The Link Reaction and Krebs Cycle

After glycolysis, if oxygen is available, the two molecules of pyruvate produced per glucose enter the mitochondria to undergo further oxidation. The link reaction connects glycolysis to the Krebs cycle, while the Krebs cycle completes the oxidation of the carbon skeleton derived from glucose. Both stages occur in the mitochondrial matrix.

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The Link Reaction

Location

• The mitochondrial matrix.

Process

Pyruvate (3C) is transported from the cytoplasm into the mitochondrial matrix by a specific carrier protein in the inner mitochondrial membrane. Once inside, it undergoes oxidative decarboxylation:

1. Decarboxylation — One carbon atom is removed from pyruvate as carbon dioxide (CO₂). This reduces the molecule from a 3C compound to a 2C acetyl group.

2. Oxidation — Hydrogen atoms are removed from pyruvate and transferred to NAD⁺, reducing it to NADH + H⁺ (reduced NAD). This is a dehydrogenation reaction.

3. Combination with Coenzyme A — The 2C acetyl group combines with coenzyme A (CoA) to form acetyl coenzyme A (acetyl CoA). Coenzyme A is a complex molecule containing a vitamin (pantothenic acid / vitamin B5) and acts as a carrier, transferring the acetyl group to the Krebs cycle.

Summary Equation for the Link Reaction

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

Since glycolysis produces two molecules of pyruvate per glucose, the link reaction occurs twice per glucose molecule:

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.

Exam Tip: No ATP is produced directly in the link reaction. Its main role is to feed acetyl groups into the Krebs cycle and produce reduced NAD for oxidative phosphorylation.

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The Krebs Cycle (Citric Acid Cycle / Tricarboxylic Acid Cycle)

Location

• The mitochondrial matrix, where all necessary enzymes are dissolved in the matrix fluid.

Overview

The Krebs cycle is a cyclical series of enzyme-controlled oxidation-reduction reactions. The 2C acetyl group from acetyl CoA is completely oxidised to CO₂, generating reduced coenzymes (NADH and FADH₂) and a small amount of ATP by substrate-level phosphorylation.

Steps of the Krebs Cycle

1. Acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C) — The 2C acetyl group combines with the 4C compound oxaloacetate to form the 6C compound citrate (citric acid). Coenzyme A is released and recycled to the link reaction. This reaction is catalysed by citrate synthase.

2. Citrate (6C) → 5C compound — Citrate is converted to a 5C compound through a process of decarboxylation (one CO₂ released) and dehydrogenation (hydrogen transferred to NAD⁺, forming NADH + H⁺). The 5C intermediate is alpha-ketoglutarate.

3. 5C compound → 4C compound — The 5C compound undergoes a second decarboxylation (another CO₂ released) and dehydrogenation (another NAD⁺ reduced to NADH + H⁺), forming a 4C compound (succinyl CoA, which is then converted to succinate).

4. Substrate-level phosphorylation — During the conversion of succinyl CoA to succinate, enough energy is released to synthesise one molecule of ATP (or GTP, which is readily converted to ATP) from ADP and Pi.