Describe the structure of the mitochondrion
Pyruvate from Stage 1 (glycolysis) does not enter directly into Stage 3 (TCA cycle).
What reaction occurs?
The link reaction
Where does the link reaction occur?
Mitochondrial matrix – pyruvate transported from cytoplasm across mitochondrial membrane
Which enzyme catalyses the link reaction?
Pyruvate dehydrogenase (large multi-enzyme complex)
Describe three features of the link reaction
- Reaction is sensitive to Vitamin B1 deficiency (PDH requires factors from B1)
- Reaction is irreversible (key regulatory step)
- Irreversible loss of CO2
PDH is subject to multiple regulation.
Describe its activation and inhibition in terms of phosphorylation and dephosphorylation
What is the consequence of PDH deficiency?
Describe three features of the Tricarboxylic Acid Cycle
- Acetyl CoA converted to 2CO2
- Produces some energy & precursors for biosynthesis
Where does the TCA cycle occur?
How many rounds of TCA cycle occur per glucose molecule entering from glycolysis?
What is produced after the two rounds of the TCA cycle occurring per glucose molecule?
- 6 NADH
- 2 FADH2
- 2 GTP
How is the TCA cycle regulated?
Appreciate the biological precusors molecules synthesised from the TCA cycle
Which three events occur in oxidative phosphorylation?
- Electron transport and ATP synthesis
- NADH & FADH2 re-oxidised
- Large amounts of energy (ATP) produced
Where does the ETC occur?
Inner mitochondrial membrane
What is the role of the electron carriers NADH and FAD2H in the ETC?
- NADH and FAD2H contain high energy electrons that can be transferred to O2 through a series of carrier molecules with the release of large amounts of energy
- The energy is used to drive ATP synthesis in the final stage of catabolism (oxidative phosphorylation)
Identify and describe the two uses of reducing power in ATP synthesis
- Electron transport: electrons on NADH and FAD2H transferred through a series of carrier molecules to oxygen
- Oxidative phosphorylation: free energy released used to drive ATP synthesis
Describe what happens in the ETC
- NADH and FADH2 supply electrons from metabolic substrates
- Electrons pass through the ETC and reduce oxygen to form H2O at Complex IV
What do proton translocating complexes do?
- Complexes I, III and IV act as proton translocating complexes
- They use free energy derived from ETC to move protons from the inside to the outside of the inner mitochondrial membrane to build up a proton motive force
Explain the concept of the proton motive force
- The inner mitochondrial membrane is impermeable to protons and the H+ concentration in the intermembrane space builds up
- A [H+] gradient (membrane potential) across inner mitochondrial membrane forms i.e. the proton motive force
Oxidative phosphorylation involves electron transport coupled to ATP synthesis.
In three steps, explain how this occurs
⇒ Electrons are transferred from NADH and FAD2H to molecular oxygen
⇒ Energy released is used to generate the PMF
⇒ Energy from the dissipation of the proton motive force is coupled to the synthesis of ATP from ADP
How many proton translocating complexes are used by NADH and FADH2 respectively?
Electrons in NADH have more energy than in FAD2H:
- NADH uses 3 PTCs
- FADH2 uses only 2 PTCs
State the amounts of ATP synthesised from the oxidation of two moles of NADH and FADH2 respectively
- Oxidation of 2 moles of NADH → synthesis of 5 moles of ATP (P/O = 2.5)
- Oxidation of 2 moles of FADH2 → synthesis of 3 moles of ATP (P/O = 1.5)
In 4 steps, explain how a high [ATP] regulates oxidative phosphorylation
⇒ Low [ADP] is low means no substrate for ATP synthase
⇒ Inward flow of H+ stops
⇒ [H+] in intermitochondrial space increases
⇒ Stops electron transport
Explain the inhibition of oxidative phosphorylation
Inhibitors block electron transport e.g. cyanide prevents acceptance of electrons by O2
In three steps, explain how the uncoupling of oxidative phosphorylation occurs
⇒ Uncouplers increase the permeability of inner mitochondrial membrane to H+
⇒ Dissipate proton gradient (reducing PMF)
⇒ No drive for ATP synthesis
Identify an example of an uncoupler
Thermogenin (UCP1) is a naturally-occurring uncoupling protein found in brown adipose tissue.
In 6 steps, explain its role in response to cold
⇒ Noradrenaline activates lipase
⇒ Lipase releases fatty acids from triacylglycerol
⇒ Fatty acids activate UCP1
⇒ UCP1 uncouples electron transport from ATP synthesis
⇒ H+ are transported back into mitochondria
⇒ Energy of PMF is released as extra heat
Where is brown adipose tissue found and what is its function?
- Newborn infants – to maintain heat, particularly around vital organs
- Hibernating animals – to generate heat to maintain body temperature
Compare and contrast oxidative phosphorylation with substrate level phosphorylation in terms of the following:
- Energy coupling
- O2 dependency
- ATP synthesis