describe the link reaction, including the role of coenzyme A in the transfer of acetyl (2C) groups

Respiration – Overview (Cambridge IGCSE/A‑Level 9700 Topic 12)

Cellular respiration converts the chemical energy of glucose into ATP through four linked stages. The location of each stage and its main products are shown below.

Stage	Location	Main products (per molecule of glucose)
Glycolysis	Cytoplasm	2 pyruvate, 2 ATP (substrate‑level), 2 NADH
Link reaction (pyruvate decarboxylation)	Mitochondrial matrix	2 acetyl‑CoA, 2 CO₂, 2 NADH
Citric‑acid (Krebs) cycle	Mitochondrial matrix	Per acetyl‑CoA: 2 CO₂, 3 NADH, 1 FADH₂, 1 GTP (≈ATP)
Oxidative phosphorylation	Inner mitochondrial membrane	ATP from NADH & FADH₂ via the electron‑transport chain & chemiosmosis

1. Glycolysis (cytoplasm)

• Glucose (6 C) is split into two molecules of pyruvate (3 C each).
• Energy‑yielding steps:
  • Substrate‑level phosphorylation produces a net 2 ATP (4 made, 2 used).
  • Two molecules of NADH are generated by reduction of NAD⁺.
• Overall glycolytic equation (per glucose): \[ \text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \;\longrightarrow\; 2\text{Pyruvate} + 2\text{NADH} + 2\text{H}^+ + 2\text{ATP} + 2\text{H}_2\text{O} \]

2. Link Reaction – Pyruvate Decarboxylation (mitochondrial matrix)

Location

The reaction takes place in the mitochondrial matrix after pyruvate has been transported across the inner membrane.

Overall chemical equation (per pyruvate)

\[ \text{Pyruvate} + \text{CoA‑SH} + \text{NAD}^+ \;\longrightarrow\; \text{Acetyl‑CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+ \]

Enzyme complex – Pyruvate Dehydrogenase Complex (PDC)

• Pyruvate dehydrogenase (E1) – decarboxylates pyruvate, forming a hydroxyethyl‑lipoamide attached to E2.
• Dihydrolipoamide transacetylase (E2) – transfers the acetyl group to Coenzyme A.
• Dihydrolipoamide dehydrogenase (E3) – re‑oxidises the lipoamide cofactor using NAD⁺, producing NADH + H⁺.

Step‑by‑step mechanism

1. Decarboxylation (E1): Pyruvate (3 C) loses one carbon as CO₂, generating a hydroxyethyl‑lipoamide intermediate bound to E2.
2. Acetyl transfer (E2): The acetyl (2 C) group is transferred from the hydroxyethyl‑lipoamide to the thiol group of CoA‑SH, forming acetyl‑CoA (a thioester).
3. Lipoamide regeneration (E3): The reduced lipoamide is oxidised by NAD⁺, yielding NADH + H⁺ and restoring the active lipoamide for another catalytic cycle.

Role of Coenzyme A (CoA‑SH)

• CoA contains a reactive –SH (thiol) group that forms a **thioester bond** with the acetyl group, producing acetyl‑CoA.
• The thioester bond is a high‑energy linkage; its hydrolysis releases enough free energy to drive the first step of the Krebs cycle.
• CoA‑SH is regenerated after each cycle, allowing it to repeatedly accept acetyl groups.

Quantitative yield from the link reaction (per glucose)

• 2 NADH (one from each pyruvate) → feed oxidative phosphorylation.
• 2 CO₂ are released as waste.
• 2 acetyl‑CoA are produced, providing the “fuel” for the Krebs cycle.

What you need to know for the exam – Link reaction

Summary table – Link reaction components

Component	Function in the link reaction	Main product(s)
Pyruvate dehydrogenase (E1)	Decarboxylates pyruvate → hydroxyethyl‑lipoamide	CO₂ (released)
Dihydrolipoamide transacetylase (E2)	Transfers acetyl group to CoA‑SH	Acetyl‑CoA
Dihydrolipoamide dehydrogenase (E3)	Re‑oxidises lipoamide using NAD⁺	NADH + H⁺
Coenzyme A (CoA‑SH)	Accepts the acetyl group, forming a high‑energy thioester	Acetyl‑CoA (enters the Krebs cycle)

3. Citric‑acid (Krebs) Cycle (mitochondrial matrix)

Overall picture

• Acetyl‑CoA (2 C) combines with oxaloacetate (4 C) to form citrate (6 C).
• Through a series of eight enzyme‑catalysed steps the six‑carbon skeleton is fully oxidised to two molecules of CO₂.
• During the cycle high‑energy carriers are produced that will later drive oxidative phosphorylation.

Key products per acetyl‑CoA

Product	Number per acetyl‑CoA
CO₂	2
NADH	3
FADH₂	1
GTP (or ATP)	1

Step‑by‑step outline (concise – exam level)

1. Citrate synthase: Acetyl‑CoA + oxaloacetate → citrate.
2. Aconitase: Citrate ⇌ isocitrate (via cis‑aconitate).
3. Isocitrate dehydrogenase: Isocitrate + NAD⁺ → α‑ketoglutarate + CO₂ + NADH.
4. α‑Ketoglutarate dehydrogenase complex: α‑ketoglutarate + NAD⁺ + CoA‑SH → succinyl‑CoA + CO₂ + NADH.
5. Succinyl‑CoA synthetase: Succinyl‑CoA + GDP + Pᵢ → succinate + GTP + CoA‑SH.
6. Succinate dehydrogenase: Succinate + FAD → fumarate + FADH₂.
7. Fumarase: Fumarate + H₂O → malate.
8. Malate dehydrogenase: Malate + NAD⁺ → oxaloacetate + NADH + H⁺ (regenerates the starter molecule).

Why the cycle matters

• Each turn releases two CO₂ molecules – the complete oxidation of the original glucose carbon atoms.
• The high‑energy carriers (3 NADH, 1 FADH₂, 1 GTP) feed oxidative phosphorylation, producing the bulk of cellular ATP.
• Regeneration of oxaloacetate ensures the cycle can continue as long as acetyl‑CoA is supplied.

4. Oxidative Phosphorylation (inner mitochondrial membrane)

• Electrons from NADH and FADH₂ travel through the electron‑transport chain (Complex I‑IV), releasing energy.
• The energy is used to pump protons from the matrix to the inter‑membrane space, creating an electro‑chemical gradient.
• ATP synthase (Complex V) allows protons to flow back into the matrix, driving synthesis of ATP from ADP + Pᵢ (chemiosmosis).
• Approximate ATP yield (per glucose):
  • From NADH (including the 2 from glycolysis, 2 from the link reaction, 6 from the Krebs cycle): ~2.5 ATP each.
  • From FADH₂ (2 from the Krebs cycle): ~1.5 ATP each.
  • Total ≈ 30‑32 ATP.

Quick‑Recall Box – Whole‑cell Respiration (exam checklist)