Test Bank for Biochemistry 7th Edition By Jeremy M. Berg

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Test Bank for Biochemistry 7th Edition By Jeremy M. Berg




Test Bank Of Biochemistry 7th Edition By Jeremy M. Berg

Chapter 7   Hemoglobin: A Portrait of a Protein in Action



Matching Questions

Use the following to answer questions 1-10:


Choose the correct answer from the list below. Not all of the answers will be used.

  1. a) cooperative
  2. b) Bohr effect
  3. c) thalassemia
  4. d) carbamate
  5. e) metmyoglobin
  6. f) superoxide
  7. g) myoglobin
  8. h) bicarbonate ion
  9. i) sickle-cell anemia
  10. j) protoporphyrin
  11. k) fetal
  12. l) carbonic acid


1. Carbon dioxide reacts with the amino terminal groups of deoxyhemoglobin to form __________ groups.


  Ans:  d
  Section:  7.3



2. ____________ This is the organic portion of the heme group in hemoglobin.


  Ans:  j
  Section:  7.1


3. ____________ This is a genetic disease due to the decreased production of one of the subunits of hemoglobin.


  Ans:  c
  Section:  7.4


4. ____________ This is the chemical form in which most of the carbon dioxide is transported in the blood.


  Ans:  h
  Section:  7.3


5. ____________ This substance is produced when carbon dioxide reacts with water.


  Ans:  l
  Section:  7.3


6. ____________ This type of hemoglobin is composed of two chains and two chains.


  Ans:  k
  Section:  7.2


7. ____________ This is the molecule whose function is to store oxygen in muscle cells.


  Ans:  g
  Section:  Introduction


8. ____________ This oxidized hemeprotein does not reversibly bind oxygen.


  Ans:  e
  Section:  7.1


9. ____________ This type of binding is indicated by a sigmoidal-shaped binding curve.


  Ans:  a
  Section:  7.2


10. ____________ This condition is a result of a single point mutation in the chain of hemoglobin.


  Ans:  i
  Section:  7.4



Fill-in-the-Blank Questions


11. Under normal conditions, the heme iron in myoglobin and hemoglobin is in the ____________ oxidation state.
  Ans:  ferrous, or Fe+2     Section:  7.1


12. The ability of myoglobin to bind oxygen depends on the presence of a bound prosthetic group called _____________.
  Ans:  heme     Section:  7.1


13. In hemoglobin, the iron of the heme is bonded to the four nitrogens of porphyrin and to the proximal ______________ residue of the globin chain.
  Ans:  histidine     Section:  7.1


14. The binding of 2-3-bisphosphogycerate to hemoglobin ____________ (increases, decreases) its affinity of oxygen binding.
  Ans:  decreases     Section:  7.2


15. The effect of pH on oxygen-binding of hemoglobin is referred to as the _____________.
  Ans:  Bohr effect     Section:  7.3


16. Deoxyhemoglobin is stabilized through ___________________ interactions between the carbamates and positively charged amino acids at the interface between dimmers.
  Ans:  salt-bridge     Section:  7.3


17. The T-state of hemoglobin is stabilized by a salt bridge between 1 Asp 94 and the C-terminal ___________________ of the 1 chain.
  Ans:  histidine     Section:  7.3


18. In normal adult hemoglobin, HbA, the 6 position is a glutamate residue, whereas in sickle-cell hemoglobin, HbS, it is a ____________ residue.
  Ans:  valine     Section:  7.4


19. As the partial pressure of carbon dioxide increases, the affinity of oxygen binding to hemoglobin ______________.
  Ans:  decreases     Section:  7.3


20. 2,3-Bisphosphoglycerate binds only to the __________________ form of hemoglobin.
  Ans:  T-, or deoxy     Section:  7.2



Multiple-Choice Questions


21. What factor(s) influence(s) the binding of oxygen to myoglobin?
  A) The concentration of bicarbonate ion, HCO3
  B) The partial pressure of oxygen, pO2
  C) The concentration of hemoglobin present
  D) The concentration of 2,3-BPG
  E) Both b and d
  Ans:  B     Section:  7.2


22. Which of the following is correct concerning the differences between hemoglobin and myoglobin?
  A) Both hemoglobin and myoglobin are tetrameric proteins.
  B) Hemoglobin exhibits a hyperbolic O2 saturation curve while myoglobin exhibits a sigmoid shaped curve.
  C) Hemoglobin exhibits cooperative binding of O2 while myoglobin does not.
  D) Hemoglobin exhibits a higher degree of O2 saturation at all physiologically relevant partial pressures of O2 than does myoglobin.
  E) All of the above.
  Ans:  C     Section:  7.2


23. Which of the following is not correct concerning myoglobin?
  A) The globin chain contains an extensive -helix structure.
  B) The heme group is bound to the globin chain by two disulfide bonds to cysteine residues.
  C) The iron of the heme group is in the Fe+2 oxidation state.
  D) The diameter of the iron ion decreases upon binding to oxygen.
  E) The function of myoglobin is oxygen storage in muscle.
  Ans:  B     Section:  7.1


24. The structure of normal adult hemoglobin can be described as
  A) a tetramer composed of four myoglobin molecules.
  B) a tetramer composed of two dimers.
  C) a tetramer composed of two 2 and two 2 dimers.
  D) a tetramer composed of two 2 and two 2 dimers.
  E) None of these accurately describe hemoglobin.
  Ans:  B     Section:  7.1


25. Which of the following is correct concerning fetal hemoglobin?
  A) Fetal hemoglobin is composed of two and two subunits.
  B) Fetal hemoglobin binds 2,3-BPG more tightly than normal adult hemoglobin.
  C) Fetal hemoglobin binds oxygen less than HbA at all pO2.
  D) Fetal hemoglobin does not exist in the T-form.
  E) None of the above.
  Ans:  A     Section:  7.2


26. Hemoglobin-binding of oxygen is best described as a
  A) concerted model.
  B) Michaelis-Menten model.
  C) sequential model.
  D) combination of sequential and concerted models.
  E) None of the above.
  Ans:  D     Section:  7.2


27. 2-3 Bisphosphoglycerate
  A) binds in the central cavity in the T-form of hemoglobin.
  B) preferentially binds to deoxyhemoglobin and stabilizes it.
  C) is present in the red blood cells.
  D) All of the above.
  E) None of the above.
  Ans:  D     Section:  7.2


28. What is the Bohr effect?
  A) the ability of hemoglobin to retain oxygen when in competition with myoglobin
  B) the regulation of hemoglobin-binding by hydrogen ions and carbon dioxide
  C) the alteration of hemoglobin conformation during low oxygen stress
  D) All of the above.
  E) None of the above.
  Ans:  B     Section:  7.3


29. Why is the HbS mutation so prevalent in Africa and other tropical regions?
  A) The oxygen binds with greater affinity to the proximal histidine residue of HbS.
  B) Bonding of carbon dioxide to HbS molecules increases the binding of oxygen.
  C) Hemoglobin binds more oxygen with the reduction of the hemoglobin S chain.
  D) Hemoglobin binds more oxygen with aggregations of chains found in sickle-cell hemoglobin.
  E) People with sickle-cell trait are resistant to malaria, increasing the prevalence of the HbS allele.
  Ans:  E     Section:  7.4


30. Which of the following describes the Bohr effect?
  A) Lowering the pH results in the release of O2 from oxyhemoglobin.
  B) Increasing the pressure of CO2 results in the release of O2 from oxyhemoglobin.
  C) Increasing the pH increases the T-form of hemoglobin.
  D) All of the above.
  E) a and b.
  Ans:  E     Section:  7.3


31. Which of the following is correct concerning the following equilibria?

CO2  +  H2O    H2CO3

  A) An increase in the pressure of CO2 will result in a decrease of pH.
  B) This reaction is catalyzed by carbonic anhydrase.
  C) The H2CO3 dissociates to H+ and bicarbonate ion, HCO3.
  D) The majority of CO2 is transported to the lungs in the form of HCO3.
  E) All of the above.
  Ans:  E     Section:  7.3


32. Carbon dioxide forms carbamate groups in proteins by reaction with
  A) aspartate residues.
  B) cysteine residues.
  C) N-terminal amino groups.
  D) tyrosine residues.
  E) heme groups.
  Ans:  C     Section:  7.3


33. Sickle-cell anemia is caused by
  A) a decreased production of chains of hemoglobin.
  B) a substitution of a Glu residue for a Phe residue at the 6 position.
  C) the loss of the heme group because the proximal His is oxidized.
  D) a substitution of a Val residue for a Glu residue at the 6 position.
  E) a substitution of Glu residue for His at the C-terminal of the chain.
  Ans:  D     Section:  7.4



34. Which of the following is correct concerning the oxygenation plot of proteins X and Y shown below?
  A) Protein Y exhibits tighter oxygen-binding than protein X.
  B) Protein Y corresponds to fetal hemoglobin, and protein X corresponds to normal adult hemoglobin.
  C) Protein X corresponds to fetal hemoglobin, and protein Y corresponds to normal adult hemoglobin.
  D) Protein X corresponds to myoglobin, and protein Y corresponds to hemoglobin.
  E) None of the above.
  Ans:  C     Section:  7.2


35. Which of the following is not correct concerning the oxygenation plot of proteins X and Y shown below?
  A) Protein X exhibits tighter oxygen binding than protein Y.
  B) Protein Y would function as a better transport protein than protein X.
  C) Protein X exhibits cooperative binding, whereas Y does not.
  D) Protein X corresponds to myoglobin, and protein Y corresponds to hemoglobin.
  E) Protein Y contains multiple binding sites.
  Ans:  C     Section:  7.2


36. Which is not correct concerning the models that are accepted to describe cooperative binding?
  A) In the sequential model, the binding of a ligand changes the conformation of the subunit to which it binds, which in turn induces a change in neighboring subunits.
  B) All known allosteric proteins exhibit either the concerted or sequential model exclusive of the other.
  C) Both models incorporate a low affinity T-state and a higher affinity R-state.
  D) Both models explain the sigmoid-shaped binding curve.
  E) In the concerted model, all molecules exist either in the T-state or the R-state.
  Ans:  B     Section:  7.2


37. Consider the oxygen-binding profile at three different pH values of 7.6, 7.4, and 7.2.  Which statement is most correct?


  A) Curve X most likely corresponds to pH 7.2.
  B) Curve Z most likely corresponds to pH 7.6.
  C) Hb has a higher affinity for oxygen at the pH of curve Z.
  D) Curve Y most likely corresponds to pH 7.4.
  E) pH has no effect on the oxygenation of hemoglobin.
  Ans:  D     Section:  7.3


38. What would be the expected result of a Lys residue being substituted with a Ser residue in the BPG binding site of hemoglobin?
  A) BPG would bind tighter because of the loss of a positive charge.
  B) BPG would bind tighter because of the gain of a positive charge.
  C) BPG would bind less tightly because of the loss of a positive charge
  D) BPG would bind less tightly because of the gain of a positive charge.
  E) This substitution would have no effect on the binding of BPG.
  Ans:  C     Section:  7.2



Short-Answer Questions


39. Why is it advantageous for hemoglobin to have allosteric properties?
  Ans: Hemoglobin binds oxygen in a positive cooperative manner. This allows it to become saturated in the lungs, where oxygen pressure is high. When the hemoglobin moves to tissues, the lower oxygen pressure induces it to release oxygen and thus deliver oxygen where it is needed.
  Section:  7.2


40. What is fetal hemoglobin? How does it differ from adult hemoglobin?
  Ans: Fetal hemoglobin contains two a and two g chains, in contrast to adult hemoglobin with two a and two b chains. The fetal hemoglobin g chain is probably a result of gene duplication and divergence. The difference in the chains results in a lower binding affinity of 2-3 BPG to fetal hemoglobin. Thus, the fetal hemoglobin has a higher affinity for oxygen, and the oxygen is effectively transferred from the mothers hemoglobin to fetal hemoglobin.
  Section:  7.2


41. What is metmyoglobin?
  Ans: Metmyoglobin is formed when the heme iron ion, which is normally in the +2 oxidation state, is oxidized to the +3 oxidation state.  This oxidized form of myoglobin does not bind molecular oxygen and is not functional.
  Section:  7.1


42. Describe the recurring structure called the globin fold.
  Ans: Each of the four subunits of hemoglobin consists of a set of helices in the same arrangement as the helices of myoglobin. The arrangement is known as the globin fold.
  Section:  7.1


43. What functional role does the distal histidine play in the function of myoglobin and hemoglobin?
  Ans: The bonding between the iron and oxygen can be described as a combination of resonance structures, one with Fe2+ and dioxygen and another with Fe3+ and superoxide.  The distal histidine donates a hydrogen bond to this complex, stabilizing the complex, and inhibits the oxidation of the iron to the ferric state.
  Section:  7.1


44. Draw the oxygen-binding curve of myoglobin and that of hemoglobin.  Indicate the partial pressure of oxygen in the lungs and the range of pressure in tissue.




20 40 torr



  Section:  7.2 and Figure 7.8


45. Describe the structure of normal adult hemoglobin.
  Ans: Normal adult hemoglobin, HbA, is a tetramer.  It is composed of two subunits and two subunits.  Each subunit has a structure very similar to myoglobin.  It can be best described as a pair of identical dimers.  Each subunit contains a heme group.  So, each molecule of hemoglobin can bind up to four molecules of oxygen.
  Section:  7.2


46. What is neuroglobin and what is its suspected role?
  Ans: Neuroglobin is expressed in the brain, and at especially high levels in the retina. It may play a role in protecting neural tissues from hypoxia.
  Section:  7.4


47. Describe the concerted model to explain allosteric cooperative binding.
  Ans: The protein exists in two conformations: a T-state (for tense) that has a lower affinity for the ligand and an R-state (for relaxed) that has a higher affinity for the ligand.  In the concerted model, all of the molecules exist either in the T-state or in the R-state.  At each ligand concentration, there is an equilibrium between the two states.  An increase in the ligand concentration shifts the equilibrium from the T- to the R-state.
  Section:  7.2


48. Describe the role of 2,3-bisphosphoglycerate in the function of hemoglobin.
  Ans: 2,3-bisphosphoglycerate, 2,3-BPG, is a relatively small, highly anionic molecule found in the RBC.  2,3-BPG only binds to the center cavity of deoxyhemoglobin (T-state).  The size of the center cavity decreases upon the change to the R-form so that it cannot bind to the R-state.  Thus, the presence of 2,3-BPG shifts the equilibrium toward the T-state. The  T-state is unstable, and without BPG, the equilibrium shifts so far toward the R-state that little oxygen would be released under physiological conditions.
  Section:  7.2


49. Describe the chemical basis of the Bohr effect.
  Ans: The effect observed by Christian Bohr is that hemoglobin becomes deoxygenated as the pH decreases.  In deoxyhemoglobin, three amino acid residues form two salt bridges that stabilize the T-state.  One of these is formed between the C-terminal His 146 and an Asp residue (94).  As the pH increases, this stabilizing salt bridge is broken because His becomes deprotonated and loses its positive charge.  At lower pH values, this His is positively charged.  The formation of the salt bridge shifts the equilibrium from the R-state to the T-state, thus releasing oxygen.
  Section:  7.3


50. Describe how carbon dioxide affects the oxygenation of hemoglobin.
  Ans: Increased levels of carbon dioxide cause hemoglobin to release oxygen.  The more active the tissue, the more fuel is burned and the more CO2 is produced.  These active tissue cells have the greatest need for oxygen to produce more energy.  The CO2 combines with the N-terminal amino groups to form negatively charged carbamate groups.  The negatively charged carbamate groups form salt bridges that stabilize the T-state.  Thus, the increase of carbon dioxide causes the conversion of the R-state to the T-state, releasing the bound oxygen to the tissues producing the most CO2.
  Section:  7.3


51. Briefly describe the cause of sickle-cell anemia.
  Ans: Sickle-cell anemia is a genetic disorder that is the result of a single substitution of 6 Glu with a Val.  This changes a negatively charged side chain to a nonpolar, hydrophobic side chain.  This Val binds into a hydrophobic pocket on the chain of an adjacent molecule whose 6 Val binds to another molecule, thus hemoglobin aggregates.  These aggregates form long fibers that strain the RBC and force into a sickled shape.  The distorted red blood cells clog capillaries and impair blood flow, resulting in the sickle-cell crisis.  The sickled cells are then destroyed, resulting in the anemia.
  Section:  7.4


52. What is thalassemia?
  Ans: Thalassemia is caused by the substantial decreased production of one of the subunits of hemoglobin.  In -thalassemia, the decreased production of the chain results in the formation of tetramers of only the chain.  This 4 binds oxygen more tightly than HbA and does not exhibit cooperative binding.  In -thalassemia, the chains form insoluble aggregates in the immature red blood cells.
  Section:  7.4


53. What is the role of -hemoglobin stabilizing protein?
  Ans: Four genes express the chains, and only two genes express the chain.  Thus, there is an excess of chains, which if allowed, would aggregate and become insoluble.  Red blood cells produce -hemoglobin stabilizing protein (AHSP), which binds to the chain monomers to from a soluble complex.  This prevents the aggregation of the subunits.
  Section:  7.4

Chapter 17   The Citric Acid Cycle



Matching Questions

Use the following to answer Questions 110:


Choose the correct answer from the list below. Not all of the answers will be used.

  1. a) cytosol
  2. b) phosphorylation
  3. c) anaplerotic
  4. d) mitochondria
  5. e) cis-aconitate
  6. f) arsenite
  7. g) metabolon
  8. h) oxaloacetate
  9. i) inner membrane
  10. j) flavoproteins
  11. k) carbon dioxide
  12. l) glyoxylate cycle


1. ____________ Where does the citric acid cycle take place in the cell?


  Ans:  d
  Section:  Introduction


2. ____________ These enymes are tightly associated with FAD or FMN.


  Ans:  j
  Section:  17.1


3. ____________ This is the intermediate between citrate and isocitrate.


  Ans:  e
  Section:  17.2


4. ____________ This is the location of succinate dehydrogenase.


  Ans:  i
  Section:  17.2


5. ____________ This intermediate is both at the beginning and at the end of the citric acid cycle.


  Ans:  h
  Section:  17.2


6. ____________ This is one of the products of the citric acid cycle.


  Ans:  k
  Section:  Introduction and 17.2


7. ____________ A pathway that allows glucose synthesis from acetyl-CoA.


  Ans:  l
  Section:  17.5


8. ____________ This substance is toxic because it reacts with the neighboring sulfhydryl groups of dihydrolipoyl groups and blocks its reoxidation to lipoamide.


  Ans:  f
  Section:  17.4


9. ____________ This type of enzyme regulation process inhibits the pyruvate dehydrogenase complex.


  Ans:  b
  Section:  17.3


10. ____________ This is the name applied to metabolic reactions that replenish citric acid cycle intermediates that are depleted because they were used for biosynthesis.


  Ans:  c
  Section:  17.4



Fill-in-the-Blank Questions


11. Carbons from carbohydrate enter the citric acid cycle in the form of _______________.
  Ans:  acetyl CoA     Section:  Introduction


12. In the citric acid cycle, the __________ is produced by a substrate-level phosphorylation.
  Ans:  GTP     Section:  17.2


13. E1 of the pyruvate dehydrogenase complex requires the coenzyme ________________ for proper activity.
  Ans:  thiamine pyrophospate     Section:  17.1


14. E2 of the pyruvate dehydrogenase complex contains a lipoyl group that is covalently attached to a _______________ residue of the enzyme.
  Ans:  lysine     Section:  17.1


15. _______________ is a citric acid cycle enzyme that is also an example of an iron-sulfur protein.
  Ans:  Aconitase or succinate dehydrogenase     Section:  17.2


16. The ____________ cycle is a process by which plants and some bacteria can convert two-carbon acetyl units into four-carbon units (succinate) for glucose synthesis, energy production, and biosynthesis.
  Ans:  glyoxylate     Section:  17.5


17. Most organisms cannot convert ______________________ into glucose because of the two decarboxylations in the citric acid cycle.
  Ans:  acetyl CoA    Section:  17.5


18. In general, the citric acid cycle is inhibited under ________ (high, low) energy conditions.
  Ans:  high    Section:  17.3


19. ________________ is the first citric acid cycle intermediate to be oxidized.
  Ans:  Isocitrate     Section:  17.2


20.  Beri-beri is caused by a deficiency of __________________.
  Ans:  thiamine     Section:  17.4



Multiple-Choice Questions


21. The citric acid cycle is also known as the
  A) Krebs cycle. D) A and C.
  B) Cori cycle. E) A, B, and C.
  C) tricarboxylic acid cycle.    
  Ans:  D     Section:  Introduction


22. What molecule initiates the citric acid cycle by reacting with oxaloacetate?
  A) pPyruvate D) All of the above
  B) Acetyl CoA E) None of the above
  C) Oxaloacetate    
  Ans:  B     Section:  17.2


23. What enzyme(s) is (are) responsible for the following reaction?

Pyruvate + CoA + NAD+ acetyl CoA + NADH + H+  + CO2

  A) Acetyl CoA synthetase D) A and B
  B) Pyruvate decarboxylase E) A, B, and C
  C) Pyruvate dehydrogenase complex    
  Ans:  C     Section:  17.1


24. What are the steps involved (in order) in the conversion of pyruvate to acetyl CoA?
  A) Decarboxylation, oxidation, transfer to CoA
  B) Decarboxylation, transfer to CoA, oxidation
  C) Oxidation, decarboxylation, transfer to CoA
  D) Oxidation, transfer to CoA, decarboxylation
  E) None of the above
  Ans:  A     Section:  17.1


25. Which of the following vitamins are precursors to coenzymes that are necessary for the formation of acetyl CoA from pyruvate?
  A) Thiamine, riboflavin, niacin, lipoic acid, and pantothenic acid
  B) Thiamine, riboflavin, niacin, lipoic acid, pantothenic acid, and biotin
  C) Thiamine, riboflavin, niacin, and biotin
  D) Thiamine, riboflavin, and lipoic acid
  E) None of the above
  Ans:  A     Section:  17.1  and Table 17.1


26. Which of the following functions as a flexible swinging arm when it transfers the reaction intermediate from one active site to the next?
  A) FAD
  B) NAD+
  C) Lipoamide
  D) Thiamine pyrophosphate
  E) Coenzyme A
  Ans:  C     Section:  17.1


27. Formation of citrate from acetyl CoA and oxaloacetate is a(n) _________ reaction.
  A) oxidation D) ligation
  B) reduction E) None of the above
  C) condensation    
  Ans:  C     Section:  17.2


28. What is/are the chemical change(s) involved in the conversion of citrate into isocitrate?
  A) Hydration followed by dehydration D) Dehydration followed by hydration
  B) Oxidation E) A and B
  C) Oxidation followed by reduction    
  Ans:  D     Section:  17.2


29. In which reaction is GTP (or ATP) directly formed in the citric acid cycle?
  A) Conversion of succinyl CoA to succinate
  B) Decarboxylation of a-ketoglutarate
  C) Conversion of isocitrate to a-ketoglutarate
  D) All of the above
  E) None of the above
  Ans:  A     Section:  17.2


30. The enzymes in the glyoxylate cycle are the same as the citric acid cycle except for?
  A) Malate synthase
  B) Glyoxylate synthase
  C) Isocitrate lyase
  D) A and B
  E) B and C
  Ans:  D    Section:  17.5


31. Which of these compounds is oxidized by a multienzyme complex that requires five different coenzymes?
  A) D)
  B) E)
  Ans:  B     Section:  17.2


32. Which of the following conditions will activate pyruvate dehydrogenase kinase, which catalyzes the phorphorylation and inactivation of E1 in the pyruvate dehydrogenase complex?
  A) Elevated concentrations of NADH and ATP
  B) Elevated concentrations of NAD+ and ADP
  C) Ca2+
  D) Insulin
  E) Elevated concentrations of acetyl-CoA
  Ans:  A     Section:  17.3


33. Approximately how many ATP or GTP equivalents are produced during one turn of the citric acid cycle?
  A) 10     B) 6     C) 9     D) 12     E) None of the above
  Ans:  A     Section:  17.2


34. In addition to pyruvate dehydrogenase, what other enzymes are key regulatory sites in the citric acid cycle?
  A) Malate dehydrogenase D) A and B
  B) a-ketoglutarate dehydrogenase E) B and C
  C) Citrate synthase (in bacteria)    
  Ans:  E     Section:  17.3


35. The glyoxylate cycle enables plants to survive using only
  A) pyruvate. D) All of the above.
  B) acetate. E) None of the above.
  C) oxaloacetate.    
  Ans:  B     Section:  17.5



Short-Answer Questions


36. Give the net equation of the citric acid cycle.
  Ans: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi
2 CO2 + 3 NADH + 3 H+ + FADH2 + GTP + CoA
  Section:  17.2


37. Why is the isomerization of citrate to isocitrate a necessary step of the citric acid cycle?
  Ans: Citrate is a tertiary alcohol that cannot be oxidized.  The isomerization converts the 3 alcohol into isocitrate, which is a 2 alcohol that can be oxidized.
  Section:  17.2


38. List the five coenzymes that are required for the oxidative decarboxylation of pyruvate and ketoglutarate and give the essential nutrient (vitamin) that is required for each.
  Ans: 1.      Thiamine pyrophosphate:  thiamine, vitamin B1

2.      Lipoamide:  lipoic acid

3.      NAD+:  niacin

4.      FAD:  riboflavin, vitamin B2

5.      Coenzyme A:  pantothenic acid


  Section:  17.1 and 17.2


39. Explain why a GTP is energetically equivalent to an ATP in metabolism.
  Ans: The enzyme nucleoside diphosphokinase reversibly transfers a phosphoryl group from GTP to ADP according the reaction:

GTP + ADP     GDP  +  ATP

Conversly, a phosphoryl group can be transferred from ATP to a GDP forming GTP.

  Section:  17.2


40. Give the reaction in the citric acid cycle by which the energy is conserved in the formation of a phosphoanhydride bond by substrate level phosphorylation.  Give the name of the enzyme that catalyzes this reaction and give the structures of the reactants and products of this reaction.
  Section:  17.2


41. Why is it necessary that there be a mechanism to replenish oxaloacetate?
  Ans: During periods of biosynthesis, oxaloacetate may be converted to amino acids for protein synthesis. Even if acetyl CoA levels are high, the citric acid cycle will operate at reduced levels until new oxaloacetate is formed.
  Section:  17.4


42. Starting with oxaloacetate in the glyoxylate cycle, identify what molecules enter and exit the glyoxylate cycle.
  Ans: The cycle begins with the condensation of acetyl CoA and oxaloacetate to form citrate resulting in a net two carbon entry into the cycle. When isocitrate is hydrolyzed to succinate and glyoxylate, succinate leaves the cycle to serve as a biosynthetic intermediate in other pathways. Glyoxylate then condenses with another Acetyl CoA to form malate, allowing for another two carbon fragment to enter the cycle.
  Section:  17.5


43. What is the energy source that drives the condensation of oxaloacetate and and acetyl CoA to produce citrate?
  Ans: Citrate synthase catalyzes the condensation of acetyl CoA and oxalacetate to form citryl CoA.  This reaction is easily reversible.  The hydrolysis of the thioester of citryl CoA forms citrate and regenerates the CoA.  The hydrolysis of the high energy thioester drives the reaction toward citrate.
  Section:  17.2


44. How does the decarboxylation of a-ketoglutatarate resemble that of pyruvate decarboxylation?
  Ans: Both are a-ketoacids, which are decarboxylated, and involve formation of a thioester with CoA, which has high transfer potential. The enzymatic complexes and mechanisms are similar, and the dihydrolipoyl dehydrogenase components are identical.
  Section:  17.1 and 17.2


45. How many ATP equivalents are produced from the total oxidation of one pyruvate to 3 CO2?
  Ans: The total oxidation of one pyruvate by pyruvate dehydrogenase and the citric acid cycle produces 4 NADH, 1 FADH2 and one GTP.  2.5 ATPs are produced when two electrons are transferred from NADH to oxygen by the electron transport chain.  1.5 ATPs are produced when two electrons are transferred from FADH2 to oxygen by the electron transport chain.  Energetically, a GTP is equal to an ATP.  So a total of 12.5 ATP equivalents are produced (4 2.5 + 1.5 + 1 = 12.5).
  Section:  17.2


46. The G = -21 kJ/mol for the reaction catalyzed by isocitrate dehydrogenase, yet the G = +29.7 kJ/mol for the reaction catalyzed by malate dehydrogenase.  Both of these reactions involve the oxidation of a secondary alcohol.  Give an explanation as to why the oxidation of isocitrate is so exergonic.
  Ans: The

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