reaction will proceed at the same maximum velocity as in the absence of inhibitor.

Question 66

Natural inhibitors often are regulators of metabolic reactions.

Question 67

Select the correct class of enzyme for each of these reactions.
Glucose-6-phosphate →
Fructose-6-phosphate

Question 68

Increasing the concentration of a classic competitive inhibitor has no effect on the maximum velocity of an enzyme-substrate reaction.

Question 69

Select the correct class of enzyme for each of these reactions.
Polypeptide + H₂O → Amino acids

Question 70

Very few catalytic proficiency values are known because many nonenzymatic reaction rates are so slow that they are difficult to measure.

Question 71

In bi-substrate reactions the substrates always bind to the enzyme in a specific order.

Question 72

The most common enzyme inhibitors are competitive inhibitors.

Question 73

All irreversible inhibitors known are from man-made sources.

Question 74

Affinity labels are radioactive competitive inhibitors used to identify the most important residues in an enzymeʹs active site.

Question 75

Select the correct class of enzyme for each of these reactions.
Lactate + NAD⁺ → Pyruvate +
NADH + H⁺

Question 76

Select the correct class of enzyme for each of these reactions.
L-aspartic acid +
a-ketoglutarate → oxaloacetic acid + L-Glutamate

Question 77

Select the correct class of enzyme for each of these reactions.
Pyruvate + H⁺ → Acetaldehyde + Carbon dioxide

Question 78

Select the correct class of enzyme for each of these reactions.
L-Glutamate + ATP + NH₄⁺
→ L-glutamine + ADP + P_i

Question 79

Select the correct class of enzyme for each of these reactions.
Sucrose + H₂O → Fructose +
Glucose

Question 80

Allosteric modulators bind covalently to the regulatory site of an enzyme and change its conformation.