Chapter 5 - Generation of ATP from Metabolic Fuels and Oxygen Toxicity

Question 1

Consider the reaction catalyzed by fumarase: fumarate + H2O malate

When measured in the absence of fumarase, the ΔGoº' for this reaction is 0 kcal/mol (neglecting any terms associated with H2O). The equilibrium constant for this reaction would therefore be which one of the following?
(A) 0
(B) 0.5
(C) 1.0
(D) 10.0
(E) 50.0

Answer 1

The answer is C. If ΔGº’ = 0, then–RT ln Keq = 0, since ΔGº’ =–RT ln Keq. For–RT ln Keq to be equal to 0, the ln Keq must be 0, which means that Keq = 1 (the natural log of 1 = 0).

Question 2

Consider the reaction catalyzed by fumarase: fumarate + H2O malate

Fumarase was added to a solution that initially contained 20 μM fumarate. After the establishment of equilibrium, the concentration of malate was which one of the following?
(A) 2 μM
(B) 5 μM
(C) 10 μM
(D) 20 μM
(E) 50 μM

Answer 2

The answer is C. From the answer to Question 1, we know that Keq = 1 = [Malate]/[Fumarate] = X/(20–X). Therefore, (20–X) = X, 20 = 2X, and X = 10 μM.

Question 3

Reaction / Approximate ΔGº’ (kcal/mol)

Acetate + 2 O2 → 2 CO2 + 2 H2O / –243
NADH + H+ + 1⁄2 O2 → NAD+ + H2O / –53
FADH2 + 1⁄2 O2 → FAD + H2O / –41
GTP → GDP + Pi / –8
ATP → ADP + Pi / –8

Of the total energy available from the oxidation of acetate, what percentage is transferred via the TCA cycle to NADH, FADH2, and GTP?
(A) 38%
(B) 42%
(C) 81% 
(D) 86%
(E) 100%

Answer 3

The answer is D. In the TCA cycle, each turn of the cycle produces 3 NADH, 1 FADH2,
and 1 GTP. Each NADH releases 53 kcal/mol; the 3 NADH thus yield 159 kcal/mol of energy. FADH2 releases 41 kcal/mol, and GTP 8 kcal/mol. The energy captured is 159 + 41 + 8, or 208 kcal/mol. The total energy available is 243 kcal/mol, so the fraction of energy captured is 208/243, or 86%.

Question 4

Reaction / Approximate ΔGº’ (kcal/mol)

Acetate + 2 O2 → 2 CO2 + 2 H2O / –243
NADH + H+ + 1⁄2 O2 → NAD+ + H2O / –53
FADH2 + 1⁄2 O2 → FAD + H2O / –41
GTP → GDP + Pi / –8
ATP → ADP + Pi / –8

What percentage of the energy available from the oxidation of acetate is converted to ATP?
(A) 3%
(B) 30%
(C) 40%
(D) 85%
(E) 100%

Answer 4

The answer is B. About 10 ATP (7.5 from NADH, 1.5 from FADH2, and 1 from GTP) are pro- duced by the TCA cycle (10 × 8 kcal = 80 kcal). The percentage of the total energy available from oxidation of acetate that is converted to ATP is 80/243, or 33%.

Question 5

A genetic mutation caused the cellular concentration of an enzyme to increase 100-fold for a biochemical reaction. Therefore, the equilibrium constant for the reaction catalyzed by the enzyme would change in which one of the following ways?
(A) It would decrease two-fold.
(B) It would remain the same.
(C) It would increase in proportion to the
enzyme concentration.
(D) It would change inversely with the enzyme
concentration.
(E) It would decrease 100-fold.

Answer 5

The answer is B. An enzyme increases the rate at which a reaction reaches equilibrium but does not change the concentration of the reactants and products at equilibrium; that is, the Keq is not affected by an enzyme, so a change in enzyme concentration will have no effect on the Keq.

Question 6

Consider the section of the TCA cycle in which isocitrate is converted to fumarate. This segment of the TCA cycle can be best described by which one of the following?
(A) These reactions yield 5 moles of high- energy phosphate bonds per mole of isocitrate.
(B) These reactions require a coenzyme synthesized in the human from niacin (nicotinamide).
(C) These reactions are catalyzed by enzymes located solely in the mitochondrial membrane.
(D) These reactions produce 1 mole of CO2 for every mole of isocitrate oxidized.
(E) These reactions require GTP to drive one of the reactions.
170

Answer 6

The answer is B. In the conversion of isocitrate to fumarate, 2 CO2, 2 NADH (which contains niacin), 1 GTP, and 1 FADH2 are produced. A total of approximately 7.5 ATP are generated. The enzymes for these reactions are all located in the mitochondrial matrix except succinate dehydrogenase, which is an inner mitochondrial membrane protein. GTP is not required in any of the reactions but is produced in the conversion of succinyl-CoA to succinate.

Question 7

In the TCA cycle, a role for thiamine pyrophosphate is which one of the following?
(A) To accept electrons from the oxidation of pyruvate and α-ketoglutarate
(B) To accept electrons from the oxidation of isocitrate
(C) To form a covalent intermediate with the α-carbon of α-ketoglutarate
(D) To form a thioester with the sulfhydryl group of CoASH
(E) To form a thioester with the sulfhydryl group of lipoic acid

Answer 7

The answer is C. Thiamine pyrophosphate is involved in the making and breaking of carbon- carbon bonds. It is a necessary cofactor for oxidative decarboxylation reactions, in which a carbon-carbon bond is broken and carbon dioxide is released. Mechanistically, thiamine pyrophosphate forms a covalent intermediate with the α-carbon of an α-keto acid substrate, which, in the TCA cycle, is α-ketoglutarate. Thiamine pyrophosphate is not involved in redox reactions, or in thioester formation.

Question 8

Which one of the following is a property of pyruvate dehydrogenase?
(A) The enzyme contains only one polypeptide chain.
(B) The enzyme requires thiamine pyrophosphate as a cofactor.
(C) The enzyme produces oxaloacetate from pyruvate.
(D) The enzyme is converted to an active form by phosphorylation.
(E) The enzyme is activated when NADH levels increase.

Answer 8

The answer is B. Pyruvate dehydrogenase converts pyruvate to acetyl-CoA. It contains multiple subunits: a dehydrogenase component that oxidatively decarboxylates pyruvate, a dihydrolipoyl transacetylase that transfers the acetyl group to coenzyme A, and a dihydrolipoyl dehydrogenase that reoxidizes lipoic acid. Thiamine pyrophosphate, lipoic acid, coenzyme A, NAD+, and FAD serve as cofactors for these reactions. In addition, a kinase is present that phosphorylates and inactivates the decarboxylase component. Acetyl-CoA and NADH activate this kinase, thus inactivating pyruvate dehydrogenase. A phosphatase dephosphorylates the decarboxylase subunit, thereby reactivating pyruvate dehydrogenase.

Question 9

Which one of the following components of the electron transport chain only accepts electrons, and does not donate them?
(A) Cytochrome b
(B) Oxygen
(C) Coenzyme Q
(D) FMN
(E) Cytochrome C1

Answer 9

The answer is B. Under physiological conditions, oxygen is the terminal electron acceptor in the electron transport chain, and water will not donate electrons to other substrates to regener- ate oxygen. The cytochromes, FMN, and coenzyme Q both accept and donate electrons during the course of electron flow through the electron transport chain.

Question 10

Which one of the following tissues of the eye relies almost solely on anaerobic metabolism instead of the TCA/electron transport cycle?
(A) Cornea
(B) Lens
(C) Ciliary muscle
(D) Retina
(E) All of the tissues of the eye use only
anaerobic glycolysis as an energy source.

Answer 10

The answer is B. Aerobic metabolism requires an O2 supply. Oxygen is usually obtained from the blood, which is circulating through the blood vessels. However, transparent tissue cannot have an extensive network of blood vessels since these would create opacities that would block the transmission of light. The cornea is exposed to air and gets its oxygen by diffusion from air. The lens has no capillaries and is not exposed to the air, so it utilizes anaerobic metabolism. Glucose and lactate diffuse from and into aqueous and vitreous humor. The ciliary muscle and retina have extensive blood vessel systems, and can carry out oxidative phosphorylation in order to generate energy.

Question 11

A 43-year-old female has been on a “grapefruit and potatoes” diet for several months in an effort to lose weight. She now complains of a rash
covering most of her body, a large, beefy tongue, nausea and diarrhea, and some confusion.

Which one of the following cofactors or enzyme complexes would be most affected by this condition?
(A) The concentration of NAD+
(B) The concentration of FAD
(C) The concentration of coenzyme Q
(D) The functioning of the FMN components
of complex I
(E) The functioning of the cytochrome-containing components of complex III

Answer 11

The answer is A. This patient has the classic symptoms of pellegra, a vitamin B3 (niacin) deficiency. NAD+ is derived from niacin. Pellagra leads to the four Ds–dermatitis, dementia, diarrhea, and death. Riboflavin is the precursor for both FAD and FMN. Coenzyme Q is synthesized from acetyl-CoA, and its levels would not be affected as much as those of NAD+. Heme is synthesized from succinyl-CoA and glycine, and a reduction in heme levels would lead to an anemia and not the symptoms as described for this patient.

Question 12

A 43-year-old female has been on a “grapefruit and potatoes” diet for several months in an effort to lose weight. She now complains of a rash
covering most of her body, a large, beefy tongue, nausea and diarrhea, and some confusion.

To reverse the symptoms described in the patient, a diet high in which one of the following should be recommended?
(A) Green, leafy vegetables
(B) Whole grains and meat
(C) Citrus fruits
(D) Orange and yellow vegetables
(E) Chocolate cake

Answer 12

The answer is B. While green, leafy vegetables are rich in other B vitamins, whole grains, meats, fish, and liver are the best sources of niacin. Citrus fruits are high in vitamin C. Orange and yellow vegetables are high in vitamin A. Chocolate cake is high in flavonoids, an antioxidant, fats, and carbohydrates.

Question 13

An alcoholic presents with swelling and fissuring of the lips, cracking at the angles of the mouth, red eyes, and an oily, scaly rash of his scrotum.

Which one of the following cofactors of enzyme complexes would be most affected by this condition?
(A) The concentration of NAD+
(B) The concentration of NADP+
(C) The concentration of coenzyme Q
(D) The functioning of the FMN components
of complex I
(E) The functioning of the cytochrome-
containing components of complex III

Answer 13

The answer is D. This patient has vitamin B2 (riboflavin) deficiency, ariboflavinosis, as indicated by the symptoms displayed by him. Both FAD and FMN require vitamin B2 to be produced. NAD+ and NADP+ are derived from niacin. Coenzyme Q is derived from acetyl-CoA, and vitamin B2 is not needed in the synthesis of the heme ring, which is derived from succinyl-CoA and glycine.

Question 14

An alcoholic presents with swelling and fissuring of the lips, cracking at the angles of the mouth, red eyes, and an oily, scaly rash of his scrotum.

Which of the following foods would best help reverse the symptoms described in the above patient?
(A) Broccoli
(B) Carrots
(C) Grapefruits
(D) Wheat
(E) Chocolate cake

Answer 14

The answer is A. Dark green vegetables, especially broccoli, meats, and dairy products are all high in riboflavin. Carrots are high in vitamin A, grapefruits in vitamin C, and whole grains in niacin. Chocolate cake is high in flavonoids, an antioxidant, fats, and carbohydrates.

Question 15

A firefighter is brought to the emergency room (ER) from the scene of a fire complain- ing of headaches, weakness, confusion, and difficulty in breathing. His skin and mucous membranes appear very pink/red. The causative agent of these symptoms inhibits electron transport and oxidative phosphorylation by which one of the following mechanisms?
(A) Uncoupling of electron transport and phosphorylation
(B) Combining with NADH dehydrogenase
(C) Combining with cytochrome oxidase
(D) Inhibiting an adequate supply of ADP
(E) Combining with coenzyme Q

Answer 15

The answer is C. The symptoms experienced by the firefighter could be caused by either cyanide or carbon monoxide, both of which inhibit cytochrome c oxidase. Both carbon monoxide and cyanide are byproducts of fuel oxidation, and would be generated during a fire. The firefighter most likely inhaled smoke that contained one or both of these compounds. Both agents will block the reduction of oxygen to water, thereby halting the electron transfer chain and oxidative phosphorylation. Neither agent is an uncoupler, nor do they block the ANT (so ADP levels will not be decreased). Rotenone, a fish poison, complexes with NADH dehydrogenase (complex I) to inhibit electron flow from complex I to coenzyme Q. Neither cyanide nor carbon monoxide will bind to coenzyme Q and block its ability to either accept or donate electrons.

Question 16

A patient is undergoing an appendectomy under general anesthesia (succinylcholine and an inhaled anesthetic) when she begins to develop muscle rigidity, tachycardia, and hyperthermia. Which of the following best describes the mechanism of this process?
(A) Uncoupling of electron transport and phosphorylation
(B) Inhibition of NADH dehydrogenase
(C) Inhibition of cytochrome oxidase
(D) Inhibiting an adequate supply of ADP
(E) Combining with coenzyme Q

Answer 16

The answer is A. The patient is experiencing malignant hyperthermia, which is similar in symptoms to the uncoupling of the electron transfer chain from ATP synthesis. Succinylcholine and several inhaled anesthetics can act as uncouplers of electron transport in susceptible individuals. An inhibition of complex I (NADH dehydrogenase), or cytochrome oxidase, would block both electron flow and ATP synthesis, and muscle rigidity and hyperthermia would not result. The same is true if coenzyme Q were prevented from accepting and donating electrons. An uncoupler will not lead to a decrease in ADP levels as ATP cannot be synthesized under these conditions, and ADP levels would be expected to increase.

Question 17

A patient is in septic shock and his tissues are poorly perfused and oxygenated. The major end product of glucose metabolism in these tissues will be an accumulation of which one of the following?
(A) Pyruvate
(B) Acetyl-CoA
(C) Lactate
(D) Urea
(E) Citrate

Answer 17

The answer is C. Without oxygen, aerobic metabolism cannot function, so the electron transfer chain will stop (without oxygen, cytochrome oxidase cannot remove electrons from the chain to reduce oxygen to water). ATP synthesis in the mitochondria will stop because of the coupling of oxidation and phosphorylation. The TCA cycle will stop because of the accumulation of NADH in the mitochondria (as the electron transfer chain is fully reduced, owing to the lack of oxygen, NADH cannot donate electrons to a reduced complex I), and NADH inhibits key enzymes of the TCA cycle. However, the tissues still need energy, and so they use anaerobic glycolysis to generate ATP. The end product, pyruvate, is converted to lactate to regenerate NAD+ such that glycolysis can continue. An accumulation of lactate (which is a metabolic dead end) can lead to lactic acidosis in the patient.

Question 18

Which one of the following cell types cannot utilize the TCA cycle or electron transport chain?
(A) Brain
(B) Red blood cells
(C) Liver
(D) Kidney
(E) Heart

Answer 18

The answer is B. Cellular respiration occurs in the mitochondria. Red blood cells lack mitochondria, and can only utilize anaerobic glycolysis for energy production. The brain, liver, kidney, and heart cells contain mitochondria, and can carry out oxidative phosphorylation, as well as anaerobic glycolysis.

Question 19

Metformin is the standard first-line oral medication for Type 2 diabetes. The use of the drug has the potential side effect of lactic acidosis. Which of the following explains why this lactic acid buildup is rarely seen clinically?
(A) The red blood cells utilize the lactate as fuel.
(B) The renal cell utilizes the lactate as fuel.
(C) The cardiac muscle cells utilize the lactate
as fuel.
(D) The large, voluntary muscle groups utilize
the lactate as fuel.
(E) The lactate directly enters the TCA cycle to
be oxidized.

Answer 19

The answer is C. Metformin can increase glucose uptake by tissues and lead to increased lactate formation. In addition, metformin, through unknown mechanisms, appears to block lactate uptake by the liver (this could occur because of the reduced gluconeogenesis occurring in the liver in the presence of metformin, as lactate is a key substrate for gluconeogenesis). The cardiac muscles, with their massive amount of mitochondria, will utilize lactate as fuel and
can overcome a lactate buildup from therapeutic doses of metformin. It is only in the rare case (about 1 in 10,000) that metformin treatment will lead to lactic acidosis. Lactate does not enter the TCA cycle to be oxidized, as it first has to be converted to pyruvate, then to acetyl-CoA before it can enter the cycle. Red blood cells generate lactate, but do not utilize it as a fuel. Kidney cells, and other muscles, do not utilize lactate to an appreciable extent as a fuel, as compared to the heart muscle.

Question 20

In which one of the following scenarios should a patient with Type 2 diabetes, who is taking metformin for glucose control, be advised to discontinue the metformin owing to an increased risk of lactic acidosis resulting from continuation of the metformin?
(A) Severe anemia
(B) Early pyelonephritis
(C) Severe loss of cardiac tissue from a myo-
cardial infarction
(D) Severe tear of the quadriceps muscle
(E) Significant weight gain

Answer 20

The answer is C. Metformin tends to increase circulating blood lactate levels owing to reduced use of lactate by the liver for gluconeogenesis, which is inhibited in the presence of metformin. The excess lactate, however, can be used by the heart as an energy source, which reduces circulating lactate levels. If the loss of cardiac muscle cells (and their mitochondria) is significant due to a myocardial infarction, the elevated lactate due to metformin use will accumulate since it is no longer being metabolized by the heart. This could lead to lactic acidosis, which may be fatal. Lactate can also accumulate in renal failure; however, none of the other conditions listed will lead to renal failure. Pyelonephritis usually does not lead to renal failure. Torn muscles may cause myoglobinuria, and this may lead to renal failure, but it would be an uncommon out- come. Red blood cells produce lactate, and so anemia would cause reduced lactate formation. A significant weight gain would not be the cause to stop taking metformin.

Question 21

The most potent ROS is which one of the following?
(A) Hydrogen peroxide (B) Superoxide
(C) Hydroxyl radical
(D) Nitric oxide
(E) Coenzyme Q

Answer 21

The answer is C. The hydroxyl radical is the most potent ROS. It creates chain reactions that produce lipid peroxides and organic radicals. Hydrogen peroxide is not a radical, but it can generate hydroxyl radicals through interactions with transition metals. Superoxide is a potent ROS, but its potency is diminished by its reduced solubility. NO and coenzyme Q are not ROS, although NO is a radical, and will give rise to reactive nitrogen–oxygen species (RNOS).

Question 22

Using Internet sources, a patient has developed his own diet plan in an attempt to prevent macular degeneration. For breakfast every morning, he has poached eggs, orange and carrot juice, and red wine. Which of the following in his diet can potentially protect against the patient develop- ing macular degeneration?

Carotenoids / Flavonoids / Vitamin C / Vitamin D / Vitamin E

(A) Yes / Yes / Yes / No / Yes
(B) Yes / No / Yes/ Yes / Yes
(C) Yes / Yes / No / No / Yes
(D) No / No / No / Yes / Yes
(E) No / Yes / No / No / No
(F) No / No / Yes / Yes / No

Answer 22

The answer is A. Macular degeneration can come about through oxidative damage to the macula, so the patient is trying to increase his intake of antioxidant compounds to protect against the generation of ROS. There are no sources of vitamin D in the patient’s diet. Carotenoids, found in the carrot juice, are antioxidants. Flavonoids, another potent antioxidant, are found in wine. Orange juice contains vitamin C, another antioxidant. Vitamin E, a very potent antioxidant, is found in egg yolks. The role of vitamin D as an antioxidant is controversial, as its primary role is to maintain calcium homeostasis, although there are some reports of it being utilized as an antioxidant. In either event, none of the patient’s dietary sources is providing for vitamin D.

Question 23

Which of the following medications, given chronically, could create a physiologic response that would be a major source of free-radical production?
(A) Ciprofloxacin
(B) Isoniazide
(C) Cimetidine
(D) Ketaconazole
(E) None of these drugs has the potential to
increase free-radical formation.

Answer 23

The answer is B. Ciprofloxacin, cimetidine, and ketaconazole all inhibit cytochrome P450. Their mode of detoxification does not require the actions of cytochrome P450. Isoniazide, however, induces cytochrome P450 formation as a means of oxidizing the drug to remove it from the body. Cytochrome P450 enzymes are a major source of free-radical production that can occur when electrons are accidentally leaked from reactions and react with molecular oxygen.

Question 24

A contestant on a TV reality show, in which the contestants had to survive off the land for an extended period of time, developed recur- rent diarrhea, dermatitis, and had trouble remembering things. These symptoms could be brought about due to the lack of which one of the following in the contestant’s diet?
(A) Niacin
(B) Thiamine
(C) Riboflavin
(D) Vitamin C
(E) Vitamin D

Answer 24

The answer is A. The contestant has the symptoms of pellagra, which is characterized by the four Ds (i.e., diarrhea, dermatitis, dementia, and eventual death). Pellagra is due to a lack of niacin in the diet. A thiamine deficiency will lead to beriberi; a riboflavin deficiency to ariboflavinosis; a vitamin C deficiency to scurvy; and a vitamin D deficiency to rickets. Only pellagra would yield the symptoms observed by the patient.

Question 25

A 40-year-old chronic alcoholic enters the hospital because of a variety of symptoms, including loss of feeling in his hands and feet, nystagmus, and difficulty with his balance when walking. This patient would have difficulty catalyzing which one of the following reactions?
(A) α-Ketoglutarate dehydrogenase
(B) Succinate dehydrogenase
(C) Fumarase
(D) Malate dehydrogenase
(E) Pyruvate carboxylase

Answer 25

The answer is A. The patient is exhibiting the symptoms of beriberi, due to a vitamin B1 deficiency. Thiamine (B1) is required for oxidative decarboxylation reactions, such as those catalyzed by pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. α-Ketoglutarate dehydrogenase requires thiamine (as thiamine pyrophosphate), lipoic acid, CoASH, FAD, and NAD+. Succinate dehydrogenase only requires FAD, fumarase has no cofactor requirement, malate dehydrogenase requires NAD+, and pyruvate carboxylase requires biotin.

Question 26

A scientist has developed a drug that, when added to eukaryotic cells, leads to elevated lactate levels. An analysis of mitochondrial
contents also demonstrated elevated α-ketoglutarate levels in drug-treated cells. This drug may be interfering with a reaction that re- quires which one of the following vitamins?
(A) Biotin
(B) Vitamin K
(C) Pantothenate
(D) Ascorbate
(E) Pyridoxine

Answer 26

The answer is C. The elevation of lactate, as well as α-ketoglutarate in the TCA cycle, suggests a defect in reactions that catalyze oxidative decarboxylations. Pyruvate would accumulate if pyruvate dehydrogenase was defective, and the increase of concentration of pyruvate would increase the production of lactate. Five cofactors are needed for oxidative decarboxylation reactions, and they are NAD+, FAD, lipoic acid, thiamine pyrophosphate, and coenzyme A (which is derived from pantothenic acid). The drug appears to be acting by blocking the conversion of pantothenic acid to coenzyme A. Biotin is used for carboxylation reactions, as is vitamin K. Ascorbate is utilized for proline hydroxylation, and vitamin B6 helps to catalyze a variety of re- actions involving amino acids.

Question 27

A man presents to the ER with an elevated temperature, sweats, and increased rate of breathing. He had been spraying insecticide and accidentally inhaled some of the poison. Using the insecticide on cultured cells, it was demonstrated that the rate of oxygen consumption by the cells was much greater than in the absence of the compound. This drug is acting most like which one of the following?
(A) Dintrophenol
(B) Cyanide
(C) Carbon monoxide
(D) Rotenone
(E) Atractyloside

Answer 27

The answer is A. The insecticide is acting as an uncoupler of oxidation and phosphorylation, reducing ATP synthesis while increasing oxygen uptake (increased rate of breathing). As the uncoupler dissipates the proton gradient, the energy used to generate a gradient is lost as heat (leading to the sweating and increased temperature observed in the worker). Of the drugs listed as possible answers, only dinitrophenol (DNP) acts as an uncoupler. Cyanide and carbon monoxide interfere with cytochrome oxidase (complex IV) and block electron transfer to oxygen. Rotenone blocks electron flow from complex I to coenzyme Q, and atractyloside inhibits the ANT, leading to reduced ADP in the mitochondria and a cessation of oxidative phosphorylation. Oxygen consumption would not continue in the presence of atractyloside owing to the loss of ATP synthesis, and the coupling of oxygen consumption and ATP production.

Question 28

A patient suffering a heart attack was brought to the ER. An atherosclerotic plaque had blocked a major coronary artery, preventing blood from reaching a region of her heart. As a result, in cells of the affected heart muscle, there was an increase in which one of the following?
(A) The rate of CO production 2
(B) The rate of electron transport by the electron transport chain
(C) The concentration of ADP
(D) The proton gradient across the inner mitochondrial membrane
(E) The rate of O2 consumption

Answer 28

The answer is C. A lack of blood flow decreased the flow of O2 to the heart, which slowed down the electron transport chain. The reduction in the rate of the electron transfer chain will lead to an increase in intramitochondrial NADH levels, which slows down the TCA cycle. A slowdown of the TCA cycle leads to a reduction in carbon dioxide production. ATP levels within the mitochondria would also drop as the ATP was transported into the cytoplasm in exchange for ADP. Since oxidative phosphorylation was blocked, the ADP that enters the mitochondria cannot be converted back into ATP, leading to an accumulation of ADP under these conditions.

Question 29

A scientist is conducting an experiment on isolated mitochondria in a buffered solution, in the presence of ADP. At time = 0, oxygen and succinate were added to the mitochondrial suspension, and oxygen consumption measured as a function of time. At various times after starting the experiment (labeled as 1, 2, 3, and 4 in the figure shown), different chemicals were added to the solution. (See page 173 question 29)

Which one of the following correctly describes which chemicals were added at times 1, 2, 3, and 4?

Time 1 / Time 2 / Time 3 / Time 4

(A) Antimycin / Oligomycin / Cyanide / Carbon monoxide
(B) Antimycin / Cyanide / Dinitrophenol / Oligomycin
(C) Rotenone / Oligomycin / Dinitrophenol / Cyanide
(D) Rotenone / Dinitrophenol / Oligomycin / Carbon monoxide
(E) Dinitrophenol / Rotenone / Oligomycin / Cyanide
(F) Dinitrophenol / Cyanide / Oligomycin / Dinitrophenol

Answer 29

The answer is C. At time = 0, succinate is donating electrons to the electron transfer chain through complex II. Succinate is converted to fumarate, and the FADH2 generated donates the electrons to coenzyme Q via complex II. The addition of a chemical at time = 1 does not affect oxygen consumption, and the rate of oxygen consumption is the same as before adding the chemical. Rotenone inhibits at complex I, and would not affect electrons being donated through complex II. Antimycin blocks the transfer of electrons from complex III to complex
IV, and would be expected to block electron transfer, and oxygen consumption, from electrons donated from complex II. Dinitrophenol is an uncoupler, and would be expected to increase the rate of oxygen consumption when added, as the uncoupler dissipates the proton gradient and makes it easier for the electron transfer chain to pump protons out of the mitochondria. At time = 2, electron flow stops, but it restarts at time = 3, and at a faster rate. Thus, at time = 3, an uncoupler (dinitrophenol) is added. The fact that electron flow can start again, after stopping, suggests that the chemical added at time = 2 inhibited phosphorylation, as phosphorylation and oxygen consumption are coupled. Inhibiting phosphorylation, via oligomycin, will block oxygen consumption without inhibiting any steps in the electron transfer chain. This is why oxygen consumption can resume again once the uncoupler is added. At time = 4, all electron flow stops, and that can be due to either the addition of cyanide or carbon monoxide, both of which inhibit at complex IV.

Question 30

A 52-year-old man suddenly collapsed at work, and was administered CPR by some co-workers until the ambulance arrived. Upon arrival at the hospital, blood work demonstrated an increased troponin I level. Tissue plasminogen activator was administered to the patient, but further damage to the affected organ resulted from this treatment. This may have happened owing to which one of the following?
(A) Release of cytochrome a from the mitochondria
(B) Increased generation of oxygen-derived radicals
(C) Elevated lactic acid levels
(D) Inhibition of the TCA cycle due to plasminogen activator administration
(E) Uncoupling of oxidation from phosphorylation

Answer 30

The answer is B. The patient is experiencing ischemic-reperfusion injury. The patient has suffered a heart attack, in which a region of the heart muscle has become anaerobic due to a lack of oxygen delivery. In these cells, the mitochondrial electron transfer chain is reduced, since the terminal electron acceptor (oxygen) is missing. When tPA is administered to dissolve the clot that is causing the blockage of oxygen delivery to the damaged heart muscle, it allows oxygen to rapidly reenter the damaged cells. In these cells, due to the hypoxia, coenzyme Q is fully reduced in the mitochondrial membrane, and the sudden influx of oxygen leads to some accidental electron transfers to oxygen, generating the superoxide radical. This leads to in- creased radical damage to the already damaged heart muscle, and further damage to the tissue. Cytochrome a is not released from mitochondria (there may be release of cytochrome c if the damage is sufficient, which would initiate apoptosis in the heart cells). Lactic acid levels are elevated owing to anaerobic glycolysis being used to generate energy, but the high lactate levels are not contributing to further damage to the heart muscle. tPA does not have a direct effect on the activity of the TCA cycle enzymes, and is not an uncoupler.

Question 31

A 23-year-old man was diagnosed as HIV positive 3 years ago. He has been on a variety of anti-HIV drugs since then, including AZT, along with some dideoxy compounds and HIV protease inhibitors. He recently devel- oped muscle weakness, to the point where he had difficulty walking. This complication has come about due to which one of the following?
(A) AZT inhibition of nuclear RNA polymerase
(B) AZT inhibition of nuclear DNA polymerase
(C) AZT inhibition of mitochondrial RNA
polymerase
(D) AZT inhibition of mitochondrial DNA
polymerase
(E) AZT uncoupling of oxidative
phosphorylation
(F) AZT induced release of cytochrome c from
the mitochondria

Answer 31

The answer is D. AZT is a DNA chain terminator, with greatest affinity for the viral reverse transcriptase. However, mitochondrial DNA polymerase will also recognize and utilize AZT as a substrate, thereby interfering with mitochondrial DNA replication, and mitochondrial division. For some patients, after prolonged AZT treatment, the reduction in mitochondrial function results in the symptoms observed in this patient. Removal of the offending agent will usually reverse this complication. AZT does not affect RNA polymerase, nor is it an uncoupler of oxidative phosphorylation. It itself does not induce the release of cytochrome c from the mitochondria (a signal to start apoptosis), although if sufficient mitochondria are damaged within a cell, the cell might decide to undergo apoptosis.

Question 32

A 4-year-old boy has had a history of skin infections, pneumonia, nausea, vomiting, and abdominal pain. He has been on antibiotics prophylactically for the past year, but still
contracts various sort of infections, both bacterial and fungal. A physical exam demonstrated hepatosplenomegaly. The boy most likely has inherited a mutation that prevents which one of the following reactions?
(A) Hydrogen peroxide conversion to hydroxyl radicals
(B) Superoxide conversion to hydrogen perox- ide and oxygen
(C) Oxidized glutathione to reduced glutathione
(D) The formation of hydrochloric acid
(E) The formation of superoxide

Answer 32

The answer is E. The boy has the symptoms of chronic granulomatosis, which in the inherited condition is due to a defect in a component of NADPH oxidase. NADPH oxidase is responsible for the respiratory burst in neutrophils, and produces superoxide from oxygen and NADPH. The superoxide formed helps to destroy the invading bacteria and fungi. In the absence of
this activity, the body has difficulty protecting itself from bacterial and fungal infections. The conversion of hydrogen peroxide to hydroxyl radicals is not enzyme-catalyzed, and so the loss of such conversion cannot be inherited. Mutations in SOD (which catalyzes the conversion of superoxide to hydrogen peroxide and oxygen) can lead to ALS, but not chronic granulomatosis. The loss of glutathione reductase can lead to hemolytic anemia, but not the observed symptoms. Neutrophils do not produce hydrochloric acid. The stomach does however, and a loss of acid production can lead to digestive disorders.

Question 33

A 42-year-old man had experienced cramping and stiffness of muscles, coupled with severe muscle weakness in his left leg. Over time, the muscle weakness becomes more severe, and his speech has become slurred, and he has to use a wheelchair to get around. He developed swallowing difficulties, and upon diagnosis was told he had less than 2 years to live. A family history indicated that the man’s father had begun to experience mild muscle weakness before he died in an automobile accident. The man’s symptoms can be best explained at the molecular level by which one of the following?
(A) Increased activity of NADPH oxidase
(B) Increased activity of catalase
(C) Elevated levels of hydrogen peroxide
(D) Elevated levels of superoxide
(E) Decreased activity of glutathione peroxidase
(F) Decreased activity of myeloperoxidase

Answer 33

The answer is D. The man is experiencing the inherited form of ALS, which is most often due to an inactivating mutation in SOD. Lack of SOD activity would lead to elevated levels of superoxide, which would then, in an unknown manner, lead to a loss of motor neuron activity. Inherited mutations in NADPH oxidase, catalase, glutathione peroxidase, or myeloperoxidase have not been correlated with ALS.

Question 34

A long-distance runner is training, and part of the training requires running multiple sets of 800-m runs in order to increase her stamina. Energy generation by the TCA cycle is enhanced under these conditions owing to which one of the following?
(A) Allosteric activation of isocitrate dehydrogenase by increased NADH
(B) Allosteric activation of fumarase by in- creased ADP
(C) A rapid decrease in the concentration of 4-carbon intermediates
(D) Product inhibition of citrate synthase
(E) Stimulation of the flux through a number of enzymes by a decreased NADH/NAD+ ratio

Answer 34

The answer is E. During aerobic exercise, the muscle needs to generate ATP via oxidative phosphorylation. For this to occur, NADH levels will decrease (an increase in NADH levels would inhibit the TCA cycle enzymes), leading to a decrease in the NADH/NAD+ ratio. While citrate is an inhibitor of citrate synthase, if this were to occur during exercise, the TCA cycle would stop, not push forward. The activity of isocitrate dehydrogenase is inhibited (not activated) by NADH, and fumarase is not a regulated enzyme. If TCA cycle intermediates were to decrease in concentration, then the cycle would slow down as well, the opposite of what is needed under conditions in which energy needs to be generated, such as during exercise.

Question 35

A man presents to the ER after ingesting an insecticide. His respiration rate is very low. Information from the Poison Control Center in- dicates that this particular insecticide binds to and completely inhibits cytochrome c. Therefore, which one of the following would occur in this man’s mitochondria?
(A) Coenzyme Q would be in the oxidized state
(B) Cytochromes a and a3 would be in the re- duced state
(C) The rate of ATP synthesis would be approximately zero
(D) The rate of CO2 production would be increased
(E) The rate of oxygen consumption would be increased

Answer 35

The answer is C. If cytochrome c cannot function, all the components of the electron transport chain between it and O2 remain in the oxidized state, and the components of the chain before cytochrome c are reduced. The electron transport chain will not function; O2 will not be consumed; a proton gradient will not be generated; and ATP will not be produced. NADH will not be oxidized, thereby increasing the NADH/NAD+ ratio. Owing to this increased ratio, the TCA cycle will slow down and, therefore, CO2 production will decrease.