Chapter 7: Enzyme Mechanisms

Binding of glucose to hexokinase causes a conformational change in the enzyme. This is an

example of the __________ model of enzyme catalysis

lock and key

Which of the following is true of the induced-fit model of enzyme catalysis but NOT of the

lock and key model of enzyme catalysis?

a. It was proposed by Emil Fischer.

b. It involves weak interactions of a substrate with an enzyme.

c. It involves a conformational change of the enzyme.

d. It involves noncovalent interactions of the substrate with the enzyme.

It involves a conformational change of the enzyme.

Enzymes usually bind substrates with __________ affinity and __________ specificity.

high; high

Enzyme active sites

are a pocket or cleft

What is the rate enhancement as a result of the presence of an enzyme if the uncatalyzed rate

of the reaction is 1.2 × 102 mmol/sec and the catalyzed rate is 2.4 × 104 mmol/sec?

200

The transition state of a reaction is

higher in free energy than the product

Which of the following is a holoenzyme?

a. pyruvate kinase with a bound K+

b. alcohol dehydrogenase

c. nitrite reductase

d. hexokinase

pyruvate kinase with a bound K+

Nitrite reductase contains two histidine amino acids that coordinate a Cu2+ ion. When the ion

is present in the enzyme, the ion is a __________ and the enzyme is a __________.

cofactor; holoenzyme

Which of the following is a prosthetic group?

a. Mg2+

b. NADH

c. Zn2+

d. heme

heme

The dihydrolipoyl transacetylase enzyme contains a lipoyl group. The lipoyl group is a(n)

prosthetic group

Which of the following is true of a coenzyme but NOT true of a prosthetic group?

a. It contains an organic component.

b. It is loosely associated with the enzyme.

c. It is necessary for enzyme function.

d. It is present in a holoenzyme but not an apoenzyme.

It is loosely associated with the enzyme.

Hexokinase catalyzes the phosphorylation of glucose to glucose 6-phosphate. Hexokinase

belongs to which enzyme class?

transferase

Which answer correctly pairs the enzyme class with the type of reaction catalyzed?

a. lyase; formation of two products by hydrolyzing a substrate

b. transferase; transfer of H or O atoms

c. isomerase; intramolecular rearrangements

d. oxidoreductase; transfer of groups within molecules

isomerase; intramolecular rearrangements

An enzyme that requires the coenzyme nicotinamide adenine dinucleotide belongs to which

enzyme class?

oxidoreductases

An enzyme can increase the rate of a reaction inside a cell by __________ the energy of the

__________.

lowering; transition state

A reaction coordinate diagram comparing an uncatalyzed reaction with an enzyme-catalyzed

reaction can directly illustrate that the enzyme __________, but will not directly illustrate

that the enzyme __________.

stabilizes the transition state; orients the substrates appropriately for the reaction to

occur

Which of the following is a way that an enzyme can increase the rate of a reaction inside

a cell?

a. increasing the temperature of the cell

b. increasing the pressure inside the cell

c. increasing the substrate concentration inside the cell

d. orienting substrates appropriately for the reaction to occur

orienting substrates appropriately for the reaction to occur

A mutation of Lys229 in aldolase leads to a loss of enzyme activity. This is most likely

because the

enzyme can no longer hold the intermediate in the correct orientation for catalysis.

Many medicinal drugs are transition state analogs. They are good drugs because they can

interact with the target enzyme active site and are

stable

Reaction coordinate diagrams clearly show that the energy of an enzyme bound to a transition

state is higher than the energies of the E + S, E + P, and ES that occur along the same reaction

coordinate. The energy of an enzyme bound to a transition state analog would lie __________

in the diagram.

below the E + S

All of the following are common catalytic reaction mechanisms in enzyme active sites

EXCEPT __________ catalysis.

a. acid-base

b. covalent

c. metal ion

d. van der Waals

van der Waals

If an enzyme carries out acid-base catalysis, which of the following amino acids could act as

general acid?

a. phenylalanine

b. glycine

c. histidine

d. alanine

histidine

When a nucleophile present in the enzyme attacks an electrophilic substrate to form an

enzyme-substrate intermediate, this is an example of __________ catalysis.

covalent

The three general categories of enzyme-mediated reactions, which are determined on the basis

of the work they accomplish, include all EXCEPT

a. coenzyme-dependent redox reactions.

b. reversible covalent modification.

c. hydrophobic collapse reactions.

d. metabolite transformation reactions.

hydrophobic collapse reactions.

The regulation of a biomolecule through the addition or removal of a molecular tag involves

__________ reactions.

reversible covalent modification

Which pair correctly matches the coenzymes most often used to mediate the described redox

reactions?

a. NAD+/NADH; redox at C-O bonds

b. NADP+/NADPH; redox at C-C bonds

c. FAD/FADH2; redox at C-O bonds

d. FMN/FMNH2; redox at C-O bonds

NAD+/NADH; redox at C-O bonds

In a reversible covalent modification reaction involving the phosphorylation of a target

protein, which of the following amino acids is LEAST likely to be modified with a phosphate

group?

a. Ser

b. Phe

c. Tyr

d. Thr

Phe

Which type of reaction does not change the molecular formula of the product compared with

that of the substrate

isomerization

The conversion of 2-phosphoglycerate to phosphoenolpyruvate is an example of which type

of reaction?

dehydration

The catalytic triad of chymotrypsin is composed of His57, Ser195, and

Asp102

The side chains of the amino acids that make up the catalytic triad of chymotrypsin contain

all EXCEPT

a. a hydroxyl group.

b. a carboxylic acid.

c. an aromatic group.

d. an imidazole.

an aromatic group

Which amino acid acts as a general acid and a general base in the mechanism of

chymotrypsin?

His57

Which of the following is true of the tetrahedral intermediate in the chymotrypsin

mechanism?

a. It is lower in free energy than the substrate.

b. It is partially positively charged.

c. All bonds are the same length.

d. It contains an oxyanion.

It contains an oxyanion.

Both the substrate and the tetrahedral intermediate, when associated with chymotrypsin

undergo a nucleophilic attack

The substrate binding pocket of __________ contains a(n) __________, which facilitates

substrate specificity

trypsin; Asp

A mutation results in the change of Ser to Asp in the substrate binding pocket of

chymotrypsin. Most likely, the mutant enzyme will

preferentially hydrolyze substrates containing phenylalanine.

The substrate binding pocket of __________ is best at accommodating substrates with small

side chains

elastase

Which of the following functions as a general base in the mechanism of enolase?

a. Lys345

b. His57

c. Mg2+

d. Glu211

Lys345

Which of the following is NOT a function of the Mg2+ ions in the mechanism of enolase?

a. orientation of substrate in the active site

b. stabilizing the intermediate

c. making the proton at the C-2 position more acidic

d. electrophilic attack on the scissile bond

electrophilic attack on the scissile bond

The role of Glu211 in the mechanism of enolase is to

act as a general acid on the intermediate

The glutamate side chain in the active site of HMG-CoA reductase acts as a general base only

after

a conformational change triggers the exchange of NADP+ for NADPH.

The hemithioacetal intermediate formed during the action of HMG-CoA reductase is stabilized

by

Lys267

The mechanism of HMG-CoA reductase involves

two NADPH

Place the following HMG-CoA reductase steps in the correct order:

A. Reduction of aldehyde

B. Breakdown of hemithioacetal

C. Reduction of thioester

D. Cofactor exchange

C, D, B, A

For a reaction of S → P, the rate constant of the reaction is equal to

k

For a reaction of Q + R → P, the rate constant

is a second-order rate constant

If the rate constant for a reaction is determined to be equal to v / [Q][R], the reaction is the

conversion of

R plus Q to a product

On a plot of [product] versus time for an enzyme-catalyzed reaction, the v0 is equal to the

slope of the line

The initial velocity of an enzyme-catalyzed reaction is followed at various substrate

concentrations. At very high substrate concentrations it is observed that the initial velocity no

longer increases as more substrate is added. The velocity under these conditions is known as

the maximum velocity

In the steady-state condition assumed in Michaelis-Menten kinetics, __________ is relatively

constant.

[ES]

Which of the following is an assumption made when using Michaelis-Menten kinetics?

a. The conversion of E + P → ES does not occur.

b. k1 > k2

c. v0 = vmax at low [S]

d. The conversion of EP → E + P is rapid.

The conversion of EP → E + P is rapid.

KM is equal to

(k−1 + k2) / k1.

The y-axis of a Lineweaver-Burk plot is

1 / vo.

A plot of 1 / v0 versus 1/[S] is called a __________ plot. Data in this plot have a slope equal

to __________.

Lineweaver-Burk; Km/vmax

The specificity constant is equal to

kcat / Km.

An experiment is performed in which the kinetics of an enzyme-catalyzed reaction at

different pHs is monitored. It is found that the Km does not change but that the kcat increases as

the pH goes above 7. Which of the following is true?

a. A chemical group within the enzyme that has a pKa of around 7 is likely involved in

the catalytic mechanism.

b. A chemical group with a pKa of around 7 must be deprotonated in order for substrate

to bind.

c. A chemical group with a pKa of around 7 must be positively charged in order for the

substrate to bind.

d. Protons are acting as positive heterotropic allosteric effectors of this enzyme.

A chemical group within the enzyme that has a pKa of around 7 is likely involved in

the catalytic mechanism.

Which of the following is NOT a primary mechanism that affects catalytic efficiency?

a. binding of regulatory molecules

b. covalent modification

c. proteolytic processing

d. cofactor degradation

cofactor degradation

A kinase adds a phosphate group to a target enzyme, altering the catalytic efficiency of the

enzyme. This is an example of

covalent modification

Acetylcholinesterase is an important enzyme in the nervous system. Acetylcholinesterase

activity is blocked by the nerve agent sarin gas, which forms a covalent bond with a Ser in the

active site of the enzyme. Sarin gas is a(n)

irreversible inhibitor

Which type of interaction is more likely to be found between an enzyme and an irreversible

inhibitor?

covalent bond

Which of the following pairs correctly matches the type of interaction observed between an

inhibitor and an enzyme with the type of inhibition?

a. ionic; irreversible

b. covalent; irreversible

c. hydrogen bonding; reversible

d. hydrophobic; irreversible

hydrogen bonding; reversible

A mixture of enzyme and inhibitor is run through a size-exclusion chromatography column.

The activity of the enzyme is assessed before and after the chromatography. The enzyme has

more activity after the chromatography step. Which of the following is true?

a. The enzyme was not eluted fully from the column.

b. The enzyme was denatured during chromatography.

c. The inhibitor is a reversible inhibitor.

d. The inhibitor is an irreversible inhibitor.

The inhibitor is a reversible inhibitor.

An inhibitor that binds only to the ES complex and not free enzyme is known as a(n)

__________ inhibitor.

uncompetitive

In the formation of an ESI complex, __________ inhibition can result

mixed

A Lineweaver-Burk plot displays parallel lines for an enzyme in the absence and presence of

increasing amounts of an inhibitor. The inhibitor in this experiment

is uncompetitive

Which answer correctly classifies the compound with its relationship to aspartate

transcarbamoylase?

a. ATP; allosteric inhibitor

b. ATP; allosteric activator

c. CTP; allosteric activator

d. CTP; substrate

ATP; allosteric activator

Which of the following statements are true?

a. ATCase is regulated by feedback inhibition.

b. CTP is an allosteric activator of ATCase.

c. ATP is an allosteric inhibitor of ATCase.

d. GTP is an allosteric inhibitor of ATCase.

ATCase is regulated by feedback inhibition.

When compared with the T state of aspartate transcarbamoylase, the R state

has greater separation of the catalytic subunits.

A plot of vo versus [S] for aspartyl transcarbamoylase displays three sigmoidal lines. If the line

in the middle represents the enzyme activity in the absence of any allosteric effectors, then

the line to the __________ represents the enzyme in the __________ when bound to

__________.

right; T state; CTP

An enzyme undergoes a mutation that causes it to lose the ability to be regulated via

phosphorylation. Which of the following mutations may lead to this loss of regulation?

Assume that the overall structure is not altered by the mutation.

a. Ser → Thr

b. Thr → Ser

c. Tyr → Phe

d. Ser → Tyr

Tyr → Phe

Phosphorylation of __________ in glycogen phosphorylase shifts the enzyme to the

__________.

Ser14; R state

When __________ is increased, __________ is activated, which acts on glycogen

phosphorylase, leading to a decrease in the activity of the enzyme.

insulin; protein phosphatase 1

What is the appropriate order of the following steps?

A. Glutamine binding to uridylyltransferase

B. Adenylation of glutamine synthetase

C. Deuridylation of glutamine synthetase adenylyltransferase

A. C, B

Glutamine synthetase is in the R state when Tyr397 is

deadenylated

Procathepsin B is a lysosomal protease that is first translated as a proenzyme. On

autocleavage it is fully activated. Procathepsin B is

a zymogen