Deck 5: Properties of Enzymes

Question 1

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

Answer 1

first order;zero order

Question 2

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

Answer 2

active site

Question 3

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 when the substrate and enzyme are first mixed.

Answer 3

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

Question 4

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 5

An enzyme that catalyzes conversions of L-sugars to D-sugars is called a(n)________.

A)lyase
B)hydrolase
C)synthetase
D)synthase
E)isomerase

Question 6

Most of the known enzymes are ________.

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

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,kcat = 103.
B)Papain,kcat = 10.
C)Carboxypeptidase,kcat = 102.
D)Catalase,kcat = 107.

Question 9

In a first order enzyme-catalyzed reaction,the velocity of the reaction is proportional to the ________,while in a zero order enzyme-catalyzed 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 10

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 11

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 12

Enzymes that join two substrates and require energy of a nucleoside triphosphate (such as ATP)to do so are called

A)isomerases.
B)lyases.
C)ligases.
D)synthetases.
E)Both c and d.

Question 13

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 14

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 substrate's concentration
C)the substrate concentrations are equal
D)it proceeds at twice the rate upon double the concentrations of both substrates

Question 15

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 16

Which is appropriate for initial rate method experiments to analyze an enzyme-catalyzed reaction?

A)Substrate concentration is constant and the rate is measured at different concentrations of enzyme.
B)Enzyme concentration is constant and the rate is measured at different substrate concentrations.
C)The substrate and enzyme concentrations are varied inversely with respect to one another and the rate is measured.
D)Both enzyme and substrate concentrations are constant.The temperature is varied and the rate is measured.
E)All of the above.

Question 17

Which of the following statements is FALSE?

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

Question 18

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 usually proteins.

Question 19

Oxidases,peroxidases,oxygenases or reductases are all

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

Question 20

Which step in an enzyme-catalyzed reaction was assumed to be negligible by Michaelis and Menton?

A)Formation of ES from E + P.
B)Formation of E + P from ES.
C)Conversion of ES to E + S.
D)Formation of ES from E + S.

Question 21

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(n)________.

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

Question 22

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 23

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 24

The ratio of kcat/Km 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 25

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 26

The difference between uncompetitive and noncompetitive inhibitors is that

A)uncompetitive inhibitors bind irreversibly.
B)1 / V max is the same for all concentrations of uncompetitive inhibitors.
C)an uncompetitive inhibitor binds only to ES.
D)a competitive inhibitor binds only to E.

Question 27

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

A)Measure Km 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 28

It is difficult to determine either Km or Vmax 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 29

How is the catalytic proficiency defined?

A)It is the reciprocal of the turnover number.
B)It is the logarithm of the Michaelis-Menton constant.
C)It is equal to the rate constants for a reaction in the presence of the enzyme divided by the rate constant for the same reaction in the absence of the enzyme.
D)It is the rate of the enzyme-catalyzed reaction in the presences of a standard concentration of enzyme.

Question 30

Calculate the value of the maximum velocity for an enzyme-catalyzed reaction that follows Michaelis-Menton kinetics if the initial velocity is 6 mM/s at a substrate concentration of 6 mM.The KM for the enzyme system is 2 mM.

A)4.5 mM
B)8 mM
C)8.75 mM
D)12 mM
E)66 mM

Question 31

What is the shape of a typical plot of initial rate vs.substrate concentration for an enzyme catalyzed reaction that follows Michaelis-Menton kinetics?

A)Sigmoidal.
B)Parabolic.
C)Sinusoidal.
D)Bell curve.
E)Hyperbolic.

Question 32

The enymatic rate constant (kcat/Km)of orotidine 5'-phosphate decarboxylase is 6 × 10⁷ M-1s-¹ and the nonenzymatic rate constant (kn)is 3 × 10-¹⁶ s-¹.What is the value of the enzyme's 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 calcuate the profeciency without knowing the value Vmax.

Question 33

The (lower,higher)the value of Km,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 34

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 35

The catalytic proficiency of an enzyme is the ________.

A)turnover number
B)Km/[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 36

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 37

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 38

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 Vmax and Km.

Question 39

The expression: Vmax = 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 40

Which statement is true about catalytic proficiencies?

A)They are very useful for determining second-order rate constants.
B)From them you can calculate the amount of enzyme needed to achieve a specific rate of reaction.
C)They can easily be determined from the Lineweaver-Burk plot.
D)They are difficult to determine because the reactions without the enzyme are extremely slow.

Question 41

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 42

Ethanol (CH₃CH₂OH)is an 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 acts as a competitive inhibitor with respect to methanol as a substrate 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 43

In ________ inhibition the inhibitor binds at a site other than the active site,but alters the active site to prevent binding of the normal substrate.

A)classical competitive
B)non-classical competitive
C)noncompetitive
D)uncompetitive
E)irreversible

Question 44

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 45

Which equilibrium below applies to noncompetitive inhibition?

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

Question 46

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 47

Which are correlated with allosteric enzymes that do not follow typical Michaelis-Menten kinetics?

A)Cooperative binding of the substrate.
B)Sigmoidal curves of velocity when S is varied.
C)Changes in conformation of the enzyme when S binds.
D)All of the above.

Question 48

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

A)Competitive.
B)Uncompetitive.
C)Noncompetitive.
D)Irreversible.

Question 49

Enzyme reactions that require all the substrate to be present before any product is released are called ________.

A)bisubstrate
B)sequential
C)ping-pong
D)substituted

Question 50

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 51

The following data were obtained in the presence and absence of inhibitor.What type of inhibition is shown?

A)Competitive.
B)Uncompetitive.
C)Noncompetitive.
D)Irreversible.
E)Cannot tell inhibition type from the information given.

Question 52

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 53

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 true.

A)X is an activator of the enzyme.
B)The catalysis shows Michaelis-Menten kinetics.with or without X.
C)X increases the activation energy for the catalytic reaction.
D)X could be a competitive inhibitor.

Question 54

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 enzyme's substrate.

Question 55

Which might be most useful for determining the amino acid residues in an enzyme's active site?

A)Use an irreversible inhibitor that reacts with the side chains of specific amino acids.
B)Determine the entire primary structure of the protein.
C)Use a competitive inhibitor and plot the rate vs concentration of inhibitor.
D)Unfold the enzyme with urea and 2-mercaptoethanol.Then incubate and crystallize it in the presence of the substrate followed by X-ray crystallography.

Question 56

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 57

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 58

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 59

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 60

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 can often be retrieved (purified)from solutions of enzymes,but irreversible inhibitors will react with enzyme and no longer be retrievable in their original form.
D)All of the above.
E)A and B only.

Question 61

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

Question 62

Phosphorylation that changes an enzyme's activity is an example of ________.

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

Question 63

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

Question 64

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 65

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 66

The "T" 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 67

Reversible inhibitors can be separated from solutions of enzymes by dialysis.

Question 68

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 69

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 70

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 71

The most common enzyme inhibitors are competitive inhibitors.

Question 72

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 73

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

Question 74

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 75

Natural inhibitors often are regulators of metabolic reactions.

Question 76

Examples of multienzyme complexes include

A)fatty acid synthase.
B)tryptophan synthase.
C)isomerase.
D)A and B.
E)All of the above.

Question 77

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

Question 78

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

Question 79

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 80

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.