The Link Reaction and Krebs Cycle

This lesson is mapped to AQA 7402 Section 3.5.1 — Aerobic respiration: link reaction and Krebs cycle (refer to the official AQA specification document for exact wording). After glycolysis, if oxygen is available, the two molecules of pyruvate produced per glucose are imported into the mitochondrial matrix and oxidised to completion. The link reaction (also called the pyruvate dehydrogenase reaction) bridges glycolysis to the Krebs cycle by converting pyruvate into the activated 2-carbon acetyl group, while the Krebs cycle progressively oxidises that acetyl group to CO₂, generating the bulk of the reduced coenzymes (NADH and FADH₂) that will subsequently donate electrons to the inner-membrane electron transport chain.

Both stages were elucidated in the 1930s by Hans Krebs and his colleagues, working initially in Sheffield, England. Krebs's contribution — for which he shared the 1953 Nobel Prize in Physiology or Medicine with Fritz Lipmann (the discoverer of coenzyme A) — was to recognise that the previously disconnected oxidation reactions of carbohydrate, fat, and amino acid catabolism converged on a single cyclical pathway. Lipmann's complementary insight, paraphrased, was that the key activated intermediate at the entry point is not free acetate but acetate covalently linked to a thiol carrier (CoA), which makes the carbon energetically reactive. The cycle is sometimes called the citric acid cycle (after its first stable intermediate) or the tricarboxylic acid (TCA) cycle (because citrate carries three carboxylic acid groups).

Key Definition: The link reaction is the oxidative decarboxylation of pyruvate to acetyl-CoA in the mitochondrial matrix, releasing CO₂ and reducing NAD⁺ to NADH + H⁺. The Krebs cycle is a cyclical eight-step pathway that oxidises the acetyl group of acetyl-CoA to two CO₂, generating three NADH, one FADH₂, and one ATP (or GTP) per turn.

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Mitochondrial Architecture: Where the Reactions Happen

Both the link reaction and the Krebs cycle occur in the matrix (the fluid-filled compartment enclosed by the inner mitochondrial membrane). The matrix contains:

• Soluble enzymes of the Krebs cycle and the pyruvate dehydrogenase complex (link reaction).
• A small population of circular DNA molecules (mitochondrial DNA, mtDNA) plus 70S ribosomes — evidence for the endosymbiotic origin of mitochondria (covered in detail in lesson 9).
• Substrates, intermediates, and the soluble pool of NAD⁺ / NADH and CoA / acetyl-CoA.

The inner mitochondrial membrane is highly folded into cristae and contains the electron transport chain (covered in lesson 3); the link reaction and Krebs cycle deliver their reduced coenzymes directly to this membrane.

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The Link Reaction

Location

The mitochondrial matrix.

Process

Pyruvate (3C) is transported from the cytoplasm across the outer mitochondrial membrane (via porins) and through the inner mitochondrial membrane via the mitochondrial pyruvate carrier (MPC), a specific symporter that co-transports a proton. Once inside the matrix, pyruvate is acted on by the pyruvate dehydrogenase complex (PDH) — a vast multi-enzyme assembly of three sequential enzymes plus five coenzymes. PDH catalyses oxidative decarboxylation:

1. Decarboxylation — One carbon atom is removed from pyruvate as carbon dioxide (CO₂), reducing the 3C molecule to a 2C acetyl group.
2. Oxidation (dehydrogenation) — Two hydrogen atoms (2H = 2e⁻ + 2H⁺) are removed from the residual fragment. Two electrons and one proton are transferred to NAD⁺, reducing it to NADH + H⁺; the second proton enters solution. This is a true oxidation: the substrate has lost both hydrogens and a carbon (as CO₂).
3. Combination with Coenzyme A — The 2C acetyl group is transferred to coenzyme A (CoA-SH) via a thioester bond, forming acetyl-CoA. Coenzyme A is derived from pantothenic acid (vitamin B5) and acts as a high-energy activated carrier: the thioester bond stores significant chemical energy that drives the subsequent reaction with oxaloacetate.

Summary Equation for the Link Reaction

Pyruvate (3C) + NAD⁺ + CoA-SH → Acetyl-CoA (2C) + CO₂ + NADH + H⁺

Since glycolysis produces two pyruvates per glucose, the link reaction occurs twice per glucose:

Product per glucose	Quantity
Acetyl-CoA (2C)	2 molecules
CO₂	2 molecules
Reduced NAD (NADH + H⁺)	2 molecules
ATP	0

Key Definition: Oxidative decarboxylation is the simultaneous removal of CO₂ (decarboxylation) and hydrogen atoms (oxidation / dehydrogenation) from a substrate, with the hydrogens being transferred to a coenzyme such as NAD⁺.

Regulation of the PDH Complex

PDH is allosterically inhibited by its products (acetyl-CoA, NADH) and by ATP, and activated by ADP, Pi, NAD⁺, and CoA. It is also covalently regulated by phosphorylation: PDH kinase inactivates the complex by phosphorylating it, while PDH phosphatase activates it by removing the phosphate. This kinase/phosphatase pair allows hormonal regulation of pyruvate entry into the mitochondrion — insulin promotes PDH activity (encouraging glucose oxidation when fed), while elevated fatty acid oxidation inhibits PDH (sparing glucose during fasting).

Exam Tip: No ATP is produced directly in the link reaction. Its purpose is to (i) deliver activated 2-carbon units to the Krebs cycle, (ii) release one CO₂ per pyruvate, and (iii) produce one NADH per pyruvate for oxidative phosphorylation.

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The Krebs Cycle (Citric Acid Cycle / TCA Cycle)

Location

The mitochondrial matrix, where all eight cycle enzymes are dissolved in the matrix fluid. (Succinate dehydrogenase, the enzyme catalysing step 5, is exceptional in being embedded in the inner mitochondrial membrane; it doubles as Complex II of the electron transport chain.)

Overview

The cycle is a closed loop of enzyme-controlled oxidation-reduction reactions. The 2C acetyl group from acetyl-CoA is completely oxidised to two CO₂, generating reduced coenzymes (NADH and FADH₂), and a small amount of ATP (or GTP) by substrate-level phosphorylation. The 4C carrier molecule oxaloacetate is regenerated at the end of each cycle, ready to accept the next acetyl group.

Steps of the Krebs Cycle (per turn, one acetyl-CoA in)

1. Condensation: Acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C). Catalysed by citrate synthase. Coenzyme A is released as free CoA-SH and recycles to the link reaction.
2. Isomerisation: Citrate is isomerised to isocitrate by aconitase (a positional shift of a hydroxyl group).
3. First oxidative decarboxylation: Isocitrate (6C) → α-ketoglutarate (5C) + CO₂ + NADH + H⁺. Catalysed by isocitrate dehydrogenase. One CO₂ released; one NAD⁺ reduced.
4. Second oxidative decarboxylation: α-ketoglutarate (5C) → succinyl-CoA (4C) + CO₂ + NADH + H⁺. Catalysed by α-ketoglutarate dehydrogenase (mechanistically closely related to the PDH complex). One CO₂ released; one NAD⁺ reduced.
5. Substrate-level phosphorylation: Succinyl-CoA (4C) → succinate (4C) + CoA-SH + ATP (or GTP). Catalysed by succinyl-CoA synthetase. The thioester bond of succinyl-CoA is hydrolysed, and its energy is used to phosphorylate GDP (in many tissues) or ADP (in plant and bacterial cells) to GTP/ATP.
6. Oxidation by FAD: Succinate (4C) → fumarate (4C) + FADH₂. Catalysed by succinate dehydrogenase (Complex II of the ETC, embedded in the inner mitochondrial membrane). FAD is reduced to FADH₂.
7. Hydration: Fumarate (4C) + H₂O → malate (4C). Catalysed by fumarase.
8. Final oxidation: Malate (4C) → oxaloacetate (4C) + NADH + H⁺. Catalysed by malate dehydrogenase. Oxaloacetate is regenerated, ready to accept the next acetyl-CoA.

graph TD
    A["Pyruvate (3C)"] -->|"Link reaction<br/>NAD⁺ → NADH, CO₂"| B["Acetyl-CoA (2C)"]
    B -->|"+ Oxaloacetate (4C)<br/>citrate synthase"| C["Citrate (6C)"]
    C -->|"isomerisation"| D["Isocitrate (6C)"]
    D -->|"NAD⁺ → NADH<br/>CO₂"| E["α-Ketoglutarate (5C)"]
    E -->|"NAD⁺ → NADH<br/>CO₂"| F["Succinyl-CoA (4C)"]
    F -->|"ADP/GDP → ATP/GTP<br/>(SLP)"| G["Succinate (4C)"]
    G -->|"FAD → FADH₂"| H["Fumarate (4C)"]
    H -->|"+ H₂O"| I["Malate (4C)"]
    I -->|"NAD⁺ → NADH"| J["Oxaloacetate (4C)"]
    J --> B

    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style F fill:#e67e22,color:#fff
    style J fill:#9b59b6,color:#fff

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Products of One Turn of the Krebs Cycle

Product	Quantity per turn
CO₂	2 molecules
Reduced NAD (NADH + H⁺)	3 molecules
Reduced FAD (FADH₂)	1 molecule
ATP (or GTP — by SLP)	1 molecule

Since two acetyl-CoA molecules enter per glucose (from two pyruvates), the Krebs cycle turns twice per glucose:

Product per glucose	Quantity
CO₂	4 molecules
Reduced NAD (NADH + H⁺)	6 molecules
Reduced FAD (FADH₂)	2 molecules
ATP	2 molecules

Key Point: The Krebs cycle itself produces only a small amount of ATP directly (by SLP). Its principal purpose is to generate reduced coenzymes (NADH and FADH₂) that donate electrons to the inner-membrane electron transport chain for oxidative phosphorylation — which is where the bulk of ATP is actually made.

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The Role of Coenzymes in the Cycle

NAD⁺ (Nicotinamide Adenine Dinucleotide)

• Acts as a hydrogen acceptor (oxidising agent).
• NAD⁺ + 2H → NADH + H⁺ (two electrons + one proton transferred to NAD⁺, second proton released).
• Reduced once in the link reaction; reduced three times per turn of the Krebs cycle (steps 3, 4, and 8).
• NADH carries its electrons to Complex I of the electron transport chain.

FAD (Flavin Adenine Dinucleotide)

• Also a hydrogen acceptor, but with a slightly lower reduction potential than NAD⁺.
• FAD + 2H → FADH₂ (both electrons and both protons retained on the coenzyme).
• Reduced once per turn of the Krebs cycle (step 6 — succinate to fumarate by succinate dehydrogenase).
• FAD is tightly (in fact covalently) bound to its enzyme; this enzyme (succinate dehydrogenase) is anchored in the inner mitochondrial membrane.
• FADH₂ feeds electrons into the ETC at Complex II — bypassing Complex I — and therefore yields fewer pumped protons and fewer ATP per molecule than NADH (~1.5 vs ~2.5).

Coenzyme A (CoA)

• Carries acetyl groups from the link reaction to the Krebs cycle (and from β-oxidation of fatty acids to the Krebs cycle).
• The thioester bond between the acetyl group and the sulphur of CoA is high-energy: its hydrolysis provides the energy that drives condensation with oxaloacetate.
• CoA-SH is released at step 1 of the cycle and recycles to the link reaction.
• Contains the vitamin pantothenic acid (vitamin B5).

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Carbon Dioxide Release: Accounting for All Six Carbons of Glucose

Across the link reaction and the Krebs cycle, six CO₂ molecules are released per glucose:

• 2 CO₂ from the link reaction (one per pyruvate; two pyruvates per glucose).
• 4 CO₂ from the Krebs cycle (two CO₂ per turn; two turns per glucose).

This accounts for all six carbon atoms originally present in glucose. By the time the Krebs cycle has turned twice, the glucose skeleton has been completely dismantled into CO₂; what remains is the captured chemical energy, now stored in 10 NADH, 2 FADH₂, and 4 ATP (counting glycolysis, link reaction, and Krebs cycle together).

Exam Tip: When asked "where is CO₂ produced in aerobic respiration?", the answer is the link reaction and the Krebs cycle, both in the mitochondrial matrix. CO₂ is NOT produced during glycolysis or oxidative phosphorylation. A common error is to write "oxidative phosphorylation produces CO₂" — it does not; it produces water.

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Significance of the Krebs Cycle

1. Reduced coenzymes: Produces 6 NADH and 2 FADH₂ per glucose for oxidative phosphorylation — the bulk of the cell's ATP yield depends on this output.
2. CO₂ waste: Releases CO₂, transported in blood as bicarbonate (HCO₃⁻) and exhaled via the lungs in mammals.
3. Modest SLP yield: Produces 2 ATP per glucose by substrate-level phosphorylation.
4. Biosynthetic hub: Krebs cycle intermediates serve as starting materials for anabolic pathways. α-Ketoglutarate and oxaloacetate are transaminated to glutamate and aspartate respectively, feeding into amino acid synthesis. Citrate is exported to the cytoplasm for fatty acid synthesis. Succinyl-CoA is a precursor of haem biosynthesis. The cycle is therefore amphibolic — it functions catabolically and anabolically simultaneously.
5. Universality: The cycle operates in all aerobic prokaryotes and eukaryotes, indicating an evolutionary origin predating the divergence of these domains.

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Synoptic Links

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