Chapter 15—Enzyme Regulation
MULTIPLE CHOICE
1. All of the following are correct statements about enzyme regulation EXCEPT:
a.
Enzymes can be inhibited by the products they produce.
b.
Enzymes can be inactivated by the addition of a functional group.
c.
Coenzyme and substrate availability can regulate enzyme reaction rate.
d.
The reaction rate slows as equilibrium is approached.
e.
The activity of an enzyme is covalently affected by allosteric regulators.
2. All of the following are examples of a zymogen and its activating protease EXCEPT:
a.
chymotrypsinogen and chymotrypsin.
b.
procarboxypeptidase and elastase.
c.
proelastase and elastase.
d.
pepsinogen and pepsin.
e.
trypsinogen and trypsin.
3. Proinsulin is converted into insulin by:
a.
proteolytic excision of a specific peptide.
b.
allosteric binding of glucose.
c.
phosphorylation to the active form.
d.
removal of phosphate by converter enzymes.
e.
none of the above.
4. Regulation of metabolism by activation of the gene encoding a particular enzyme is called ____.
a.
covalent modification
b.
repression
c.
induction
d.
allosteric activation
e.
none of the above
5. Usually the quickest method of influencing an enzymatic activity is by:
a.
allosteric regulation.
b.
covalent modification.
c.
enzyme induction.
d.
activation of a zymogen.
e.
enzyme destruction.
Garrett/Grisham 5e Test Bank 2
6. Which of the following statements is correct regarding isozymes?
a.
they catalyze the same reaction but have vastly different structures
b.
they are always monomeric proteins
c.
their differences are based upon the type of tissue in which they are present
d.
they often respond to different inhibitors and activators
e.
both c and d are correct
7. All are true about isozymes of lactate dehydrogenase (LDH) that are present in a number of different
tissues EXCEPT:
a.
They have differing Km values for lactate.
b.
The amount of subunit phosphorylation differs.
c.
They have differing Km values for pyruvate.
d.
The ratios of A and B subunits differ depending upon the tissue type
e.
They have different kinetic parameters.
8. All are characteristic of allosteric enzymes EXCEPT:
a.
Effectors may show stimulatory or inhibitory activity.
b.
They have multiple subunits.
c.
They obey Michaelis-Menten kinetics.
d.
The regulatory effect is by altering conformation and interaction of subunits.
e.
Binding one subunit impacts binding of substrate to other subunits.
9. All are properties of regulatory enzymes EXCEPT:
a.
pathway end-products may act as allosteric inhibitors.
b.
v vs [S] plots are sigmoid- or S-shaped.
c.
substrate binding is cooperative.
d.
monomeric enzymes with a single regulated active site.
e.
may be stimulated by allosteric activators.
10. Which of the following statements regarding enzyme regulation is true?
a.
addition of an inhibitor to a V system results in kinetics similar to addition of a
competitive inhibitor to a typical hyperbolic system
b.
allosteric effectors are always more powerful than covalent modification
c.
addition of an allosteric activator to a K system changes the plot of V vs. [S] from a
sigmoidal curve to a more hyperbolic curve
d.
the T state of an enzyme generally has more activity than the R state
e.
none of the above are true
11. When binding one equivalent of S to an allosteric protein enhances the binding of additional
equivalents of S to the same protein molecule, it is termed a(n):
a.
positive heterotropic effector.
b.
positive homotropic effector.
c.
negative heterotropic effector.
d.
negative homotropic effector.
e.
none of the above.
12. The presence of a negative allosteric effector on an allosteric protein would:
a.
cause a shift to the left in the sigmoidal curve.
b.
increase the number of R conformations.
c.
decrease the cooperativity of the substrate.
d.
raise the apparent value of the equilibrium constant, L.
e.
increase the likelihood of the binding of S.
13. An enzyme that displays negative cooperativity has ____ activity when the substrate is present at
concentrations ____ than the value of K0.5 when compared with enzymes that display no
cooperativity.
a.
increased; greater
b.
increased; less
c.
decreased; greater
d.
decreased; less
e.
both b and c are correct
14. The addition of an effector to an enzyme that acts as a K system of the MWC model causes the half-
maximum velocity (K0.5) to ____ and Vmax to ____.
a.
remain constant; remain constant
b.
remain constant; change
c.
change; remain constant
d.
change; change
e.
none of the above
15. The function of glycogen phosphorylase is:
a.
the conversion of glucose-1-phosphate to glucose-6-phosphate.
b.
to break down ATP.
c.
to catalyze the phosphorolysis of glucose-1-phosphate from glycogen molecules.
d.
to inhibit the production of glucose-1-phosphate.
e.
to stimulate the build up of glycogen.
16. Characteristics of glycogen phosphorylase include all EXCEPT:
a.
covalently linked pyridoxal phosphate.
b.
an active site at the center of each of two identical subunits.
c.
regulatory phosphorylation site on a Ser residue.
d.
allosteric effector sites for ATP and glucose-6-phosphate.
e.
all are true.
17. For muscle glycogen phosphorylase, Pi (inorganic phosphate) is a:
a.
positive homotropic effector.
b.
negative homotropic effector.
c.
positive heterotropic effector.
d.
negative heterotropic effector.
e.
none of the above.
18. The correct sequence for the hormone-activated enzymatic cascade that leads to activation of glycogen
phosphorylase is:
A.
Phosphorylation to activate phosphorylase kinase
B.
Activation of G-protein
C.
Activation of adenylyl cyclase to produce cAMP
D.
Phosphorylation of glycogen phosphorylase
E.
cAMP activation of protein kinase A (PKA)
a.
A, B, C, D, E
b.
B, C, E, A, D
c.
C, B, A, D, E
d.
B, D, E, A, C
e.
E, A, D, C, B
19. The G subunit of GTP-binding protein (G protein) has all of the following characteristics EXCEPT:
a.
activates adenylyl cyclase.
b.
hormone-receptor complex promotes G release from G.
c.
has a very active GTPase activity.
d.
binds GDP in the G complex.
e.
GTP hydrolysis leads to G-adenylyl cyclase dissociation.
20. All are true for cAMP-dependent protein kinase EXCEPT:
a.
also known as PKA.
b.
phosphorylase kinase is a substrate.
c.
consists of a pair of catalytic subunits.
d.
two regulatory subunits block catalytic activity without cAMP binding.
e.
phosphorylates glycogen phosphorylase.
21. Glycogen phosphorylase displays allosteric activation and inhibition by multiple modes. Which of the
following is a correct relation?
a.
phosphate: positive heterotropic effector
b.
AMP: negative heterotropic effector
c.
ATP: positive heterotropic effector
d.
glucose-6-phosphate: negative heterotropic effector
e.
phosphorylation: covalent inhibitor
22. Adenylyl cyclase catalyzes the synthesis of cAMP, and cAMP hydrolysis is catalyzed by a 5′-
phosphodiesterase. If caffeine inhibits 5′-phosphodiesterases, drinking a caffeinated beverage would
____ cAMP levels and ____ glycogen phosphorylase activity.
a.
increase; decrease
b.
increase; increase
c.
decrease; decrease
d.
decrease; increase
e.
have no effect; have no effect
23. The polypeptide of myoglobin serves all of the following functions EXCEPT:
a.
protects the heme iron from oxidation.
b.
cradles the heme group.
c.
provides a pocket for O2 to fit.
d.
keeps the heme iron in the ferric form.
e.
none of the above.
24. Which statement below about contrasting Hb and Mb is FALSE?
a.
Hb shows sigmoidal, whereas Mb shows hyperbolic oxygen saturation curves.
b.
Hb shows cooperativity, whereas Mb does not.
c.
Hb binds O2 more tightly than Mb.
d.
Oxygen binds to a ferrous ion in both proteins.
e.
Hb-oxygen binding is dependent on physiological changes in pH, whereas Mb-oxygen
binding is not.
25. When O2 binds to heme in hemoglobin, the ____ ion is drawn into the plane of the ____ causing a
conformational change that is transmitted to adjacent subunits enhancing the ____ for additional O2
binding.
a.
Mg2+; globin; planarity
b.
Fe3+; heme; folding
c.
Mg2+; globin; attraction
d.
Fe3+; porphyrin; affinity
e.
Fe2+; porphyrin; affinity
26. We are protected to some extent against low level CO poisoning because:
a.
His F8 on Hb distorts the heme iron atom out of the heme plane and attracts O2, but not
CO.
b.
Affinity of Hb for O2 is much greater than for CO.
c.
His E7 blocks perpendicular binding of CO to heme iron allowing for better binding
competition by O2.
d.
The competition of O2 for heme is much greater than CO for heme.
e.
None of the above.
27. The T form (tense or taut form) of deoxyhemoglobin differs from oxyhemoglobin (the R form or
relaxed form) by all EXCEPT:
a.
covalent linkages between subunits.
b.
specific intrachain hydrogen bonds.
c.
between -subunit salt links (ion-pair bonds).
d.
between -subunits salt links (ion-pair bonds).
e.
intrachain salt bridges.
28. Which of the following is true regarding effectors of oxygen-hemoglobin binding?
a.
actively metabolizing tissues produce acid which increases hemoglobin’s affinity for
oxygen
b.
the presence of CO2 enhances the release of oxygen from hemoglobin
c.
2,3-bisphosphoglycerate must dissociate from hemoglobin before oxygen can be released
d.
the presence of protons will counter the effect of CO2 on the realease of oxygen by
hemoglobin
e.
none of the above
29. Carbon dioxide affects O2 binding to Hb by:
a.
Hb competing with carbonic anhydride for CO2.
b.
directly binding to heme-Fe in the oxygen binding site.
c.
forming iron carbonate with the heme-iron.
d.
forming H+ + HCO3− where the H+ is an antagonist to oxygen binding to Hb.
e.
forming HCO3− that combines with H+ to increase CO2 binding.
30. All of the following are characteristics of hemoglobin’s binding of oxygen EXCEPT:
a.
CO2 promotes dissociation of O2 from hemoglobin by lowering the pH.
b.
Protons promote binding of oxygen by Hb.
c.
2,3-Bisphosphoglycerate (BPG) promotes release of O2 by Hb.
d.
CO2 can bind with Hb’s free amino groups and stabilize deoxy-Hb.
e.
BPG and O2 are mutually exclusive allosteric effectors of Hb.
31. BPG shifts the oxygen saturation curve of Hb to the ____ because BPG binds to ____ making Hb an
O2 delivery system eminently suited for ____.
a.
right; deoxyHb; humans and other primates
b.
right; deoxyHb; cattle, sheep and goats
c.
left; oxyHb; cattle, sheep and goats
d.
left; oxyHb; humans and other primates
e.
none of the above
32. Fetal hemoglobin (Hb F) has an intrinsically greater affinity for O2
than adult hemoglobin (Hb A) because:
a.
Hb F has a diminished capacity to bind BPG compared to Hb A.
b.
Hb A has a greater affinity for oxygen than does Hb F.
c.
BPG binds Hb F with greater affinity than it binds Hb A.
d.
The pH of fetal blood is less than the pH of maternal blood.
e.
All of the above are correct.
33. The cause of cell sickling in sickle cell anemia is:
a.
interaction of oxy-Hb S with the cell membrane.
b.
precipitation of deoxy-Hb S into long, chain-like fibers.
c.
formation of oxy-Hb S complexes and subsequent cell disruption.
d.
precipitation of Hb S – Hb A hybrid molecules.
e.
none of the above.
34. Which of the following will cause an increase in the activity of glycogen phosphorylase?
a.
binding of ATP to the allosteric site
b.
phosphorylation of the enzyme
c.
high levels of glucose-6-phosphate
d.
conversion from the relaxed to tense state
e.
drinking coffee
35. Which of the following statements regarding enzyme regulation is true?
a.
the conversion of trypsinogen to trypsin is an example of zymogen activation
b.
allosteric effectors are always more powerful than covalent modification
c.
addition of an inhibitor to a V system results in kinetics similar to addition of a
competitive inhibitor to a typical hyperbolic system
d.
the T state of an enzyme generally has more activity than the R state
e.
none of the above
36. Glycogen phosphorylase displays allosteric activation and inhibition by multiple modes. Which of the
following is a correct relation?
a.
phosphate: positive heterotropic effector
b.
AMP: negative heterotropic effector
c.
ATP: positive heterotropic effector
d.
glucose-6-phosphate: negative heterotropic effector
e.
phosphorylation: covalent inhibitor
37. Of the following, which promotes the release of oxygen by hemoglobin?
a.
increase in [H+]
b.
decrease in [CO2]
c.
decrease in [2,3-bisphosphoglycerate]
d.
Tyr HC2-Val FG5 hydrogen bond breaking
e.
none of the above