TOPIC 5: ETC

Question 1

Recap

Answer 1

• 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

Question 2

How do mitochondria generate ATP from NADH and FADH2?

Answer 2

• 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

Question 3

How does the electron transport chain work?

Answer 3

• 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
• The electrons are transferred progressively from a high free energy level to a lower free energy level
  o Energy is liberated

Question 4

The ETC components are arranged in 4 major complexes within the inner mitochondrial membrane

Answer 4

1. Complex I: NADH oxidoreductase
2. Complex II: succinate dehydrogenase (TCA cycle enzyme)
3. Complex III: cytochrome b/cytochrome c 1
4. Complex IV: cytochrome oxidase

Question 5

Complex I

Answer 5

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)

Question 6

Complex II:

Answer 6

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

Question 7

Complex III:

Answer 7

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

Question 8

Complex IV:

Answer 8

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

Question 9

Proton transfer occurs at complex

Answer 9

I, III and IV

Question 10

How is flow of electrons coupled to ATP generation: Coupling ETC to ATP generation

Answer 10

• 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

Question 11

The H + gradient is a potential source of

Answer 11

energy

Question 12

Where does it occur in the mitochondria?

Answer 12

• The components of the ETC and ATP synthase are embedded in the inner mitochondrial membrane

Question 13

Respiratory control

Answer 13

• 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