mitochondrial respiratory chain & oxidative phosphorylation

Question 1

what is the function of the electron transport chain?

Answer 1

• comprises 4 large multi-unit proteins intrinsic to the inner mitochondrial membrane
• catalyse a series of reactions: NADH + H+ + 0.5 O2 = NAD+ + H20
• energy released from this reaction not released as heat but tightly coupled to the production of ATP

Question 2

what are the components of the electron transport chain?

Answer 2

• four components: complex I, II, III and IV
• these are linked by 2 soluble proteins: ubiquinone and cytochrome c
• these are free to move in the membrane by diffusion

Question 3

what is the function of complex I (NADH dehydrogenase)?

Answer 3

• initially electrons are passed to FMN to produce FMNH2
• subsequently transfer to a series of iron-sulphur clusters
• then transfer to coenzyme Q (ubiquinone)
• so the enzyme catalyses the overall reaction: NADH + H+ + Q = NAD+ + QH2
• it is a proton pump, moving protons from matrix into the intramitochondrial space

Question 4

what is the function of complex II (succinate dehydrogenase)?

Answer 4

• FAD with complex II is reduced to FADH2 by electrons gained from the conversion of succinate to demarcate in the TCA cycle
• complex II passes electron to ubiquinone
• other substrates for mitochondrial dehydrogenase also pass on their electrons to ubiquinone by not through complex II

Question 5

what is the function of complex III?

Answer 5

• ubiquinone: cytochrome c oxidoreductase

- second of 3 proton pumps in the respiratory chain

Question 6

what is the function of complex IV?

Answer 6

• cytochrome oxidase
• third and final proton pump
• carries electrons from cytochrome c to molecular oxygen
• produces water

Question 7

what are the 3 specific systems in the membrane that allow charges species to pass through?

Answer 7

1) transport of ADP and Pi into the matrix
2) synthesis ATP
3) transport ATP into the cytosol

Question 8

how does adenine nucleotide translocase work?

Answer 8

• integral protein of the inner mitochondrial membrane
• transports ADP3- from the intramitochondrial membrane space into the matrix
• in exchange for an ATP4- molecule transported in the other direction
• known as an ‘anti porter’

Question 9

how does phosphate translocase work?

Answer 9

• a second membrane transporter is essential for oxidative phosphorylation and synthesis of ATP
• transports both phosphate and hydrogen ions into the matrix: a ‘symporter’
• favoured by the transmembrane proton gradient

Question 10

what are the 2 functional domains of ATP synthase?

Answer 10

1) F0, an oligomycin-sensitive proton channel

2) F1, an ATP synthase

Question 11

what is the structure of ATP synthase?

Answer 11

• F0 comprises 3 different subunits: a, b and c
• forms a complex of 13-15 subunits
• subunits C1-10 arranged in a circle
• F1 comprises 5 different types of subunit: alpha 3, beta 3, gamma, delta and upsilon
• forms a complex of 9 subunits
• the 3 beta subunits have catalytic sites for ATP synthesis
• beta subunits are arranged alternatively with alpha subunits like segments of an orange
• gamma subunit integrates with the 2 ‘b’ subunits of F0

Question 12

what is the theory of rotational catalysis?

Answer 12

• 3 beta subunits take it in turns catalysis the synthesis of ATP
• any given beta subunit starts in a conformation for binding ADP and Pi
• then changes conformation so the active site now binds the product ATP slightly
• then changes conformation to give the active site very low affinity for ATP so the ATP is released

Question 13

how do uncoupling reagents work?

Answer 13

• normally electron flow and phosphorylation of ADP are tightly coupled
• uncouplers dissipate the pH gradient by transporting H+ back into the matrix of the mitochondria so bypassing the ATP synthase
• this an uncoupled servers the link between electron flow and ATP synthesis, with the energy being released as heat
• can occur naturally

Question 14

how does uncoupling work with brown adipose tissue (BAT)?

Answer 14

• high numbers of mitochondria
• mitochondria contain thermogenin
• specialised for heat generation
• important in new borns, possible roll in obesity/diabetes

Question 15

how does DNP work as an exogenous uncoupler?

Answer 15

• weak acid that crosses membranes ‘ferrying’ H+ across
• each DNP molecule collects a proton from the IMS and moves through the membrane with it, depositing it in the matrix
• can then return through the membrane to collect another proton
• toxicity arises from liver damage, respiratory acidosis and hypothermia