Deck 15: Enzyme Regulation

Question 1

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

Answer 1

B

Question 2

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.

Answer 2

E

Question 3

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.

Answer 3

E

Question 4

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.

Question 5

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

Question 6

The addition of an effector to an enzyme that acts as a K system of the MWC model causes the half-maximum velocity (K_0.5) to ____ and V_max to ____.

A) remain constant; remain constant
B) remain constant; change
C) change; remain constant
D) change; change
E) none of the above

Question 7

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.

Question 8

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

Question 9

All are true about isozymes of lactate dehydrogenase (LDH) that are present in a number of different tissues EXCEPT:

A) They have differing K_m values for lactate.
B) The amount of subunit phosphorylation differs.
C) They have differing K_m values for pyruvate.
D) The ratios of A and B subunits differ depending upon the tissue type
E) They have different kinetic parameters.

Question 10

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.

Question 11

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

Question 12

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.

Question 13

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.

Question 14

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

Question 15

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

Question 16

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.

Question 17

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.

Question 18

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.

Question 19

The Gα subunit of GTP-binding protein (G protein) has all of the following characteristics EXCEPT:

A) activates adenylyl cyclase.
B) hormone-receptor complex promotes Gα release from Gβγ.
C) has a very active GTPase activity.
D) binds GDP in the Gαβγ complex.
E) GTP hydrolysis leads to Gα-adenylyl cyclase dissociation.

Question 20

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

Question 21

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

Question 22

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.

Question 23

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.

Question 24

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.

Question 25

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.

Question 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.

Question 27

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

Question 28

Explain allosteric inhibition with an example.​

Question 29

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.

Question 30

Explain the KNF model of allosteric inhibition.​

Question 31

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.

Question 32

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

Question 33

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

Question 34

Explain how 2,3-bisphosphoglycerate (BPG) and O₂ are mutually exclusive allosteric effectors for hemoglobin (Hb).​

Question 35

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

Question 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

Question 37

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 β-subunit salt links (ion-pair bonds).
D) between α-subunits salt links (ion-pair bonds).
E) intrachain salt bridges.

Question 38

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

Question 39

Discuss zymogens.​

Question 40

Discuss acetylation as a prominent modification for the regulation of metabolic enzymes.​