Question 1

You are studying the effects of a compound on the respiratory activity of isolated mitochondria. Your experiments demonstrate that oxygen consumption is normal when pyruvate and malate are used as well as when succinate is added. However, you find that the production of ATP is severely impaired with addition of the compound. These results most closely resemble the effects that would be seen by the addition of the following electron transport inhibitor to your system?
A) antimycin A
B) azide
C) dinitrophenol (DNP)
D) oligomycin
E) rotenone

Accepted Answer

The normal flow of electrons, through the proteins of the oxidative phosphorylation machinery, is coupled with the establishment of a proton gradient across the inner mitochondrial membrane. The establishment of this pH gradient is the chemiosmotic potential that is coupled with the production of ATP. If the process of proton movement is uncoupled from the normal pathway through the ATP synthase complex, oxygen can be consumed but no ATP will be synthesized. This is the effect of uncoupling agents such as DNP, which act to discharge the proton gradient.

Question 2

A laboratory analysis of isolated mitochondria demonstrates that oxygen consumption is normal when succinate is added but extremely low when pyruvate and malate are used. The mitchondria are subsequently shown to have normal cytochromes but a reduced iron content. The reduced pyruvate/malate oxidation is due to a defect in which of the following respiratory components?
A) cytochrome c
B) cytochrome oxidase
C) NADH dehydrogenase
D) succinate dehydrogenase
E) ubiquinone

Accepted Answer

NADH dehydrogenase (Complex I) requires iron-sulfur clusters to function properly. The reduced iron content affects its activity, impairing the oxidation of NADH generated from pyruvate and malate, but not succinate, which enters the electron transport chain at Complex II. During the process of oxidative phosphorylation, electrons from reduced electron carriers enter the pathway at either complex I or complex II. The reduced electron carrier, derived from the oxidation of pyruvate and malate (NADH), enters the oxidative phosphorylation machinery at complex I, NADH-coenzyme Q reductase (also identified as NADH-ubiquinone oxidoreductase). Whereas, the reduced electron carrier generated during the oxidation of succinate (FADH₂) enters the oxidative phosphorylation machinery at complex II. Thus, a defect in the activity of complex I would inhibit oxygen consumption in the presence of pyruvate or malate but not succinate. The pathway of electron flow through the electron transport assembly have been determined through the uses of compounds termed antimetabolites. Some of these agents are inhibitors of electron transport at specific sites in the electron transport assembly, while others stimulate electron transport by discharging the proton gradient (uncouplers). An important example of an uncoupler is 2,4-dinitrophenol (DNP). Uncoupling agents such as DNP act as lipophilic weak acids, associating with protons on the exterior of mitochondria, passing through the membrane with the bound proton, and dissociating the proton on the interior of the mitochondrion. These agents cause maximum respiratory rates but the electron transport generates no ATP, since the translocated protons do not return to the interior through ATP synthase.

Question 3

Which of the following statements best characterizes ATP synthase?
A) it couples ATP export from the mitochondrial matrix to ATP synthesis
B) it is soluble protein found in the mitochondrial matrix
C) its catalytic function is to synthesize ATP in a reaction driven by a chemiosmotic potential
D) oligomycin binds to ATP synthase, directly preventing ATP export
E) the low H+ ion concentration outside mitochondria establishes an electrochemical gradient that drives ATP synthesis

Accepted Answer

ATP synthase synthesizes ATP using the energy from a chemiosmotic potential, which is created by the proton gradient across the mitochondrial membrane. ATP synthase is the enzyme associated with oxidative phosphorylation that utilizes the energy from the chemiosmotic potential generated by electron flow coupled to the pumping of protons out of the matrix of the mitochondria across the inner mitochondrial membrane. ATP synthase uses this potential energy to synthesize ATP from ADP and P_i.

Question 4

Which of the following enzymes catalyzes the reaction shown? Fe²⁺ + O₂ + 4H⁺ → 2H₂O + Fe³⁺ A) catalase B) cytochrome c oxidase C) cytochrome P450 D) peroxidase E) superoxide dismutase

Accepted Answer

Cytochrome c oxidase catalyzes the reduction of oxygen to water using electrons from cytochrome c, which involves the oxidation of Fe²⁺ to Fe³⁺.

Question 5

You are studying the effects of a compound on the respiratory activity of isolated mitochondria. Your experiments demonstrate that oxygen consumption increases as expected when pyruvate and malate are used as well as when succinate is added. Addition of the test compound results in a significant rate of oxygen consumption. These results most closely resemble the effects that would be seen by the addition of which of the following to your system?
A) antimycin A
B) carbon monoxide
C) cyanide
D) dinitrophenol
E) rotenone

Accepted Answer

Dinitrophenol uncouples oxidative phosphorylation, leading to increased oxygen consumption as the electron transport chain works harder to maintain the proton gradient.  Dinitrophenol, DNP, is a chemical uncoupler of electron transport. Uncouplers cause increased oxygen consumption. DNP functions by allowing protons to flow back into the matrix of the mitochondria through the inner mitochondrial membrane which dissipates the electrochemical gradient and preventing ATP synthesis. Since the proton motive force (PMF) across the membrane is lost, there is no longer a gradient to restrict the rate of electron flow; as a result electron transport accelerates resulting in increased oxygen consumption.

Question 6

A 9-year-old girl is brought to her physician for an assessment of potential causes for her increased vision problems. Examination demonstrates that her eye movement is impaired and she has pigmentary retinopathy. Her physician suspects the child has Kearns-Sayre syndrome which is caused by a defect in complex II of the electron transport chain. If this diagnosis is correct, oxidation of which of the following substrates would be associated with impaired electron transfer?
A) acetyl-CoA
B) α-ketoglutarate
C) malate
D) pyruvate
E) succinate

Accepted Answer

Kearns-Sayre syndrome is associated with defects in mitochondrial DNA, often affecting complexes I, III, or IV, but not complex II. However, if complex II were affected, succinate oxidation would be impaired, as succinate dehydrogenase is part of complex II. Electrons from the oxidation of succinate, via succinate dehydrogenase, enter the electron transport chain (ETC) from FADH₂ at complex II. Therefore, an impairment of complex II function as in this patient would impair electron flow. Oxidation of each of the other substrates generates NADH which feeds its electrons to the ETC at complex I.

Question 7

Reduced ubiquinone, generated during the oxidation of NADH, passes its electrons to which of the following?
A) ATP synthase
B) cytochrome c oxidase (complex IV)
C) cytochrome b
D) cytochrome c
E) molecular oxygen

Accepted Answer

Reduced ubiquinone (ubiquinol) passes its electrons to cytochrome b, which is part of Complex III (cytochrome bc1 complex) in the electron transport chain. Reduced CoQ (CoQH₂) diffuses in the lipid phase of the membrane and donates its electrons to complex III, whose principal components are the heme proteins cytochromes b and a non-heme protein, known as the Rieske iron-sulfur protein.

Question 8

In cases of acute cyanide poisoning, cyanide binds to the Fe³⁺ of a cytochrome. Which statement best describes this cytochrome? A) binds carbon monoxide B) directly oxidized by cytochrome c C) directly oxidizes cytochrome b D) found in the mitochondrial matrix E) reduces cytochrome c

Accepted Answer

The answer of In cases of acute cyanide poisoning, cyanide...

Question 9

You are the physician examining a 6-year-old boy who has been experiencing progressive muscle weakness. You perform assays on muscle biopsy tissue from this patient and find that mitochondrial oxidation of succinate is normal but that pyruvate
Oxidation is severely impaired. Mitochondrial extracts demonstrate that the activity of the pyruvate dehydrogenase complex (PDHc) is normal as is the activity of malate dehydrogenase. These results are consistent with the patient harboring a defect in which of the following mitochondrial components?
A) ATP synthase
B) complex I
C) complex II
D) complex III
E) complex IV

Accepted Answer

The normal oxidation of succinate indicates that complex II is functioning properly. However, the impaired oxidation of pyruvate suggests a defect in complex I, which is involved in the initial step of the electron transport chain where electrons from NADH (produced from pyruvate oxidation) are transferred.  Since the PDHc activity of the patient's mitochondria was shown to be normal but the NADH produced by this reaction could not be reoxidized via electron transport, it suggests that the defect causing the muscle weakness is in mitochondrial complex I. Succinate oxidation was normal indicating that electron flow to complex II is not impaired, further strengthening the argument that the defect lies in complex I. If ATP synthase were defective neither pyruvate nor succinate oxidation would be normal.

Question 10

You are carrying out experiments on isolated skeletal muscle mitochondria. For these experiments you are using malate as the oxidizable substrate. Following the addition of malate you add the electron transport inhibitor cyanide. You then examine the oxidation state of various components of the electron transport chain. Which of the following would be expected to be in an oxidized state following this experiment?
A) coenzyme Q
B) complex I
C) complex II
D) complex III
E) cytochrome c

Accepted Answer

Cyanide inhibits the transfer of electrons through complex IV (cytochrome oxidase). In order for any of the components of electron transport to be in an oxidized state they must have either transferred electrons to the next component downstream or never have received electrons in the first place. Oxidation of malate generates NADH which gives its electrons to complex I which then gives the electrons to coenzyme Q which then gives the electrons to complex II, then to cytochrome c, and finally they are passed to complex IV. Since cyanide blocks the transfer of electrons from complex IV to oxygen, all of the upstream electron acceptors will be "blocked" in the reduced state. Only complex II will have never accepted electrons and thus will be in the oxidized state under the conditions of this experiment.

Question 11

A 6-year-old boy is brought to his pediatrician because his parents are concerned about the appearance of seizures and stroke-like episodes. Upon examination the parents reveal that their son has recently begun to exhibit a progressive incoordination in his walking and also difficulty picking up his toys. The physician notes that the patient is short for his age and an eye examination reveals pigmentary retinopathy. Analysis of blood shows elevated levels of lactic acid. These signs and symptoms are most indicative of which of the following disorders?
A) erythrocyte pyruvate kinase deficiency
B) Kearns-Sayre syndrome (KSS)
C) Leigh syndrome
D) mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS)
E) Type 2 diabetes

Accepted Answer

The symptoms described, including seizures, stroke-like episodes, progressive incoordination, short stature, pigmentary retinopathy, and elevated lactic acid levels, are characteristic of MELAS syndrome, a mitochondrial disorder.  Mitochondrial biogenesis disorders are similar to lysosomal storage diseases in that there are symptom similarities between several different diseases of the class. Many of the mitochondrial encephalomyopathies are associated with hearing loss, vision problems such as pigmentary retinopathy, and neurological dysfunction. MELAS is characterized by the fact that it is also associated with the potential for severe lactic acidosis stroke-like episodes. Leigh syndrome is normally fatal before age 5 and so could be eliminated solely on the basis of the patient's age.

Question 12

Which of the following components of mitochondrial electron transport can act as 1-electron or 2-electron carriers?
A) coenzyme Q and flavins
B) cytochrome c, coenzyme Q, and flavins
C) NADH, non-heme iron, cytochrome c, coenzyme Q, and flavins
D) non-heme iron, cytochrome c, coenzyme Q, and flavins

Accepted Answer

Coenzyme Q (ubiquinone) and flavins (such as FAD and FMN) can both act as either 1-electron or 2-electron carriers in the mitochondrial electron transport chain. Only coenzyme Q and the flavins are capable of both 1- and 2-electron transfer reactions. Ubiquinone can undergo either 1- or 2-electron reactions leading to formation of the reduced quinol, the oxidized quinone, and the semiquinone intermediate. NADH and FADH₂ can only carry out obligatory 2-electron reactions and the cytochromes can only carry out 1-electron reactions.

Question 13

Which of the following components of the electron transport system is inhibited by oligomycin?
A) ATP synthase
B) complex I
C) complex II
D) complex III
E) complex IV

Accepted Answer

Oligomycin specifically inhibits ATP synthase, preventing the synthesis of ATP by blocking the proton channel of the enzyme. Oligomycin is a natural antibiotic isolated from Streptomyces diastatochromogenes which inhibits mitochondrial ATP synthase. It inhibits the ATPsynthase by binding to the oligomycin sensitivity-conferring protein (OSCP) of the F₀ portion of the complex.

Question 14

The standard reduction potentials (E0') for the following half reactions are given. Fumarate + 2H⁺ + 2^e− → succinate E^0' = + 0.031 V FAD + 2H⁺ + 2^e− → FADH₂ E^0' = −0.219 V If you mixed succinate, fumarate, FAD, and FADH₂ together, all at 1-M concentrations and in the presence of succinate dehydrogenase, which of the following would happen initially? A) fumarate and succinate would become oxidized; FAD and FADH₂ would become reduced B) fumarate would become reduced, FADH2 would become oxidized C) no reaction would occur because all reactants and products are already at their standard concentrations D) succinate would become oxidized, FAD would become reduced E) succinate would become oxidized, FADH₂ would be unchanged because it is a cofactor

Accepted Answer

The answer of The standard reduction potentials (E0') for the...

Question 15

You are examining a 1-year-old infant who is presenting with liver failure and suffers from frequent seizures. An analysis of mitochondrial function from a skeletal muscle biopsy shows a significant respiratory deficit. A molecular analysis shows that there is a deletion of the mtDNA encompassing the gene encoding DNA polymerase g. This patient is most likely suffering from which of the following diseases?
A) Alpers disease
B) Kearns-Sayre syndrome
C) Leigh syndrome
D) MELAS
E) myoclonic epilepsy and ragged red fiber disease (MERRF)

Accepted Answer

Alpers disease is associated with mutations in the POLG gene, which encodes the mitochondrial DNA polymerase gamma, leading to liver failure, seizures, and mitochondrial dysfunction.  Alpers disease, which usually manifests in infancy, is associated with intractable seizures and a failure to meet meaningful developmental milestones. Primary symptoms of the disease are developmental delay, progressive mental retardation, hypotonia (low muscle tone), spasticity (stiffness of the limbs), and progressive dementia. The typical seizures are of a type that consists of repeated myoclonic (muscle) jerks. Experimental evidence indicates that Alpers disease is associated with deficiencies in the mtDNA-encoded DNA polymerase g.

Question 16

Obesity is associated with hyperlipidemia resulting in an increased delivery of fatty acid to most tissues including the pancreas. Excess fatty acid oxidation in β-cells of the pancreas results in increased reactive oxygen species (ROS) production. The increased ROS production ultimately contributes to β-cell apoptosis and the need for exogenous insulin in poorly controlled Type 2 diabetes. The deleterious effects of ROS within the pancreas are amplified due to limited expression of which of the following?
A) caspase 9
B) catalase
C) glutathione peroxidase
D) peroxiredoxin
E) superoxide dismutase

Accepted Answer

The pancreas has limited expression of catalase, an enzyme that helps in breaking down hydrogen peroxide, a reactive oxygen species, into water and oxygen, thus reducing oxidative stress. The oxidation of fatty acids results in the generation of ROS both via mitochondrial and peroxisomal pathways. When fatty acid levels rise significantly, as in obesity, peroxisomal oxidation of fatty acids increases. The initial enzyme in peroxisomal fatty acid oxidation generates H₂O₂. The primary antioxidant enzyme tasked with removal of H₂O₂ is catalase. Pancreatic β-cells are highly susceptible to the damaging effects of H₂O₂ due to a significantly reduced level of expression of catalase compared to other tissue.

Question 17

Which of the following best explains how the brown adipose tissue in newborns generates large amounts of heat?
A) decreased availability of carnitine
B) increased availability of carnitine
C) increased availability of fat
D) increased formation of ATP
E) mitochondrial leak of hydrogen ions

Accepted Answer

Brown adipose tissue generates heat through a process called non-shivering thermogenesis, which involves the mitochondrial leak of hydrogen ions. This is facilitated by the protein UCP1 (uncoupling protein 1), which allows protons to re-enter the mitochondrial matrix without generating ATP, thus releasing energy as heat.  The uncoupling of proton flow releases the energy of the electrochemical proton gradient as heat. This process is a normal physiological function of brown adipose tissue (BAT). Newborn babies contain BAT in their neck and upper back that serves the function of nonshivering thermogenesis. The mitochondria in BAT contain a protein called uncoupling protein 1, UCP1 (also called thermogenin). UCP1 acts as a channel in the inner mitochondrial membrane to control the permeability of the membrane to protons. When norepinephrine is released in response to cold sensation it binds to β-adrenergic receptors on the surface of brown adipocytes triggering the activation of adenylate cyclase. Activated adenylate cyclase leads to increased production of cAMP and the concomitant activation of PKA with the result being phosphorylation and activation of hormonesensitive lipase. The released free fatty acids bind to UCP1 triggering an uncoupling of the proton gradient and the release of the energy of the gradient as heat.

Question 18

A 21-year-old man is brought to the ER after being found unconscious. He is gasping, and respirations are 24/min. There is evidence that he has vomited recently. Two capsules containing potassium cyanide are found in his shirt pocket. Blood lactate concentration is 3.3 mEq/L (normal = 0.8 ± 0.4 mEq/L). Which of the following processes is most likely inhibited in this patient?
A) conversion of pyruvate to alanine
B) glycolysis
C) mitochondrial electron transport
D) mobilization of fatty acids from adipose tissue
E) release of insulin from pancreas

Accepted Answer

Cyanide inhibits the mitochondrial electron transport chain, specifically by binding to cytochrome c oxidase, leading to impaired oxidative phosphorylation and increased anaerobic metabolism, which results in elevated lactate levels.  Cyanide is a poison that acts by inhibiting electron flow through complex IV of the oxidative phosphorylation machinery. This prevents further mitochondrial electron transport leading to rapid decline in ATP synthesis capacity and ultimately cellular death.

Question 19

Addition of an uncoupling agent to respiring mitochondria carrying out oxidative phosphorylation has which of the following metabolic effects? A) activity of the TCA cycle ceases B) ATP formation ceases, but O₂ consumption continues C) fatty acid oxidation ceases D) formation of ATP continues, but O₂ consumption ceases E) NADH oxidation ceases

Accepted Answer

Uncoupling agents disrupt the proton gradient across the mitochondrial membrane, halting ATP synthesis while allowing electron transport and oxygen consumption to continue.

Question 20

The electron transport chain of mammalian mitochondria contains 3 multienzyme protein complexes that convert the energy of oxidationreduction reactions into a form that can be used for the synthesis of ATP and for the uptake of solutes. This form of energy is most likely an example of which of the following?
A) high-energy phosphorylated ester
B) phosphocreatine
C) proton gradient only
D) sodium gradient only
E) sodium and potassium gradient

Accepted Answer

The energy from the electron transport chain is primarily used to create a proton gradient across the inner mitochondrial membrane, which is then used to drive ATP synthesis through chemiosmosis.  The normal flow of electrons, through the proteins of the oxidative phosphorylation machinery, is coupled with the establishment of a proton gradient across the inner mitochondrial membrane. The establishment of this pH gradient is the chemiosmotic potential that is coupled with the production of ATP.

Quiz 17: Mitochondrial Functions and Oxidative Phosphorylation

Which of the following statements best characterizes ATP synthase?

Which of the following enzymes catalyzes the reaction shown? Fe2+ + O2 + 4H+ → 2H2O + Fe3+

Reduced ubiquinone, generated during the oxidation of NADH, passes its electrons to which of the following?

In cases of acute cyanide poisoning, cyanide binds to the Fe3+ of a cytochrome. Which statement best describes this cytochrome?

Which of the following components of mitochondrial electron transport can act as 1-electron or 2-electron carriers?

Which of the following components of the electron transport system is inhibited by oligomycin?

Which of the following best explains how the brown adipose tissue in newborns generates large amounts of heat?

Addition of an uncoupling agent to respiring mitochondria carrying out oxidative phosphorylation has which of the following metabolic effects?

Which of the following enzymes catalyzes the reaction shown? Fe²⁺ + O₂ + 4H⁺ → 2H₂O + Fe³⁺

In cases of acute cyanide poisoning, cyanide binds to the Fe³⁺ of a cytochrome. Which statement best describes this cytochrome?