One of the enzymes involved in glycolysis, aldolase, requires Zn2+ for catalysis. Under conditions of zinc deficiency, when the enzyme may lack zinc, it would be referred to as the:
A) apoenzyme.
B) coenzyme.
C) holoenzyme.
D) prosthetic group.
E) substra
A) apoenzyme.
Which one of the following is not among the six internationally accepted classes of enzymes?
A) Hydrolases
B) Ligases
C) Oxidoreductases
D) Polymerases
E) Transferases
E) Transferases
Enzymes are potent catalysts because they:
A) are consumed in the reactions they catalyze.
B) are very specific and can prevent the conversion of products back to substrates.
C) drive reactions to completion while other catalysts drive reactions to equili
E) lower the activation energy for the reactions they catalyze.
The role of an enzyme in an enzyme-catalyzed reaction is to:
A) bind a transition state intermediate, such that it cannot be converted back to substrate.
B) ensure that all of the substrate is converted to product.
C) ensure that the product is more stabl
D) increase the rate at which substrate is converted into product.
Which one of the following statements is true of enzyme catalysts?
A) Their catalytic activity is independent of pH.
B) They are generally equally active on D and L isomers of a given substrate.
C) They can increase the equilibrium constant for a given re
D) They can increase the reaction rate for a given reaction by a thousand fold or more.
Which one of the following statements is true of enzyme catalysts?
A) They bind to substrates, but are never covalently attached to substrate or product.
B) They increase the equilibrium constant for a reaction, thus favoring product formation.
C) They in
D) They lower the activation energy for the conversion of substrate to product.
Which of the following statements is false?
A) A reaction may not occur at a detectable rate even though it has a favorable equilibrium.
B) After a reaction, the enzyme involved becomes available to catalyze the reaction again.
C) For S ? P, a catalyst sh
C) For S ? P, a catalyst shifts the reaction equilibrium to the right.
Enzymes differ from other catalysts in that only enzymes:
A) are not consumed in the reaction.
B) display specificity toward a single reactant.
C) fail to influence the equilibrium point of the reaction.
D) form an activated complex with the reactants.
E)
B) display specificity toward a single reactant.
Which of the following is true of the binding energy derived from enzyme-substrate interactions?
A) It cannot provide enough energy to explain the large rate accelerations brought about by enzymes.
B) It is sometimes used to hold two substrates in the opt
B) It is sometimes used to hold two substrates in the optimal orientation for reaction.
The concept of "induced fit" refers to the fact that:
A) enzyme specificity is induced by enzyme-substrate binding.
B) enzyme-substrate binding induces an increase in the reaction entropy, thereby catalyzing the reaction.
C) enzyme-substrate binding induc
D) substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.
The benefit of measuring the initial rate of a reaction V0 is that at the beginning of a reaction:
A) [ES] can be measured accurately.
B) changes in [S] are negligible, so [S] can be treated as a constant.
C) changes in Km are negligible, so Km can be tre
B) changes in [S] are negligible, so [S] can be treated as a constant.
Which of the following statements about a plot of V0 vs. [S] for an enzyme that follows Michaelis-Menten kinetics is false?
A) As [S] increases, the initial velocity of reaction V0 also increases.
B) At very high [S], the velocity curve becomes a horizont
B) At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.
The steady state assumption, as applied to enzyme kinetics, implies:
A) Km = Ks.
B) the enzyme is regulated.
C) the ES complex is formed and broken down at equivalent rates.
D) the Km is equivalent to the cellular substrate concentration.
E) the maximum v
C) the ES complex is formed and broken down at equivalent rates.
An enzyme-catalyzed reaction was carried out with the substrate concentration initially a thousand
times greater than the Km for that substrate. After 9 minutes, 1% of the substrate had been converted
to product, and the amount of product formed in the re
C) 27 min
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) If enough substrate is added, the normal Vmax of a reaction
D) 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.
The following data were obtained in a study of an enzyme known to follow Michaelis-Menten
kinetics:
V0 Substrate added
(?mol/min) (mmol/L)
�������������
217 0.8
325 2
433 4
488 6
647 1,000
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The Km for this enzyme is approximately:
A) 1 mM.
B)
C) 2 mM.
For enzymes in which the slowest (rate-limiting) step is the reaction
k2
ES ? P
Km becomes equivalent to:
A) kcat.
B) the [S] where V0 = Vmax.
C) the dissociation constant, Kd, for the ES complex.
D) the maximal velocity.
E) the turnover number.
C) the dissociation constant, Kd, for the ES complex.
Michaelis and Menten assumed that the overall reaction for an enzyme-catalyzed reaction could be
written as
k1 k2
E + S <--> ES ? E + P
k-1
Using this reaction, the rate of breakdown of the enzyme-substrate complex can be described by the
expression:
A) k
D) k-1 [ES] + k2 [ES].
The Lineweaver-Burk plot is used to:
A) determine the equilibrium constant for an enzymatic reaction.
B) extrapolate for the value of reaction rate at infinite enzyme concentration.
C) illustrate the effect of temperature on an enzymatic reaction.
D) solv
D) solve, graphically, for the rate of an enzymatic reaction at infinite substrate concentration.
The double-reciprocal transformation of the Michaelis-Menten equation, also called the Lineweaver-Burk plot, is given by
1/V0 = Km /(Vmax[S]) + 1/Vmax.
To determine Km from a double-reciprocal plot, you would:
A) multiply the reciprocal of the x-axis inte
A) multiply the reciprocal of the x-axis intercept by ?1.
To calculate the turnover number of an enzyme, you need to know:
A) the enzyme concentration.
B) the initial velocity of the catalyzed reaction at [S] >> Km.
C) the initial velocity of the catalyzed reaction at low [S].
D) the Km for the substrate.
E) bot
E) both A and B.
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) dissociation constant.
B) half-saturation constant.
C) maximum velocity.
D) Michaelis-Menten number.
E) turn
E) turnover number.
In a plot of l/V against 1/[S] for an enzyme-catalyzed reaction, the presence of a competitive inhibitor
will alter the:
A) curvature of the plot.
B) intercept on the l/[S] axis.
C) intercept on the l/V axis.
D) pK of the plot.
E) Vmax.
B) intercept on the l/[S] axis.
In competitive inhibition, an inhibitor:
A) binds at several different sites on an enzyme.
B) binds covalently to the enzyme.
C) binds only to the ES complex.
D) binds reversibly at the active site.
E) lowers the characteristic Vmax of the enzyme.
D) binds reversibly at the active site.
Vmax for an enzyme-catalyzed reaction:
A) generally increases when pH increases.
B) increases in the presence of a competitive inhibitor.
C) is limited only by the amount of substrate supplied.
D) is twice the rate observed when the concentration of subst
D) is twice the rate observed when the concentration of substrate is equal to the Km.
Enzyme X exhibits maximum activity at pH = 6.9. X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4. One likely interpretation of this pH activity is that:
A) a Glu residue on the enzyme is involved in the reaction.
B) a H
B) a His residue on the enzyme is involved in the reaction.
Both water and glucose share an �OH that can serve as a substrate for a reaction with the terminal phosphate of ATP catalyzed by hexokinase. Glucose, however, is about a million times more reactive as a substrate than water. The best explanation is that:
B) the larger glucose binds better to the enzyme; it induces a conformational change in hexokinase that brings active-site amino acids into position for catalysis.
A good transition-state analog:
A) binds covalently to the enzyme.
B) binds to the enzyme more tightly than the substrate.
C) binds very weakly to the enzyme.
D) is too unstable to isolate.
E) must be almost identical to the substrate.
B) binds to the enzyme more tightly than the substrate.
A transition-state analog:
A) is less stable when binding to an enzyme than the normal substrate.
B) resembles the active site of general acid-base enzymes.
C) resembles the transition-state structure of the normal enzyme-substrate complex.
D) stabilizes
C) resembles the transition-state structure of the normal enzyme-substrate complex.
The role of the metal ion (Mg2+) in catalysis by enolase is to
A) act as a general acid catalyst
B) act as a general base catalyst
C) facilitate general acid catalysis
D) facilitate general base catalysis
E) stabilize protein conformation
D) facilitate general base catalysis
Which of the following statements about allosteric control of enzymatic activity is false?
A) Allosteric effectors give rise to sigmoidal V0 vs. [S] kinetic plots.
B) Allosteric proteins are generally composed of several subunits.
C) An effector may eithe
E) Heterotropic allosteric effectors compete with substrate for binding sites.
A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic
site is termed a(n):
A) allosteric inhibitor.
B) alternative inhibitor.
C) competitive inhibitor.
D) stereospecific agent.
E) transition-state analog.
A) allosteric inhibitor.
Allosteric enzymes:
A) are regulated primarily by covalent modification.
B) usually catalyze several different reactions within a metabolic pathway.
C) usually have more than one polypeptide chain.
D) usually have only one active site.
E) usually show str
C) usually have more than one polypeptide chain.
A metabolic pathway proceeds according to the scheme, R ? S ? T ? U ? V ? W. A regulatory
enzyme, X, catalyzes the first reaction in the pathway. Which of the following is most likely correct for this pathway?
A) Either metabolite U or V is likely to be a
C) The last product, W, is likely to be a negative modulator of X, leading to feedback inhibition.
Which of the following has not been shown to play a role in determining the specificity of protein kinases?
A) Disulfide bonds near the phosphorylation site
B) Primary sequence at phosphorylation site
C) Protein quaternary structure
D) Protein tertiary st
A) Disulfide bonds near the phosphorylation site
How is trypsinogen converted to trypsin?
A) A protein kinase-catalyzed phosphorylation converts trypsinogen to trypsin.
B) An increase in Ca2+ concentration promotes the conversion.
C) Proteolysis of trypsinogen forms trypsin.
D) Trypsinogen dimers bind a
C) Proteolysis of trypsinogen forms trypsin.