L3 - TCA Cycle

Question 1

Key Points of TCA Cycle:

• Reaction i.e. substrate to product
• Location
• Tissues
• Function

Answer 1

Reaction: Oxidation of Acetyl CoA to CO2 and water

Location: Inside mitochondria (mitochondrial matrix)

Tissues: All tissues with mitochondria (not RBC or white muscle fibres)

Functions:

• Energy trapping i.e. ATP synthesis
• Biosynthesis of intermediates

Question 2

Conversion of pyruvate to acetyl CoA (Link’s Reaction):

Answer 2

Pyruvate + NAD+ —> Acetyl CoA + NADH + H+ + CO2
PYRUVATE DEHYDROGENASE

CoA forms thioester linkage with acetate - condensation reaction.

(NAD+ converted to NADH + H+)
Occurs in mitochondria
Oxidative decarboxylation (remove CO2 and oxidise pyruvate and reduce NAD)

Question 3

Stages of TCA Cycle:

Answer 3

1) Oxaloacetate + Acetyl CoA→ Citrate
CITRATE SYNTHETASE (Condensation reaction)

2) Citrate ⇔ Isocitrate
ACONITASE (Isomerisation)

3) Isocitrate + NAD+ → a-ketoglutarate + NADH + H+ + CO2
ISOCITRATE DEHYDROGENASE

4) a-ketoglutarate + CoA-SH + NAD+ → succinyl-CoA + CO2+ NADH + H+
a-KETOGLUTARATE DEHYDROGENASE

5) Succinyl-CoA + GDP + Pi ⇔ Succinate + CoA + GTP
SUCCINATE THIOLASE

6) Succinate + FAD ⇔ Fumarate + FADH2
SUCCINATE DEHYDROGENASE

7) Fumarate + H2O ⇔ Malate
FUMARASE

8) Malate + NAD+ ⇔ Oxaloacetate + NADH + H+
MALATE DEHYDROGENASE

Question 4

Role of FAD as H acceptor:

Answer 4

FAD can accept H+ and become reduced to form FADH2. This is also reoxidised in ETC with NADH.

Question 5

What is the Electron Transport Chain/ Cytochrome Chain:

Answer 5

This is where reoxidation of NADH and FADH2 occurs - it involves transfer of H+ into inner mitochondrial space generating a proton gradient and transferring electrons along a series of complexes. This allows generation of ATP Using proton gradient and H2O production (H+ and e- and O2).

Question 6

Role of NADH and FADH2 in ETC:

Answer 6

Each NADH molecule produces 2.5 molecules of ATP from 2.5 ADP and 2.5 Pi.
Each FADH2 molecule produces 1.5 molecules of ATP from 1.5 ADP and 1.5 Pi.

FADH2 produces less energy than NADH.

NADH releases e- which goes to complex I then III then IV. This also pumps H+ into IMS to generate proton gradient.
FADH2 donates e- to complex II then III then IV.

Question 7

Energy yields of TCA cycle:

Answer 7

• 3 enzyme catalysed reactions which produce NADH and H+
• 1 enzyme catalysed reaction produces FADH2
• 1 enzyme reaction produces GTP

In total, from TCA cycle alone, (3x2.5) + (1x1.5) + 1 = 10 molecules of ATP is produced.

Question 8

What are the irreversible stages (highly exergonic) of TCA cycle:

Answer 8

• Citrate synthetase
• Isocitrate dehydrogenase
• ketoglutarate dehydrogenase

Question 9

How is the rate of TCA cycle regulated by feedback inhibition on enzymes:

Answer 9

1) Citrate synthetase - inhibited by NADH and Succinyl CoA
2) Isocitrate dehydrogenase - inhibited by NADH and stimulated by ADP
3) Ketoglutarate dehydrogenase - inhibited by NADH and succinyl CoA

Question 10

Biosynthetic Role of TCA cycle:

TCA intermediates can be produced by anaplerosis where other reactions can produce these - constant supply of TCA intermediates.

Answer 10

1. Malate ⇔ Pyruvate via MALIC ENZYME
2. Citrate ⇔ fatty acids and sterols.
3. α-Ketoglutarate ⇔ Glutamate via transamination. Glutamate → other A.A and purines
4. Oxaloacetate ⇔ Aspartate. Via transamination.
   Aspartate → other A.A, purines and pyrimidines.
5. Pyruvate ⇔ Oxaloacetate via PYRUVATE CARBOXYLASE.

6) Oxaloacetate ⇔ phosphoenolpyruvate via PEP CARBOXYLASE.
PEP → Glucose. Via gluconeogenesis.