Oxidative phosphorylation Flashcards
What is the overall process of ATP synthesis by oxidative phosphorylation?
- Oxidation of NADH and FADH2 by O2 releases lots og energy.
- This energy is carried by pairs of high energy electrons, which are passed down a chain of electron transporters.
- Small amounts of energy is released at each electron transporter.
- The electron transporters use this energy to pump H+ ions from the matrix to the intermembrane space.
- This creates H+ electrochemical gradient across the inner membrane called the proton motive force.
- H+ ions diffuse back into the matrix through channels associated with ATP synthase.
- Energy of the diffusing H+ ions used to phosphorylate ADP to ATP in process known as chemiosmosis.
What are the effects of uncouplers?
- Makes the inner mitochondrial membrane more leaky to H+ ions, reducing the PMF.
- This makes the mitochondria less efficient at producing ATP from the breakdown of fuels.
What is the name of complex I?
NADH-Q reductase
What is the name of complex II?
Succinate-Q reductase (inc. succinate dehydrogenase)
What is the name of complex III?
Q-cytochrome c oxidoreductase
What is the name of complex IV?
Cytochrome c oxidase
What is the general principle of the ETC?
The complexes are simply a series of trnasmembrane enzymes catalysing redox reactions between the different mobile electron carriers in the ETC.
What is the function of complex I?
Oxidation of NADH to NAD+ with the pair of electrons being transported to CoQ.
What is the mechanism of action of complex I?
- Complex I has 2 arms:
1. Peripheral arm - Site of electron transport
2. Transmembrane arm - Site of proton pumping - In the peripheral arm, a pair of electrons are removed from NADH and passed down a series of electron carriers, starting with FMN and followed by 8 FeS groups, before being passed to CoQ.
- Conformational change induced by electron transfer in the peripheral arm causes transmembrane arm to pump 4 H+ ions from the matrix into the intermembrane space.
- 9th FeS in peripheral arm prevents formation of ROSs.
What is significant about electron transport between electron carriers in the complexes?
- Electron carriers within different complexes are usually
What is the function of complex II?
- Oxidation of succinate to fumarate.
- Re-oxidation of FADH2 to FAD and the transfer of a pair of electrons to CoQ.
What is the mechanism of action of complex II?
- Complex II contains succinate dehydrogenase, a CAC enzyme.
- Succinate is oxidised to fumarate within the complex, with FAD being reduced to FADH2.
- FADH2 is actually part of the complex itself and so is immediately reoxidised.
- Pair of electrons is released and passed down a sequence of 3 FeS complexes before being passed to CoQ.
- Haem group may be present to prevent the formation of ROSs.
- The amount of energy released from the oxidation of FADH2 is insufficient to drive the active transport of H+ ions across the membrane.
What is the function or complex III?
Transfers electrons from CoQ (reduced) to cytochrome c.
What is the mechanism of action of complex III?
- Electrons are passed to an FeS complex. Then, they are transferred to cytochrome bc1, which is closely associated with cytochrome c.
- Electrons are transferred from cytochrome c1 to cytochrome c.
- Energy released from this process is used to pump 4 H+ ions from the matrix into the intermembrane space.
What is the function of the Q-cycle?
The Q-cycle allows the net oxidation of 1 CoQ to pump 4 H+ ions from the matrix into the intermembrane space.
What is the function of complex IV?
Transfers electrons from cytochrome c to O2, which is converted to H2O. O2 + 4e- + 4H+ → 2H2O.
What is the mechanism of action of complex IV?
- Complex IV contains 4 redox centres:
1. CuA (with 2 Cu atoms)
2. Cytochrome a
3. Cytochrome a3
4. CuB - These 4 redox centres work together to ensure that O2 is fully oxidised to H2O.
- For every pair of electrons, the complex pumps 2H+ ions from the matrix into the intermembrane space. However, 2 further H+ ions are lost from the matrix in H2O production, which steepens the proton gradient.
What are the different electron carriers in the ETC?
- FAD: Capable of carrying 2 electrons as FADH2.
- FMN
- FeS complexes: Capable of carrying 1 electron each.
- CoQ: Capable of carrying 2 electrons from complex I/II to III.
- Cytochromes: Capable of carrying 1 electron each due to containing Fe2+/Fe3+.
Cu: Acts as redox centres in complex IV.
What are the different types of cytochromes?
- Cytochrome a, a3: Found in complex IV.
- Cytochrome b, c1: Found in complex III.
- Cytochrome c: Transfers electrons between complex III and IV.
How can the absorption spectrum of cytochrome c be distinguished?
Reduced cytochrome c has 2 characteristic α and β peaks between 500-600 nm.
What is proton motive force?
Force that drives the diffusion of H+ ions from the intermembrane space back into the matrix as a result of the electrochemical gradient.
What is the basis of chemiosmosis?
Energy from the diffusion of H+ ions down the electrochemical gradient, from the intermembrane space to the matrix, is used in the process of ATP synthesis. This coupling is achieved by ATP synthase.
What is the equation for the phosphorylation of ATP?
ADP + Pi → ATP + H2O
What is the gross structure of ATP synthase?
ATP synthase consists of 2 subunits:
- F0: Transmembrane domain, through which H+ ions diffuse. It is known as the ‘rotory engine’ part of ATP synthase.
- F1: Peripheral domain, site of ATP synthesis.
What is the specific structure of F0 domain?
- a subunit
- 2 b subunits forming stalk linking a subunit in F0 to δ subunit in F1.
- 10 c subunits in ring shape.
- Each subunit has H+ binding site.
- Binding and release of H+ from F0 causes its rotation, which causes conformational changes in F1 that drive ATP synthesis.
What is the specific structure of F1 domain?
- F1 consists of 9 subunits.
- 3 pairs of αβ subunits arranged around and attached to central γ subunit in a ring shape. Each pair forms ATP binding site.
- γ, δ, ε subunits all connect F1 to F0.
- γ subunit directly involved in transferring conformational change from F0 to F1.
How is energy generated by F0 for ATP synthesis in F1?
- H+ binds to a c subunit, causing a conformational change that causes c ring to rotate by 1 subunit.
- Another c subunit releases its H+ ion into the matrix, freeing up a H+ binding site for another H+ from the intermembrane space to bind to F0.
- Process repeats and the F0 subunit continuously rotates, driving conformational changes in the γ subunit of F1, which is transferred to the αβ binding sites.
- This drives the process of ATP synthesis in F1.
What are the states of the ATP binding sites on the F1 subunit?
- Open (O): Nothing bound.
- Loose (L): Bound to ADP and Pi.
- Tight (T): Catalyses reaction between ADP and Pi to form ATP.
What is the process of ATP synthesis in F1 subunit?
- ADP and Pi to binding site in L state.
- Energy from rotating F0 used to drive conformational change of binding site from L state to T state.
- ATP is formed as the T state catalyses the ADP + Pi → ATP + H2O.
- Energy is used to convert T state to O state, which causes ATP to be released.
What is the majority of energy from PMF used for during ATP synthesis?
Conversion of the T state binding site to O state, i.e. releasing the bound ATP.
What is the stoichiometry of ATP synthase?
- ~3 H+ ions need to cross the ATP synthase in order to produce 1 ATP molecule.
- ~3 ATP molecules are synthesised per turn of F0.
- Assuming 100% efficiency, ~3 ATP molecules can be synthesised per NADH molecule reoxidised and ~2 from FADH2 reoxidation.
- Taking into account proton leakage (for phosphate transport into or ATP transport out of cells), ~2.5 ATP molecules are synthesised per NADH and ~1.5 ATP molecules are synthesised per FADH2.
Which compound opposes the formation of free radicals?
Reduced glutathione (GSH)
What is an inhibitor of complex I?
Rotenone
What is an inhibitor of complex III?
Antimycin
What is an inhibitor of complex IV?
Cyanide
What is an inhibitor of ATP synthase?
Oligomycin
How is ADP transported into the mitochondria?
- Adenosine nucleotide translocase
- ADP is exchanged for ATP
How is phosphate transported into the mitochondria?
- Phosphate translocase
- By symport with H+
