Mitochondrial electron transport chain

Question 1

Describe the structure of mammalian mitochondria

Answer 1

Outer membrane - 50/50 protein and lipid, permeable due to porin protein channels

Inner membrane - 76/24 protein and lipid, very impermeable to most polar and/or charged molecules, requiring specific transport proteins, highly invaginated (cristae), enzymes of ETC and ATP-synthase

Question 2

List four metabolic processes that occur in the mitochondrial matrix

Answer 2

TCA cycle

Fatty acid oxidation

Urea cycle

Pyruvate dehydrogenase complex

Question 3

Outline the mitochondrial genome

Answer 3

Own genome, DNA and RNA polymerases, tRNAs, and ribosomes

Transcription and translation occur in the matrix

Contain several complete copies of the mitochondrial gemone

Question 4

How many proteins are encoded by the mitochondrial genome?

Answer 4

13 protein-coding genes

All involved in oxidative phosphorylation

Question 5

What is the inheritance pattern of mitochondrial mutations?

Answer 5

A mother will pass a defect to all of her children, but a father cannot

The mitochondria comes from the egg (mitochondria in sperm disintegrate before fertilization)

Question 6

Describe the structures of cytochromes and iron-sulfur centers

Answer 6

Cyotochromes contain heme prosthetic group, iron ion undergoes reversible redox, flips between 2+/3+

Iron-sulfur centers are iron ions complexed to sulfur atoms and to cysteine sulfhydryl groups

Question 7

Outline of ETC

Answer 7

NADH > Complex I > Coenzyme Q > Complex III > Cytochrome C > Complex IV

Question 8

Where does proton pumping occur in the ETC?

Answer 8

Complex I (4), complex III (4), and complex IV (2)

Question 9

Where does the energy to pump protons come from?

Answer 9

The free energy released as electrons pass from one complex to the other

(The reduction potential of each complex in the chain is lower than the previous component)

Question 10

How is pumping protons related to ATP synthesis?

Answer 10

Pumping protons into the intermembrane space allows for a proton electrochemical gradient to be established, called the proton motive force (PMF). Chemical – proton concentration in intermembrane space is higher than in the matrix. Electrical – cytosolic side of the membrane becomes positive with respect to the matrix side. ATP-synthase utilizes this PMF to generate ATP.

Question 11

Define the role of succinate dehydrogenase (Complex II)

Answer 11

FAD-containing enzyme, inner membrane, electrons flow from FADH2, through iron-sulcer centers, to coenzyme Q

Question 12

Describe the role of Electron-transferring flavoprotein (ETF)

Answer 12

matrix, FAD, accept electrons from FAD-containing dehydrogenases, re-oxidizes FADH₂ of ETF via ETF-oxidoreductase, passes electrons to coenzyme Q

Question 13

Describe the role of glycerol 3-phosphate dehydrogenase

Answer 13

outer surface of inner membrane, FAD-containing enzyme, converts glycerol 3-phosphate to dihydroxyacetone phosphate, electrons flow from FADH₂ to coenzyme Q

Question 14

Why does oxidation of 1 mol NADH yield 2.5 mol ATP, yet oxidation of 1 mol FADH₂ yields only 1.5 mol ATP?

Answer 14

There are 10 protons translocated for every 1 mol of NADH oxidized, but only 6 protons translocated during oxidation of FADH2

Question 15

How does rotenone inhibit oxidative phosphorylation?

Answer 15

isolated from woody vine known as derris, inhibits complex I, blocking transfer of electrons from NADH to coenzyme Q

Question 16

How does antimycin A inhibit oxidative phosphorylation?

Answer 16

isolated from certain spp. Streptomyces, inhibits the reduction of the cytochrome b component of complex II by coenzyme Q, prevents transfer of electrons from coenzyme Q to cytochrome c

Question 17

How does cyanide inhibit oxidative phosphorylation?

Answer 17

potent inhibitor of complex IV, binds with high affinity to heme iron within cytochrome oxidase, preventing the transfer of electrons from cytochrome c to oxygen

Question 18

How do uncoupling agents collapse the proton electrochemical gradient?

Answer 18

Provide an alternative pathway for protons to cross the membrane without passing through the ATP synthase complex

No ATP is made, energy is dissipated as heat, and mitochondrion is said to be uncoupled.

2,4-dinitrophenol (DNP) is a protonophore and transports protons across the inner mitochondrial membrane. It cycles catalytically across the membrane, collapsing the gradient.

UCP1 (thermogenin) forms a conductance pathway for protons across the mitochondrial inner membrane, dissipating the energy of the PMF as heat and warming the body

Question 19

Describe MERRF

Answer 19

Ragged red fibers

Point mutation in a gene that encodes a mitochondrial tRNA for lysine

Severe defect in mitochondrial translation

Question 20

Describe MELAS

Answer 20

Exercise intolerance, stroke, seizures, ragged red fibers, lactic acidosis

Gene encoding a mitochondrial tRNA for leucine

Question 21

Describe Leber optic atrophy

Answer 21

Sudden-onset blindness in young adults, death of optic nerve

Chronic energy insufficiency, increased prodution of reactive oxygen species

Genes encoding subunits of complex I

Question 22

Describe combined oxidative phosphorylation deficiency

Answer 22

Autosomal recessive

Nuclear, rather than mitochondrial DNA

Fatal progressive hepato-encephalopathy

Growth retardation, microcephaly, profound metabolic acidosis, encephalopathy, liver dysfunction

Gene encoding mitochondrial elongation factor G1