MCBG Session 6 - Catabolic Pathways (Part 3)

Question 1

What is the role of pyruvate dehydrogenase in glucose metabolism?

Answer 1

Pyruvate dehydrogenase converts pyruvate into AcetylCoA ready for the TCA (Krebs) cycle

Question 2

What regulates pyruvate dehydrogenase (PDH)?

Answer 2

Stimulated by:

• Pyruvate
• Low energy signals (NAD, ADP)
• Insulin (via dephosphorylation)

Inhibited by:

• Acetyl CoA
• NADH
• ATP
• Phosphorylation

Question 3

What occurs if someone has PDH deficiency?

Answer 3

Lactic acidosis - as pryuvate builds up but doesn’t go through TCA cycle, instead is diverted and acted on by LDH to form lactate.

Question 4

• What happens in the TCA cycle?

- What is produced?

Answer 4

• Acetyl converted to 2xCO2
• Oxidative reactions, so requires NAD+/FAD
• Some energy produced
• Precursors for biosynthesis produced

Question 5

What are the key principles of the TCA cycle?

Answer 5

• 2C molecule introduced into cycle and broken down to release CO2
• Reactions oxidative so NAD/FAD produced
• Small amount of energy released in the form of GTP

Question 6

How is the TCA cycle regulated?

Answer 6

Via high + low energy signal molecules:

• ADP (low energy signal) stimulates
• ATP (high energy signal) inhibits

Question 7

Where does the TCA cycle occur?

Answer 7

In the mitochondria.

Question 8

What does the TCA cycle produce precursors for?

Answer 8

For biosynthesis of other molecules, e.g.: citrate can feed out of the cycle for biosynthesis of fatty acids, oxaloacetate for amino acids and glucose etc etc

Question 9

What are the 2 processes of the final (stage 4) of catabolism?

Answer 9

1) Electron transport - electrons on NADH/FADH2 transported down carrier molecules to oxygen, releasing energy
2) Free energy released used to drive ATP synthesis

Question 10

What is the energy from movement of electrons down transport chain used for?

Answer 10

The movement of protons, driving them from the matrix to intermembrane space. This creates a proton motive force (pmf) back towards the mitochondrial matrix.

Question 11

Which enzyme (along with the proton motive force) drives the synthesis of ATP during this stage?

Answer 11

ATP synthase - protons return back across the membrane via ATP synthase which drives ATP synthesis.

Question 12

Oxidation of 2 moles NADH produces 5 moles ATP, but 2 moles FADH2 produces 3 moles ATP - why is this?

Answer 12

Electrons in NADH have more energy, and NADH uses 3 proteins translocating complexes (PTC’s) compared to FADH’s 2. The greater the pmf, the more ATP synthesised.

Question 13

How is oxidative phosphorylation regulated?

Answer 13

High ATP = inhibits

High ADP = stimulates