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Tymoczko, Biochemistry: A Short Course 3e, Launchpad - Chapter 7: Practice Quiz
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1. Vmax, the maximum velocity, of an enzyme-catalyzed reaction is
a. the rate observed when all enzyme active sites are saturated with substrate.
b. independent of the amount of enzyme present.
c. the rate observed at the highest substrate concentration that can be experimentally obtained.
d. the initial rate observed at very low substrate concentrations.
2. When k–1 > k2 (that is, when the rate constant for dissociation of the enzyme substrate complex is
greater than the rate constant for conversion to product), the KM is most analogous to
a. the Kd.
b. the Ka.
c. the kcat.
d. 1/kcat.
3. To obtain k2, the turnover number of an enzyme, one must
a. divide Vmax by 2.
b. divide v by Vmax.
c. divide Vmax by the total enzyme concentration.
d. divide Vmax by kcat.
4. In a double-reciprocal or Lineweaver-Burk plot, the KM is found from
a. y-intercept = –1/KM.
b. x-intercept = –1/KM.
c. slope = KM.
d. y-intercept = –Vmax/KM.
e. x-intercept = –Vmax/KM.
5. On what basis are enzymes and proteins with allosteric properties different from those without
allosteric properties?
a. on the basis of having more than one subunit
b. on the basis of having different responses to non-substrate molecules such as inhibitors
c. on the basis of their different dependence on substrate concentration
d. on the basis of having more than one substrate-binding site
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Tymoczko, Biochemistry: A Short Course 3e, Launchpad - Chapter 7: Practice Quiz
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6. The effects of molecules other than substrate on allosteric enzymes are called
a. heterotropic effects.
b. homotropic effects.
c. allosteric effects.
d. competitive effects.
7. A heterotropic activator will have what effect on a sigmoidal kinetic plot (v vs. [S])?
a. The curve will shift to the right.
b. The curve will shift to the left.
c.
The curve will become more sigmoidal. (The first half will be decreased and the second half
increased.)
d. The curve will become hyperbolic.
8. The study of the rates of enzyme-catalyzed reactions:
a. is called enzyme kinetics.
b. can involve determining how fast the substrate disappears as it is converted to product.
c. can involve following the appearance of product formed over time.
d. All of the above
9. Which of the following is TRUE regarding a Michaelis-Menten kinetics graph?
a. The x-axis is "time."
b. The x-axis is "substrate concentration."
c. The y-axis is "product concentration."
d. Both the first and third answers are true.
10. An allosteric enzyme can exist in two states, _____ and _____.
a. tight; relaxed
b. tight; responsive
c. turgid; relaxed
d. tense; relaxed
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Tymoczko, Biochemistry: A Short Course 3e, Launchpad - Chapter 8: Practice Quiz
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1. TPCK, a molecule with large hydrophobic groups, inactivates chymotrypsin but not trypsin because
a.
it looks like the substrate for chymotrypsin (but not trypsin) and thus can bind in its active site
and modify His-57.
b. trypsin does not have a His residue present in the active site to react with TPCK.
c. TPCK is a transition state analog for chymotrypsin but not trypsin.
d. it binds to trypsin but does not modify its active site.
2. Which of the following is NOT true of a competitive inhibitor?
a. It irreversibly inhibits the enzyme by chemically modifying a group at the active site.
b. It often resembles the substrate for the enzyme it inhibits.
c. Its effects can be overcome by increasing the concentration of substrate.
d. It competes with substrate for binding to the active site.
3. If an enzyme inhibitor decreases the apparent KM and reduces the Vmax as well, then it can be
classified as a(n)
a. competitive inhibitor.
b. noncompetitive inhibitor.
c. uncompetitive inhibitor.
d. irreversible inhibitor.
4. Diisopropyl fluorophosphate (DIFP) inactivates chymotrypsin by covalently modifying serine-195.
This occurs because
a.
DIFP looks like the substrate for chymotrypsin and binds in the active site as a competitive
inhibitor.
b.
serine-195 is in an environment which gives it a higher than normal reactivity with respect to
DIFP.
c.
DIFP randomly modifies all serine residues on the protein and if enough is added the one in
the active site will eventually be modified.
d. DIFP approaches serine-195 more closely than other substrates.
5. The mechanism of chymotrypsin can be viewed as a two-step process, acylation of the enzyme active
site followed by a deacylation reaction. The observation of "burst" kinetics in rapid kinetic studies of
the hydrolysis of p-nitrophenylphosphate by chymotrypsin is due to
a. the rate of the acylation reaction being slower than the deacylation reaction.
b. the rate of the acylation reaction being faster than the deacylation reaction.
c. the rates of acylation and deacylation being equal.
d. the change of the reaction mechanism after the "burst" phase.
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Tymoczko, Biochemistry: A Short Course 3e, Launchpad - Chapter 8: Practice Quiz
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6. You have isolated a new protease that cleaves peptide bonds on the carboxyl side of Asp and Glu.
Based on the enzyme's inactivation by DIFP, you suspect that it may utilize a mechanism similar to
chymotrypsin. The difference in specificity might be explained by
a. the presence of a positively charged residue in the S1 binding pocket.
b. the presence of a negatively charged residue in the S1 binding pocket.
c. replacement of serine-195 with a positively charged residue.
d. the absence of the S1 binding pocket.
7. Many drugs are competitive inhibitors of specific enzymes. In such cases,
a.
the amount of drug needed to achieve the same effect is increased as the substrate
concentration increases.
b.
the amount of drug needed to achieve the same effect is increased as the substrate
concentration decreases.
c. substrate must be bound to the enzyme before the drug can bind.
d. the enzymes have to be allosterically regulated.
8. The activity of an enzyme can be regulated by a:
a. competitive inhibitor binding to the active site.
b. competitive inhibitor binding to the ES complex.
c. noncompetitive inhibitor binding to the ES complex.
d. Only the first and third answers are correct.
9. As temperature increases:
a. the frequency of collision between substrate and enzyme also increases.
b. Brownian motion increases.
c. catalytic activity increases, until the enzyme is denatured by excessive heat.
d. All of the above
10. The mechanism of chymotrypsin proceeds in _____ step(s) linked by a covalently bound
intermediate.
a. one
b. two
c. three
d. six
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