Chemiosmosis Flashcards
What are some experimental evidence for the Chemiosmotic Hypothesis ?
- Coupling of electron transport and ATP synthesis
- Development of the ‘protonmotive force’ (pmf)
- Role of proton gradient in ATP synthesis
What 4 basic tenets is the Chemiosmotic hypothesis based upon ?
- An ion-impermeable inner membrane
- H+ translocating respiratory chain
- Reversible H+ translocating ATP synthase
- H+ or OH- -linked exchange diffusion system
What are the properties of the mitochondrion ?
- Outer membrane
- Freely permeable to small molecules (Mr < 5,000) and ions - Inner membrane
- Impermeable to most small molecules and ions including H+ - Matrix
- Contains enzymes for:
TCA cycle
β-oxidation
Amino acid oxidation
Many other enzymes
DNA and ribosomes
ATP, ADP, Pi, Mg2+, Ca2+, K+
Many soluble metabolic intermediates
What does the addition of weak HCl to mitochondria result in ?
An initial rapid acidification of the medium followed by an alkalination
Name the complexes of the ETC ?
Complex I - NADH dehydrogenase Complex II - Succinate dehydrogenase Complex III - Cytochrome bc1 complex Complex IV - Cytochrome aa3 oxidase Complex V - ATP synthase
Explain Complex I ?
- L-shaped, with one arm embedded in the inner membrane and the other extending into the matrix
- obligately coupled processes:
1. transfer of electrons from NADH to ubiquinone in mammals
2. transfer of four protons from the matrix to the intermembrane space
Where does Complex II function ?
In the citric acid cycle
Explain Complex II ?
- Couples the oxidation of succinate to fumarate at one site with the reduction of ubiquinone on the other.
- Electrons move (blue arrows) from succinate to FAD, then through the three Fe-S enters to ubiquinone.
- Electron transfer through Complex II is not accompanied by proton pumping across the inner membrane, although the QH2 produced by succinate oxidation will be used by Complex III to drive proton transfer
Explain Complex III ?
- Couples the transfer of electrons from dihydroquinone to cytochrome c
- Translocate protons from the matrix to the intermembrane space
- Complex III has two distinct binding sites for ubiquinone, QN and Qp, which correspond to the sites of inhibition by two drugs that block oxidative phosphorylation
1. Antimycin A, which blocks electron flow from cytochrome b to cytochrome c1
2. Myxothiazol, which prevents electron flow from QH2 to the Rieske iron-sulfur protein
Explain Complex IV ?
- carries electrons from cytochrome c to molecular oxygen, reducing it to H2O
- transport 2 molecules of protons (H+) at a time
Explain how the changes in reduction potential and free energy during the stepwise flow of electrons through the ETC ?
- Translocation of protons across the inner mitochondrial membrane
- Electrons pass through the multi protein complexes from those with a lower reduction potential to those with a higher (more positive) reduction potential (left scale), with a corresponding reduction in free energy (right scale).
- The energy released as electrons flow through three of the complexes is sufficient to power the pumping of H+ ions across the membrane, establishing a proton-motive force.
How are respirasomes formed ?
Although the four electron-transferring complexes can be separated in the laboratory, in the intact mitochondrion, the respiratory complexes tightly associate with each other in the inner membrane
How much protons does the respiratory chain translocate ?
10H+/2e-
Reduction of one molecular oxygen (O2) allow ?
Four electron flow in the ETC that translocates 20 protons (H+)
Explain Proton-motive Force ?
Electron transport chain proteins create the electrochemical proton gradient by one of these three mechanisms:
- actively transport protons across the membrane via relatively poorly understood mechanisms
- pass electrons to ubiquinone which picks up protons from the matrix to form dihydroquinone (QH2)
- protons released into the inter-membrane side when QH2 is oxidised
