S1B4 - Electron Transport Chain

Question 1

Name the protein components of the mitochondrial electron transport chain. Include the name and complex number.

Answer 1

• Complex I - NADH dehydrogenase
• Complex II - Succinate dehydrogenase
• Complex III - ubiquinone: cytochrome c oxidoreductase
• Cytochrome c
• Complex IV - Cytochrome oxidase
• Complex V - ATP synthase

Question 2

What is the main function of flavoproteins, heme containing cytochromes, copper in complex IV, and UQ?

Answer 2

Flavoproteins transfer electrons along the electron transfer chain

Question 3

What do you need to know about coenzyme Q?

Answer 3

Coenzyme Q (aka ubiquinone)

• Ubiquinoneis a lipid-soluble conjugated dicarbonyl compound that readily accepts electrons
• Upon accepting two electrons, it picks up two protons to give an alcohol, ubiquinol
• Ubiquinol can freely diffuse in the membrane, carrying electrons with protons from one side of the membrane to another side
• Coenzyme Q is a mobile electron carrier transporting electrons from Complexes I and II to Complex III

Question 4

Name the entry point in the electron transport chain for the electrons from the following biological fuels:

1. fatty acid oxidation
2. succinate oxidation (citric acid cycle)
3. NADH
4. succinate
5. Glycerol 3-phosphate
6. Acyl-CoA dehydrogenase

Answer 4

1. Q
2. Q
3. through a flavoprotein with the cofactor FMN to a series of FE-S centers (in Complex I) and then to Q
4. through a flavoprotein with the cofactor FMN to a series of FE-S centers (in Complex II) and then to Q
5. to a flaviprotein on the outer face of the inner mitochondrial membrane, then to Q
6. to electron-transferring flavoprotein (EFT), from which they pass to Q via ETF: ubiquinone oxidoreductase

Question 5

What does Myxothiazol do?

Answer 5

Myxothiazol prevents electron flow from QH₂ to the Rieske iron-sulfur protein, binds at Q_p. This all takes place in complex III.

Question 6

How many electrons from NADH to O₂ result in the translocation of how many protons across the membrane. How many protons from Complex I & IV? How many from Complex II?

Answer 6

2e from NADH to O₂ result in the translocation of 10 protons across the membrane, 4 each from Complex I & IV and 2 from Complex II.

Question 7

Describe the two main functional subunits of ATP synthase.

Answer 7

• F₁
  
  • soluble complex in the matrix
  • individually catalyzes the hydrolysis of ATP
• F₀
  
  • integral membrane complex
  • Transports protons from IMS to matrix, dissipating the proton gradient
  • Energy transferred to F₁ to catalyze phosphorylation of ADP

Question 8

Describe Leber hereditary optic neuropathy (LHON).

Answer 8

• LHON affects CNS causing sudden–onset blindness in early childhood due to death of the optic nerve
• LHON results from single base change in the mitgenes encoding three subunits of complex I (ND1, 4, & 6) lowering its activity
• Patients with lowered amount of muted mitDNAdevelop symptoms and blindness in early adulthood while patients with higher percent of mutated mitDNAdevelop sever disease in early childhood

Question 9

What are the two different shuttles used to transport NADH across the inner mitochondrial membrane? Which is more energy efficient?

Answer 9

Since NADH cannot cross the mitochondrial membrane, cytosolic electrons carried by NADH are carried across the mitochondrial membrane via two shuttle pathways:

• The primary NADH electron transport system is the malate-aspartate shuttle, which transports NADH electrons to complex I in the mitochondria.
• The less commonly used NADH electron transport system is the glycerol-3-phosphate (G3P) shuttle. In the G3P shuttle, NADH reduces dihydroxyacetone-phosphate to G3P, which can cross the inner mitochondrial membrane. G3P is then oxidized back to dihydroxyacetone-phosphate by FAD+ to form FADH2. FADH2 donates its electrons to complex II. Because NADH is converted to FADH2 in this system, it is less efficient than the malate-aspartate shuttle.

Question 10

For each of the following electron transport chain toxins, would there be an increased or decreased proton gradient and oxygen consumption compared to normal: Rotenone? Oligomycin? 2,4-Dinitrophenol?

Answer 10

Question 11

What toxin inhibits ATP synthase directly by blocking its proton channel?

Answer 11

Oligomycin (a macrolide) inhibits ATP synthase (complex V) by blocking its proton channel (specifically the Fo subunit, “o” for oligomycin).

Question 12

Reduced nicotinamide adenine dinucleotide produced during glycolysis is shuttled into mitochondria to provide electrons for oxidative phosphorylation. Which of the following molecules is NOT involved in the most common shuttle mechanism?

A) Aspartate

B) Alanine

C) Oxaloacetate

D) Malate

E) Glutamate

Answer 12

The malate-aspartate shuttle is used to shuttle NADH into mitochondria. The process starts with oxaloacetate + NADH becoming malate. Malate is able to enter the mitochondria, where it is converted back to OAA + NADH. OAA is unable to leave the mitochondria, but it can be converted to aspartate using glutamate as an amino group donor. The aspartate enters the cytosol and is converted back to OAA. Alanine is NOT involved in the malate-aspartate shuttle.

Question 13

Which two toxins can bind and disrupt NADH dehydrogenase (or complex I) of the ETC? Which toxin can do this to cytochrome c reductase (complex III)?

Answer 13

Amobarbital (known as amytal) and rotenONE bind to NADH dehydrogenase (complex I) and directly inhibit electron transport.

Antimycin A (“AnTHREEmycin”) binds to cytochrome c reductase (complex III) and directly inhibits electron transport.

Question 14

How does 2,4-dinitrophenol inhibit aerobic respiration?

Answer 14

2,4-Dinitrophenol and increased doses of aspirin increase the permeability of the inner mitochondrial membrane leading to a decreased proton gradient and increased oxygen consumption. Heat is generated instead of ATP (this explains the fever generated following toxic doses of aspirin.)

Question 15

What two tissues are most severely affected when exposed to toxins that disrupt any component of the electron transport chain?

Answer 15

Toxins that disrupt any component of the ETC disrupt the aerobic production of ATP. Tissues that depend highly on aerobic respiration, such as the CNS and the heart, are particularly affected.

Question 16

To what enzyme do carbon monoxide and cyanide bind in the electron transport chain?

Answer 16

Carbon monoxide and Cyanide bind to Cytochrome C oxidase (complex IV) and directly inhibit electron transport.

Question 17

In neonates, brown fat is metabolized for heat production. To accomplish this, the proton gradient across the mitochondrial membrane is disrupted. Which of the following toxins similarly disrupts the proton gradient?

A) Oligomycin

B) Glutamate

C) Cyanide

D) Carbon monoxide

E) 2,4-dinitrophenol

Answer 17

E) 2,4-dinitrophenol

• 2,4-dinitrophenol transports protons across the mitochondrial membrane, inhibiting the electron transport system.
• Oligomycin inhibits ATP synthase.
• CO and CN bind to cytochrome c oxidase, consuming oxygen and depriving the mitochondria of an electron receptor.

Question 18

What is the final electron acceptor of the electron transport chain?

Answer 18

Molecular oxygen, O2, is the final electron acceptor.

Question 19

What powers ATP synthase to generate ATP?

Answer 19

ATP Synthase (Complex V): uses the electrochemical proton gradient created by the ETC to produce ATP from ADP and Pi.

Question 20

What is the main function of the electron transport chain? In humans, where is the electron transport chain located?

Answer 20

Electron transport chain (oxidative phosphorylation): uses NADH and FADH2 electrons (from glycolysis, pyruvate dehydrogenase complex, and the citric acid cycle) to form a proton gradient. The proton gradient drives the production of ATP.

The ETC (electron transport chain) is composed of 5 multi-enzyme complexes, numbered I-V. It is embedded in the inner mitochondrial membrane.

Question 21

Carbon monoxide, in addition to binding heme, can also directly disrupt a process that is central to ATP synthesis. To which enzyme does carbon monoxide bind?
A) Succinate dehydrogenase

B) Pyruvate kinase

C) Cytochrome c oxidase

D) Isocitrate dehydrogenase

E) Adenosine triphosphate synthase

Answer 21

C) Cytochrome c oxidase

CO binds to cytochrome c oxidase in the electron transport chain.

Question 22

In the electron transport chain, what is the ATP yield per molecule of NADH? FADH₂?

Answer 22

1 NADH yields Ž2.5 ATP and 1 FADH2 yields Ž1.5 ATP. The reason for the lower energy yield of FADH2 is that NADH electrons are transferred to complex I while FADH2 electrons are transferred to complex II.

• Complex II is succinate dehydrogenase of the citric acid cycle.

Question 23

What are the mobile electron carriers of the electron transport chain?

Answer 23

Mobile electron carriers coenzyme Q and cytochrome c shuttle electrons between enzyme complexes of the ETC.

• coenzyme Q shuttles electrons from complexes I and II to complex III.
• cytochrome c shuttles electrons from complex III to complex IV.

Question 24

What provides the energy for creating the proton gradient in the electron transport chain?

Answer 24

The flow of electrons (provided by NADH and FADH2) through the ETC provides the energy to pump protons into the mitochondrial inter-membrane space. This creates an electrochemical proton gradient.

Question 25

Why is cobra venom so deadly?

Question 26

What kind of poisoning is thiosulfate used to treat?

Answer 26

Administration of **thiosulfate** causes **cyanide** to react with enzyme rhodanese, forminga nontoxic thiocyanate.

Question 27

What do you need to know about mitochondrial myopathies in mitochondrial tRNA genes?

Answer 27

* **Point mutations** on the genes encoding mittRNA result in two most common mitdiseases **characterized by encephalomyopathy** * Lys mutation in tRNA causes **myoclonic epilepsy and ragged red fibers (MERRF)** * **Myoclonus and ataxia with seizures and myopathy** are associated with this disease. Skeletal muscles contain abnormal mit with paracrystalline structures appearing as ragged red fiber with decreased cyt oxidases * Mutation in tRNA for Leu causes the common mitochondrial encephlopathy, lactic acidosis and stroke-like activity (MELAS). Skeletal muscle contain ragged red fiber but retain cyt oxidase activity * More than 85% mutation results in **severe CNS symptoms**, and 5%-30% mutated DNA patients show **maternally inherited diabetes mellitus and deafness** * Both tRNA mutations impair mitprotein synthesis leading to **decreased activities of complex I and cyt oxidase**

Question 28

What do you need to know about exercise intolerance in patients with mutations in cyt b?

Answer 28

* In 1993 cyt b mutation resulting in lower cyt bc1 complex activity was reported in a 25 year old man. * Other mutations G290D, G339E, G34S, and G166E observed. D/E mutations occur on cyt b close to Qo and S mutation close to Qi site for ubiquinone oxidation and reduction respectively * All the mutations involve **guanine to adenine transition** in mitDNA indicating that the mutations have **resulted from oxidative damage** * Replacement of G by larger charged molecule alter the structure of cyt b resulting in **lowered catalytic activity of bc1 complex.** * Truncation and deletion of 4 to 24 base pairs from nonsense mutations often lead to severe exercise intolerance, **lactic acidosis in resting** state and occasional myoglobinuria. * In contrast to majority of mtDNAmutations, cyt b mutations **are not maternally inherited**. Also, they are **mainly in muscle tissue suggesting somatic mutations**

Question 29

What do you need to know about NADP oxidase (NOX) in health and disease?

Answer 29

* NADPH-dependent oxidase (NOX) in phagocytic and other cells **produces large amounts of superoxide** to kill microbes. * Mutations in NOX results in **inability to produce adequate amounts of superoxide** to combat infections * There are 7 different isoforms of NOX family in all tissues of the body. Theses isoforms are thought to contribute in maintaining the vascular tone , cell proliferation, angiogenesis, and apoptosis * Despite the beneficial role of NOX in cell processes, **excess ROS generated by NOX results in pathophysiological conditions** including, endothelial cell dysfunction that contributes to atherosclerosis, hypertension, congestive heart failure, ischemic reperfusion injury, diabetic vascular problem.

Question 30

What do you need to know about myocardial reperfusion injury in relation to the electron transport chain?

Answer 30

* Early reperfusion with appropriate therapy after myocardial infarction **results in reduction of the infarct size and better clinical outcome** * **Restoring blood flow to the ischemic tissue, however, may damage heart** termed as myocardial reperfusion injury * Such a damage comes from damage to myocardium **due to rapid restoration of electron transfer through mt ETC with concomitant generation of ROS** * **ROS are also generated** by xanthine oxidase of endothelial cells and by NADPH oxidase of neutrophils

Question 31

What do you need to know about Batten's disease and ATP synthase?

Answer 31

**Batten’s disease** Lysosomal storage disease where the **subunit c of ATP synthase** has been found as a predominant storage protein

Question 32

What do you need to know about Alzheimer's disease and ATP synthase?

Answer 32

**Alzheimer’s disease** Progressive degenerative brain disease in which a **deficiency of ATP synthase** has been observed in mitochondria.

Question 33

What do you need to know about drug-resistant strains of *Mycobacterium* and ATP synthase?

Answer 33

**Drug-resistant strains of Mycobacterium** R207910, blocks the synthesis of ATP by **targeting subunit c of ATP synthase**

Question 34

What do you need to know about tumor angiogenesis and ATP synthase?

Answer 34

**Tumor angiogenesis** ATP synthase inhibitors **shown to inhibit migration and proliferation of endothelial cells with little effect on intracellular ATP**

Question 35

What do you need to know about lupus erythematosus and ATP synthase?

Answer 35

**Lupus erythematosus** Bz-423, kills pathogenic lymphocytes selectively by **inducing apoptosis** in lymphoid cells

Question 36

What do you need to know about Leigh syndrome, ataxia, and retinitis pigmentosa syndrome in relation to ATP synthase?

Answer 36

**Leigh syndrome, ataxia, and retinitis pigmentosa syndrome** Neurodegenerative disease causing neuromuscular disorder with a 50% survival rate. Severe impairment of ATP synthesis due to a **mutation in subunit a of ATP synthase.**

Question 37

What do you need to know about antiobesity and ATP synthase?

Answer 37

**Antiobesity** Inhibition of nonmitochondrial ATP synthase **inhibits the cytosolic lipid droplet accumulation**, suggesting ATP synthase as a molecular target for antiobesity drugs.

Question 38

What do you need to know about Antiangiogenic therapeutic strategy in relation to ATP synthase?

Answer 38

**Antiangiogenic therapeutic strategy** Inhibitors with the nonmitochondrial ATP synthase of endothelial cells inhibit the migration and proliferation of endothelial cells with little effect on intracellular ATP synthase, **suggesting for an antiangiogenic therapeutic strategy to block tumor angiogenesis.**

Question 39

What do you need to know about blood pressure and ATP synthase?

Answer 39

**Blood pressure** ATP synthase **F6 subunit circulating in the blood** has been recognized to be involved in the **increase of blood pressure.**

Question 40

What is the pattern of inheritance in most mitochondria based diseases?

Answer 40

maternal