Link reaction and Krebs cycle

Question 1

What is the link reaction? What does it produce? What is the key enzyme involved?

Answer 1

🔗 Link Reaction (Pyruvate → Acetyl-CoA)

🧬 Overview:

Occurs in mitochondrial matrix
Irreversible reaction (key regulation point)
Links glycolysis to the citric acid cycle
Enzyme: Pyruvate Dehydrogenase Complex (PDC) — a large multi-enzyme complex

Question 2

What is the structure and function of pyruvate dehydrogenase complex? (PDC)

Answer 2

E1 – Pyruvate dehydrogenase:
TPP (Thiamine pyrophosphate) is used
Pyruvate loses CO₂ (decarboxylation)
The remaining 2-carbon unit (hydroxyethyl) is attached to TPP and oxidised to an acyl group
E2 – Dihydrolipoyl transacetylase:
Transfers the acyl group to lipoamide
Then to Coenzyme A → Acetyl CoA (fuel for TCA)
Forms an energy-rich thioester bond
E3 – Dihydrolipoyl dehydrogenase:
Reoxidises lipoamide using FAD → FADH₂ → NAD⁺ → NADH

Question 3

What is catalytic and what is stoichiometric in the link reaction?

Answer 3

Catalytic (recycled): TPP, lipoic acid, FAD
Stoichiometric (used up): CoA, NAD⁺

Question 4

How is the link reaction regulated?

Answer 4

Activated by: ↑ ADP, ↑ Pyruvate, ↑ Ca²⁺ (e.g., during exercise)
Inhibited by: ↑ ATP, ↑ Acetyl-CoA, ↑ NADH
Enzymes involved:
PDK (kinase) – inactivates E1 (favoured by high energy)
PDP (phosphatase) – activates E1 (favoured by low energy)

Question 5

What is the citric acid cycle? What is it the central pathway of? What is the entry point?

Answer 5

🔁 Citric Acid Cycle (TCA/Krebs Cycle)

Central pathway for oxidation of:

Carbs (glucose → pyruvate)
Fats (β-oxidation → acetyl-CoA)
Proteins (aa breakdown → TCA intermediates)
🔑 Entry Point:

Acetyl-CoA (2C) combines with oxaloacetate (4C) → Citrate (6C)

Question 6

What is the first step of TCA? (citrate synthase)

Answer 6

Citrate Synthase
Combines acetyl-CoA + oxaloacetate → citrate
Aldol condensation + hydrolysis
Entry point of acetyl units

Question 7

What is the second step of TCA? (aconitase)

Answer 7

2 . Aconitase
Isomerization of citrate → isocitrate via aconitate intermediate
Prepares molecule for next oxidation step

Question 8

What is the third step of TCA? (isocitrate dehydrogenase)

Answer 8

Oxidation + decarboxylation
Produces NADH + CO₂
Regulatory point
↑ ADP activates (allosteric)
↓ ATP inhibits
↓ NADH inhibits (product feedback)
Excess citrate may inhibit PFK in glycolysis (a feedback loop)

Question 9

What is the 4th step of the TCA? (α-Ketoglutarate Dehydrogenase)

Answer 9

α-Ketoglutarate Dehydrogenase
Like pyruvate dehydrogenase (multienzyme complex)
Produces NADH + CO₂ + succinyl-CoA
Regulatory point:
Inhibited by: ↑ NADH, ↑ Succinyl-CoA
Activated when ATP is low

Question 10

What is the 5th step of the TCA? (Succinyl-CoA Synthetase)

Answer 10

Converts succinyl-CoA → succinate
Coupled to GTP (substrate-level phosphorylation)
GTP ⇄ ATP via nucleoside diphosphate kinase

Question 11

What is the 6th step of the TCA? (Succinate dehydrogenase)

Answer 11

Succinate Dehydrogenase
Oxidation: succinate → fumarate
FAD is the electron acceptor (→ FADH₂)
Part of the ETC (Complex II)
Electrons passed directly to coenzyme Q

Question 12

What is the 7th step of the TCA? (fumarase)

Answer 12

Fumarase
Adds water: fumarate → malate
Breaks the double bond

Question 13

What is the energy yield from one acetyl-CoA?

Answer 13

3 NADH → 7.5 ATP
1 FADH₂ → 1.5 ATP
1 GTP → 1 ATP
= ~10 ATP per cycle

Question 14

What is the 8th step of the TCA? (malate dehydrogenase)

Answer 14

Malate Dehydrogenase
Oxidation: malate → oxaloacetate
Produces NADH
ΔG⁰′ is positive, but driven forward because oxaloacetate is constantly used

Question 15

What are the repeating themes in the TCA?

Answer 15

Oxidation → high-energy electrons (NADH/FADH₂) → Electron Transport Chain → ATP
Many TCA intermediates are also used in biosynthesis (amino acids, lipids, glucose)
Regulation is tightly linked to cellular energy status