ML7-8: Stage III catabolism (citric acid cycle & oxidative phosphorylation)

Question 1

Where do Stage III reactions occur?

Answer 1

The mitochondria

Question 2

What are the reactions of cellular respiration?

Answer 2

The citric acid cycle removes electrons from acetyl CoA to form NADH and FADH₂

Reoxidation of NADH and FADH₂ in oxidative phosphorylation produces ATP

Question 3

What is the link between glycolysis and the citric acid cycle?

Answer 3

Pyruvate dehydrogenase

Question 4

What is pyruvate dehydrogenase?

Answer 4

Large, multi-subunit enzyme found in the mitochondrial matrix

Question 5

What is the equation for the conversion of pyruvate to acetyl-CoA? Is it reversible or irreversible?

Answer 5

It is irreversible

Question 6

What is the overall equation for the citric acid cycle?

Answer 6

CH₃CO~CoA + 3NAD⁺ + FAD + GDP + Pi + 2H₂O → 2CO₂ + CoA + 3NADH + 3H⁺ + FADH₂ + GTP

Question 7

How can the citric acid cycle be regulated?

Answer 7

• ATP/ADP ratio
• NAD⁺/NADH ratio
• Allosteric regulation
• Product inhibition
• Substrate availability

Question 8

Define ‘amphibolic’.

Answer 8

A reaction pathway containing both catabolic and anabolic reactions

Question 9

What type of reaction, overall, is the citric acid cycle?

Answer 9

Amphibolic because it has both catabolic and anabolic reactions

Question 10

What are ‘anaplerotic’ reactions?

Answer 10

Reactions that top up the pathway to allow continuation

Question 11

Does the citric acid cycle function in anaerobic conditions?

Answer 11

No

Question 12

Where does oxidative phosphorylation occur?

Answer 12

Inner mitochondrial membrane

Question 13

Give an overview of oxidative phosphorylation.

Answer 13

• Occurs in the mitochondria
• Electron transport and ATP synthesis
• NADH and FADH₂ are re-oxidised
• O₂ is required (reduced to H₂O)
• Lots of energy (ATP) is produced

Question 14

What is the equation for the complete oxidation of glucose?

Answer 14

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Question 15

Where has the energy gone that was produced at the end of the citric acid cycle?

Answer 15

It is used to drive ATP synthesis

Question 16

Explain oxidative phosphorylation.

Answer 16

• Electrons are transferred through a series of carrier molecules to O₂, with release of energy
• The release of energy results in the transfer of protons from the matrix to the intermembrane space
• This results in a proton gradients across the inner mitochondrial membrane (proton motive force (pmf))
• Protons can only return across membrane via ATP synthase and this drives ATP synthesis.

Question 17

What is the chemiosmotic hypothesis?

Answer 17

The proton motive force generated by the electron transport chain can be used to drive ATP synthesis, which is catalysed by the enzyme ATP synthase

Question 18

How does cytoplasmic NAD get into the mitochondrial matrix?

Answer 18

Via transporter molecules in the mitochondrial membrane:

1. Glycerol-3-phosphate shuttle
   NADH + H⁺ + E-FAD → NAD⁺ + E-FADH₂
2. Malate-aspartate shuttle
   NADH + NAD⁺ → NAD⁺ + NADH

Question 19

How much ATP is produced in oxidative phosphorylation?

Answer 19

Each NADH produces 2.5 molecules of ATP

Each FADH₂ produces 1.5 molecules of ATP

Question 20

How can oxidative phosphorylation be regulated?

Answer 20

The availability of ADP controls the rate of oxidative phosphorylation

High ATP:ADP ration inhibits oxidative phosphorylation and the citric acid cycle

Question 21

What are the inhibitors of oxidative phosphorylation?

Answer 21

1. Inhibition of the electron transfer chain
   Rotenone = insecticide
   Amytal
   Antimycin A = antibiotic
2. Inhibition of ATP synthase
   e.g. oligomycin, dicyclohexylcarbodiimide (DCC)
3. Inhibition of ATP export
   e.g. bongkrekic acid
4. Uncoupling electron transport from ATP synthesis
   e.g. 2,4-dinitrophenol

Causes increased permeability of the inner mitochondrial membrane to H⁺, therefore the proton motive force is dissipated to produce heat

Question 22

What is thermogenin?

Answer 22

• a.k.a. uncoupling protein 1 (UCP-1)
• Present in brown adipose tissue
• Transmembrane protein of innner mitochondrial membrane
• Allows transport of protons back into matrix without passage through ATP
• Heat generated by the dissipation of the proton gradient

Question 23

How does thermogenin work in the cold?

Answer 23

In response to cold, noradrenaline (norepinerphrine) activates:

• Lipases to release fatty acids from triacylglycerols
• Fatty acid oxidation generates NADH/FADH₂
• Electron transport chain stimulated
• Thermogenin transports H⁺ back into mitochondria
• This generates heat
• Esepcially useful in: newborn infants to maintain heat, especially around vital organs; hibernating animals to maintain body temperature

Question 24

What are the main differences between oxidative phosphorylation and substrate-level phosphorylation?

Answer 24