TOPIC 5: ETC Flashcards
Recap
- We’ve now seen in detail the route of glucose metabolism in fermentative and aerobic conditions
- In metabolising glucose by glycolysis and the TCA cycle, we have generated ATP by substrate level phosphorylation
- We have also generated reduced electron carriers (ie. NADH and FADH2)
- In this final stage of respiration, the cell converts these reduced molecules into ATP
How do mitochondria generate ATP from NADH and FADH2?
- During glycolysis and the TCA cycle, these molecules have become reduced (i.e. have gained electrons) when a substrate was oxidised.
- Now, the cell will systematically pass these electrons from the reduced electron carriers, down an electron transport chain, causing H+ ions to be transferred into the inter membrane space
- The H+ ions will be used to generate a ‘proton gradient’ which ultimately drives ATP synthesis
- This whole process is referred to as oxidative phosphorylation
How does the electron transport chain work?
- NADH and FADH 2 have acquired electrons (e.g. TCA cycle)
- They are energy-rich molecules as they contain a pair of electrons that have a high transfer potential
- These electrons are passed down the electron transport chain to the ultimate electron acceptor O2
- A large amount of energy is liberated through the chain
- This is transferred into chemical potential energy (proton gradient)
- Ultimately, this is used to generate ATP
- Each of the reactions is a redox reaction
o One component of the reaction is being oxidised and the other reduced
- Each of the reactions is a redox reaction
- The electrons are transferred progressively from a high free energy level to a lower free energy level
o Energy is liberated
The ETC components are arranged in 4 major complexes within the inner mitochondrial membrane
- Complex I: NADH oxidoreductase
- Complex II: succinate dehydrogenase (TCA cycle enzyme)
- Complex III: cytochrome b/cytochrome c 1
- Complex IV: cytochrome oxidase
Complex I
Complex I: Step 1
- The first reaction is the oxidation of NADH + H + by NADH oxidoreductase
- A tightly-bound prosthetic group, flavin mononucleotide (FMN) becomes reduced
Complex I: Step 2
- NADH oxidoreductase also contains non-heme Fe
- This is probably involved in the transfer of electrons to coenzyme Q (aka ubiquinone)
Complex II:
Step 3
- Reduced FADH 2 is generated by FAD-linked dehydrogenases
- e.g. succinate dehydrogenase from TCA cycle; fatty acyl CoA dehydrogenase from -oxidation
- FADH 2 directly reduces coenzyme Q
Complex III:
Steps 4-6
- Coenzyme Q donates electrons to complex III
- Complex III consists of a series of cytochromes
- Electron transport proteins that contain a heme prosthetic group
- Iron alternates between ferric (Fe3+) and ferrous (Fe2+) states
Complex IV:
Final step
- Cytochromes (a + a3) are the terminal members of the chain
- They exist as a complex called cytochrome oxidase
- This complex also contains copper, which undergoes cupric (Cu2+)/cuprous (Cu +) redox reaction
- This reaction is important in the final transfer of electrons to O2
- Of all the members of the ETC, only cytochrome (a + a3) can react directly with O2
Proton transfer occurs at complex
I, III and IV
How is flow of electrons coupled to ATP generation: Coupling ETC to ATP generation
- Flow of electrons down the ETC drives H + ions across the mitochondrial membrane into the intermembrane space
- This creates an electrochemical (proton) gradient
- This gradient is a potential source of energy
- Cells harness it to generate ATP
The H + gradient is a potential source of
energy
Where does it occur in the mitochondria?
- The components of the ETC and ATP synthase are embedded in the inner mitochondrial membrane
Respiratory control
- Regulation of the rates of ETC and oxidative phosphorylation by ADP levels is known as respiratory control
- This is of obvious physiological importance
- Electrons do not flow from fuel molecules to O2 unless ATP synthesis is needed
- This means that fuel molecules are not catabolised unnecessarily
