Biology – Respiration | e-Consult

The link reaction is the crucial step connecting glycolysis to the Krebs cycle (Citric Acid Cycle) in cellular respiration. It occurs in the mitochondrial matrix and bridges the gap between the breakdown of glucose and the complete oxidation of pyruvate.

Process: Each molecule of pyruvate (formed from glycolysis) is converted to acetyl-CoA. Acetyl-CoA then enters the Krebs cycle. The link reaction involves the reaction of pyruvate with Coenzyme A (CoA) to form acetyl-CoA. This reaction is catalyzed by the enzyme pyruvate dehydrogenase. This is a complex multi-enzyme complex involving several coenzymes.

Enzymes Involved: The primary enzyme is pyruvate dehydrogenase, a complex multi-enzyme. Other coenzymes, such as thiamine pyrophosphate (TPP), lipoamide, FAD, and NAD+, are essential for its activity.

Products Formed: The main products of the link reaction are:

• Acetyl-CoA: This is the key molecule that enters the Krebs cycle.
• Carbon Dioxide (CO2): Released during the conversion of pyruvate to acetyl-CoA.
• NADH: NAD+ is reduced to NADH, which carries high-energy electrons to the electron transport chain.

Role of Coenzyme A (CoA): CoA acts as a carrier molecule. It binds to pyruvate, forming acetyl-CoA. The acetyl group (a two-carbon unit) from pyruvate is covalently attached to CoA. This creates a high-energy thioester bond. This bond is crucial because it makes the acetyl group more reactive and facilitates its transfer to the Krebs cycle enzymes. Without CoA, the acetyl group would not be readily available for further oxidation.