Lecture 8 Chapter 6 & 7 Protein Function Enzymes Flashcards
The velocity of a reaction is the quantity of the:
a. reactant that appears in a specified unit of time.
b. product that disappears in a specified unit of time.
c. solution color that disappears in a specified unit of time.
d. reactant that disappears in a specified unit of time.
e. solution color that appears in a specified unit of time.
d. reactant that disappears in a specified unit of time.
What is the reaction order if the reaction rate was doubled by doubling the reactant concentration?
a. zero order
b. first order
c. second order
d. pseudo-first order
e. pseudo-second order
b. first order
What is the reaction order if the reaction rate was increased by a factor of 4 by doubling the reactant
concentration?
a. zero order
b. first order
c. second order
d. pseudo-first order
e. pseudo-second order
c. second order
Which rate equation is TRUE if doubling the concentration of reactant A doubles the reaction rate and
doubling the concentration of reactant B has no effect on the rate?
a. V = k[A]
b. V = k[A][B]
c. V = k[A]2[B]1
d. V = k[A]2[B]
e. V = k[A]2
a. V = k[A]
Which statement about pseudo-first order reactions is FALSE?
a. Bimolecular reactions can be pseudo-first-order reactions.
b. The reaction rate will not appear to depend on the concentration of reactant A, whose concentration
greatly exceeds that of reactant B.
c. The reaction has a pseudo-first order if the concentration of one reactant greatly exceeds that of the
second one and if the second reactant is present at low concentrations.
d. The reaction rate will not appear to depend on the concentration of reactant A, whose concentration is significantly less than that of reactant B.
e. Unimolecular reactions cannot be pseudo-first-order reactions.
d. The reaction rate will not appear to depend on the concentration of reactant A, whose concentration is significantly less than that of reactant B.
Choose the CORRECT statement concerning the reaction order.
a. First-order rate constants have units M–1 s–1.
b. Biochemical reactions cannot have a zero order.
c. Bimolecular reactions are always second-order reactions.
d. Second-order reactions are usually unimolecular.
e. First-order rate constants have unit s–1.
e. First-order rate constants have unit s–1.
Which statement about enzyme kinetics is TRUE?
a. Michaelis–Menten enzymes have sophisticated regulatory properties.
b. The kinetic description of enzymatic activity is required to understand the structure of enzymes.
c. The action of allosteric enzymes increases the metabolic chaos.
d. Allosteric enzymes allow the efficient integration of metabolism.
e. All enzymes obey Michaelis–Menten kinetics.
d. Allosteric enzymes allow the efficient integration of metabolism.
Which characteristics are usually the first ones to be determined when studying enzyme activity?
a. Vmax and KM
b. V0 and KM
c. Vmax and V0
d. [S] and KM
e. Vmax and k2
a. Vmax and KM
In the Michaelis–Menten model:
a. the reaction equilibrium is attained with time, and the change in the concentration of the product
allows measuring of the reaction rate.
b. a unique characteristic of the enzyme is investigated when the enzyme concentration is relatively
constant and the product formation is negligible.
c. enzyme concentration is relatively constant only near the end of the reaction, so only in the
beginning of the reaction can the velocity be measured precisely.
d. only in the beginning of the reaction, there is no net change in concentrations of S and P, so the
reaction velocity can be measured precisely.
e. the initial velocity depends on [S] only when the entire enzyme is bound to the substrate but the
product has not accumulated yet.
b. a unique characteristic of the enzyme is investigated when the enzyme concentration is relatively
constant and the product formation is negligible.
The Michaelis constant:
a. depends on the concentration of the enzyme.
b. displays the maximal rate of the reaction, when it is much less than the substrate concentration.
c. describes the properties of the enzyme.
d. displays the maximal rate of the reaction, when it is equal to the substrate concentration.
e. displays zero-order kinetics, when the it is equal to the substrate concentration.
b. displays the maximal rate of the reaction, when it is much less than the substrate concentration.
Which statement about Vmax is TRUE?
a. Vmax is independent of enzyme concentration.
b. For Michaelis–Menten enzymes, the maximal velocity is approached asymptotically.
c. Vmax can be attained only when half of the enzyme is bound to the substrate.
d. The reaction velocity is maximal when the concentration of the substrate equals KM.
e. The reaction velocity is half Vmax when the entire enzyme is bound to the substrate.
b. For Michaelis–Menten enzymes, the maximal velocity is approached asymptotically.
What is the consequence of a high KM value of an enzyme?
a. Moderate concentration of the substrate will be enough for maximal enzyme activity.
b. This enzyme has high maximal velocity.
c. The enzyme is present in a large concentration in the cell.
d. High concentration of the substrate should be reached for maximal enzyme activity.
e. The velocity will be directly proportional to the substrate concentration.
d. High concentration of the substrate should be reached for maximal enzyme activity.
How can the reciprocal plot of initial velocity NOT be applied?
a. When it is half the maximal speed, the substrate concentration equals KM.
b. It gives a way to define KM when it is plotted against the reciprocal of the substrate concentration.
c. When it is plotted against the reciprocal of the substrate concentration, it allows KM/Vmax to be
defined as a slope of the graph.
d. It allows definition of the reciprocal of the maximal velocity in a double-reciprocal plot.
e. It is part of the Lineweaver–Burk equation.
a. When it is half the maximal speed, the substrate concentration equals KM.
How can the turnover number of an enzyme be determined?
a. as Vmax
b. when the enzyme is fully saturated
c. as [E]T
d. when half of the enzyme is occupied with the substrate
e. by the initial velocity
b. when the enzyme is fully saturated
Calculate the turnover of an enzyme if the maximal velocity is 0.2 M sec–1 and the product concentration
after the reaction is 1 M.
a. 0.2 sec
b. 0.2 sec–1
c. 5 sec
d. 0.5 sec
e. 0.5 sec–1
b. 0.2 sec–1
An enzyme has specificity constant values of 1.0 × 103, 1.1 × 104, and 4.0 × 10–1 for compounds A, B, and
C, respectively. For which compound does the enzyme have a higher efficiency and why?
a. Compound C; a low kcat value accounts for the high and specific rate of catalysis, while a high KM
value lowers the required concentration of the compound.
b. Compound B; a high KM value accounts for the high and specific rate of catalysis, while a low kcat
value lowers the required concentration of the compound.
c. Compound A; higher kcat accounts for the high and specific rate of catalysis, while a low KM value
lowers the required concentration of the compound.
d. Compound C; the least amount of the compound is required to saturate the enzyme, while having the
highest rate of catalysis.
e. Compound B; the least amount of the compound is required to saturate the enzyme, while having the
highest rate of catalysis.
e. Compound B; the least amount of the compound is required to saturate the enzyme, while having the
highest rate of catalysis.
Which constant represents the nature of the enzyme–substrate interaction?
a. specificity constant
b. constant of the catalysis rate
c. Michaelis constant
d. rate constant k1
e. allosteric constant
c. Michaelis constant
The specificity constant does NOT depend on the rate constant for the:
a. formation of the ES complex (kcat).
b. dissociation of the ES complex (k–1).
c. formation of the product (k2).
d. formation of the ES complex (k1).
e. formation of the product (kcat).
a. formation of the ES complex (kcat).
KM of the cytosolic isomer of fumarase is equal to 5 μM, whereas KM of the mitochondrial isomer is equal
to 50 μM. What should the value of k2 be for a less active isomer of the enzyme to achieve kinetic perfection?
a. from 5 × 108 to 5 × 109
b. from 2 × 107 to 2 × 108
c. from 1 × 108 to 1 × 109
d. from 5 × 102 to 5 × 103
e. from 2 × 106 to 2 × 107
d. from 5 × 102 to 5 × 103
What is the ternary complex for the enzyme (E) that converts pyruvate to lactate using NADH as a
cosubstrate?
a. E (lactate) (NAD+)
b. E (NADH) (pyruvate)
c. E (pyruvate) (lactate)
d. E (NADH) (NAD+)
e. E (lactate) (NADH)
e. E (lactate) (NADH)
hat is the reaction class if all substrates must bind to the enzyme in an arranged manner before any
product is released?
a. random sequential
b. ping-pong
c. ordered sequential
d. second order
c. ordered sequential
An example of a reaction of the double-displacement class is the:
a. conversion of pyruvate and NADH to lactate and NAD+ by lactate dehydrogenase with formation of
a ternary complex.
b. conversion of creatine and ATP to phosphocreatine and ADP by creatine kinase with ATP being
released before both substrates bind the enzyme.
c. conversion of aspartate and α-ketoglutarate to oxaloacetate and glutamate with formation of a ternary
complex.
d. cleavage of a peptide bond by chymotrypsin with the formation of a substituted intermediate on a
serine residue.
e. isomerization of dihydroxyacetone phosphate to glyceraldehyde 3-phosphate.
d. cleavage of a peptide bond by chymotrypsin with the formation of a substituted intermediate on a
serine residue.
What is NOT a feature of allosteric enzymes?
a. Their activity can be modified by environmental signals.
b. They have a sigmoidal dependence of the reaction velocity on the substrate concentration.
c. Each allosteric enzyme is capable of conducting multiple reactions.
d. Their activity isn’t regulated by a threshold effect.
e. They depend on alterations in the quaternary structure.
d. Their activity isn’t regulated by a threshold effect.
Judging by the given scheme of two metabolic pathways where en denotes different enzymes, give the name
of interactions that regulate the function of these pathways in a coherent fashion.
a. K inhibits e1 and e10 while F inhibits e1, and I inhibits e10.
b. F and I inhibit e1 and e10, respectively, and stimulate e10 and e1, respectively, whereas K inhibits e1and e10.
c. F stimulates e10 and I stimulates e1 while K inhibits e1 and e10.
d. K stimulates e1 and e10 while F inhibits e1, and I inhibits e10.
e. F and I stimulate e1 and e10, respectively, and inhibit e10 and e1, respectively, whereas K inhibits e1
and e10.
b. F and I inhibit e1 and e10, respectively, and stimulate e10 and e1, respectively, whereas K inhibits e1and e10.
It is NOT characteristic for the concerted model for allosteric enzymes to:
a. catalyze multiple reactions within one enzyme.
b. maintain two distinct conformations of the enzyme that can be spontaneously interconverted.
c. bind the substrate to the more stable conformation more readily.
d. follow the symmetry rule by all of the enzyme active sites.
e. disrupt the equilibrium between the conformations in favor of the least stable form after the substrate
binding.
c. bind the substrate to the more stable conformation more readily.
What is TRUE regarding the action of allosteric effectors?
a. Negative effectors inhibit the less stable form of the enzyme.
b. Positive effectors stimulate the more stable form of the enzyme.
c. Negative effectors stabilize the less stable form of the enzyme.
d. Negative effectors inhibit the more stable form of the enzyme.
e. Negative effectors stabilize the more stable form of the enzyme.
e. Negative effectors stabilize the more stable form of the enzyme.
What is TRUE for the sequential model for allosteric enzymes?
a. The binding of a substrate disrupts the equilibrium between two states of the enzyme.
b. The initial molecule of the substrate prevents unproductive colliding between other substrate
molecules and the tense form of the enzyme.
c. The rate constant for the binding of a substrate is lower for the last subunit of the enzyme than for the
first one.
d. The affinity to the substrate increases due to the influence of neighboring subunits.
e. Cooperativity in the sequential model can only be positive.
d. The affinity to the substrate increases due to the influence of neighboring subunits.
Which enzyme of the given metabolic pathway is most likely to be an allosteric enzyme that controls the
synthesis of metabolite H?
a. e1
b. e2
c. e3
d. e5
e. e7
e. e7
Which enzyme of the given metabolic pathway is most likely to catalyze a committed step for the synthesis
of metabolite D?
a. e1
b. e2
c. e3
d. e4
e. e6
c. e3
What is the name for allosteric regulation when a molecule blocks the committed step of its own synthesis pathway?
a. committed inhibition
b. feedback inhibition
c. concerted model
d. sequential model
e. cooperativity
b. feedback inhibition
Which type of experiment allows simultaneous studying of millions of enzyme molecules?
a. ensemble studies
b. cooperative studies
c. in singulo
d. homotropic
e. heterotropic
a. ensemble studies
Which type of method allows investigation of molecular heterogeneity?
a. homotropic
b. ensemble studies
c. heterotropic
d. in singulo
e. cooperative studies
d. in singulo
What is the premise of the Michaelis–Menten equation that is achieved by assuming equal rates of formation and breakdown of the ES complex?
a. no-reverse-reaction assumption
b. investigating the reaction at time close to zero
c. steady-state assumption
d. assumption of emanations
e. equilibrium assumption
c. steady-state assumption
An enzyme will be MOST sensitive to changes in the substrate concentration when:
a. [S] is near KM.
b. [S] is below KM.
c. KM is below Vmax.
d. [S] is above KM.
e. KM is Vmax/2.
b. [S] is below KM.
Judging by the given Lineweaver–Burk plot, identify the turnover number of enzyme B if its total
concentration is 0.2 M.
a. 0.1
b. 10
c. 5
d. –1
e. 2.5
b. 10
Judging by the given Lineweaver–Burk plot, identify the concentration of substrate that is required for
enzyme A to operate at half of its maximal speed.
a. 0.5
b. –0.5
c. -2
d. 2.5
e. 0.4
a. 0.5
What is the critical feature of the Michaelis–Menten model of enzyme catalysis?
a. increasing the probability of product formation
b. shifting the reaction equilibrium
c. formation of an ES complex
d. reaching the reaction equilibrium
e. accumulation of the product
c. formation of an ES complex
Which description of the concerted model for allosteric enzymes is TRUE?
a. At a low substrate concentration, L0 is small. Upon increasing the substrate concentration, in accordance with the symmetry rule, L0 will be increasing. The disrupted equilibrium of the enzyme
states results in a sharp increase of the reaction velocity. As the concentration reaches the threshold, the enzyme becomes more active.
b. Upon increasing the substrate concentration, due to cooperativity, the number of enzyme molecules
in the T state will be decreasing. The equilibrium between enzyme states is disturbed due to a lower
stability of the R state. As the concentration reaches the threshold, the enzyme becomes more active.
c. At a low substrate concentration, L0 is large. Increasing the substrate concentration will result in accumulation of the enzyme in the R state, which will increase L0. The disrupted equilibrium of the
enzyme states results in a sharp increase of the reaction velocity. When the substrate concentration is way above KM, the enzyme activity is very sensitive to the changes in the concentration, which
results in a sharp increase of the reaction velocity.
d. Upon increasing the substrate concentration due to the symmetry rule, the number of enzyme
molecules in the T state will be increasing. The equilibrium between enzyme states is disturbed due to a lower stability of the T state. When the substrate concentration is way above KM, the enzyme activity is very sensitive to the changes in the concentration, which results in a sharp increase of the reaction velocity.
e. At a low substrate concentration, L0 is large. Upon increasing the substrate concentration, L0 will be
markedly decreasing. The disrupted equilibrium of the enzyme states results in a sharp increase of the reaction velocity. As the concentration reaches the threshold, the enzyme becomes more active.
e. At a low substrate concentration, L0 is large. Upon increasing the substrate concentration, L0 will be
markedly decreasing. The disrupted equilibrium of the enzyme states results in a sharp increase of the reaction velocity. As the concentration reaches the threshold, the enzyme becomes more active.
What is the value of [S] as a fraction of KM required to obtain 20% Vmax?
a. 0.2 KM
b. 0.25 KM
c. 0.5 KM
d. 0.75 KM
e. 0.8 KM
b. 0.25 KM
Allosteric effectors alter the equilibrium between the:
a. ES state.
b. R and T forms of a protein.
c. forward and reverse reaction rate.
d. formation of the product and its reverse reaction.
e. concentration thresholds.
b. R and T forms of a protein.
loss of allosteric regulation in the production of purine nucleotides may result in:
a. excess nucleotides for DNA.
b. a loss of PRS catalytic activity.
c. decreased urate degradation.
d. a decrease in urate concentration.
e. an excess accumulation of urates.
e. an excess accumulation of urates.
The formula V0 = Vmax ([S]/[S] + KM) indicates the relationship between the:
a. enzyme activity and the equilibrium constant.
b. rate of a catalyzed reaction and the equilibrium constant.
c. enzyme activity and substrate concentration.
d. probable substrate concentration and the normal concentration in vivo.
e. turnover number and the rate of catalysis.
c. enzyme activity and substrate concentration.
When the substrate concentration is much greater than KM, the rate of catalysis is almost equal to:
a. [S] + KM.
b. kcat.
c. Vmax.
d. Vmax/2.
e. (Vmax/KM)[S].
c. Vmax.
If the enzyme concentration is 5 nM, the substrate concentration is 5 mM, and KM is 5 μM:
a. the enzyme will be saturated with the substrate.
b. most of the enzyme won’t have the substrate bound.
c. there will be more enzyme than substrate.
d. the enzyme will be displaying first-order kinetics.
e. the rate of the reaction will be half its maximal value.
a. the enzyme will be saturated with the substrate.
Homotropic effects of allosteric enzymes:
a. are due to the effects of substrates.
b. are due to the effects of allosteric activators.
c. shift the kinetics curve to the left.
d. shift the kinetics curve to the right.
e. account for the linear nature of the kinetics curve.
a. are due to the effects of substrates.
When [S] «_space;KM, the enzymatic velocity depends on:
a. the values of kcat/KM, [S], and [E]T.
b. Vmax of the reaction.
c. the affinity of the substrate for the catalytic site.
d. the values of kcat, [S], and [E]T.
e. the formation of the ES complex.
a. the values of kcat/KM, [S], and [E]T.
.Allosteric effectors:
a. can cause large changes in enzymatic activity.
b. can lead to a decrease in the availability of a protein.
c. don’t alter the sensitivity of a metabolic pathway.
d. decrease the sensitivity of the enzyme at nearly all concentrations of the substrate.
e. alter enzyme activity by binding to the active site of an enzyme.
a. can cause large changes in enzymatic activity.
What condition is met when the amount of substrate is far greater than the amount of enzyme present?
a. [S] is much less than KM.
b. V0 is half of Vmax.
c. The enzyme is displaying second-order kinetics.
d. The enzyme is displaying first-order kinetics.
e. The enzyme is displaying zero-order kinetics.
e. The enzyme is displaying zero-order kinetics.
During the early stages of an enzyme purification protocol, when cells have been lysed but cytosolic
components have not been separated, the reaction velocity-versus-substrate concentration is sigmoidal. As you continue to purify the enzyme, the curve shifts to the right. Explain your results.
a. This is an enzyme that displays Michaelis–Menten kinetics and you purify away a homotropic
inhibitor.
b. This is an enzyme that displays Michaelis–Menten kinetics, and you must use a Lineweaver–Burk
plot to determine KM and Vmax correctly.
c. This is an allosteric enzyme and you must use a Lineweaver–Burk plot to determine KM and Vmax
correctly.
d. This is an allosteric enzyme and during purification you purify away a heterotropic activator.
e. This is an allosteric enzyme displaying a double-displacement mechanism and during purification
you purify away one of the substrates.
d. This is an allosteric enzyme and during purification you purify away a heterotropic activator.
Calculate the turnover number for the enzyme if its Mr is 75,000 and if by assaying 5 μg of the enzyme under saturating [S] concentrations, you determine that Vmax is approached asymptotically and is equal to 1.68 μmol/sec.
a. 2.25 × 106 sec–1
b. 1.50 × 105 sec–1
c. 2.50 × 105 sec–1
d. 2.52 × 104 sec–1
e. You also need to know KM for this enzyme to calculate the turnover number.
d. 2.52 × 104 sec–1
Enzymes
protein catalysts that can accelerate the rate of a reaction by factors of as much as a million or more.
Substrates
What reactants in an enzyme are called
Hydrolysis
the breaking of a chemical bond by the addition of a water molecule
proteolytic enzymes
enzymes that break down protein
Trypsin
a proteolytic enzyme that cleaves on the carboxyl side of Arg and Lys residues
Thrombin
a proteolytic enzyme that cleaves on Arg-Gly bonds in particular sequences only
oxidoreductases
Major class of enzymes that catalyze oxidation-reduction reactions
Transferases
major class of enzymes that move functional groups between molecules
Hydrolases
major class of enzymes that cleave bonds with the addition of water
lyases
major class of enzymes that remove atoms to form double bonds or add atoms to double bonds
