Link Reaction in Cellular Respiration

Overview

• Previous topic discussed: Glycolysis
  • Glycolysis results:
    • Two pyruvate molecules
    • Two reduced NAD
    • Four ATP molecules
  • Process not complete; next step: Link Reaction.

Link Reaction Location

• Occurs in the mitochondrial matrix.
• Pyruvate molecules are actively transported into the mitochondrial matrix.

Steps of the Link Reaction

• The link reaction consists of three main steps:
  1. Decarboxylation
  2. Oxidation (Dehydrogenation)
  3. Addition of Coenzyme A

Detailed Steps

1. Decarboxylation

   • Pyruvate (3-carbon molecule) undergoes decarboxylation.
   • Results in removal of one carbon as carbon dioxide, forming a 2-carbon molecule.

2. Oxidation (Dehydrogenation)

   • The 2-carbon molecule undergoes oxidation by removal of hydrogen.
   • Involves NAD as a hydrogen carrier.
   • NAD receives hydrogen to become reduced NAD.
     • Note: In A-level, terms like NADH, NADH+H, or reduced NAD are interchangeable.

3. Addition of Coenzyme A

   • The remaining 2-carbon molecule is an acetyl group.
   • Acetyl group needs to be transported to the Krebs cycle.
   • Coenzyme A attaches to the acetyl group forming acetyl-CoA.
   • Acetyl-CoA enters the Krebs cycle.

Additional Information

• Coenzyme A is not an enzyme but a molecule necessary for enzyme function.
• Coenzyme A originates from dietary vitamin B5.

Overall Summary

• Each pyruvate molecule undergoes:
  • Decarboxylation to produce CO₂.
  • Oxidation to produce reduced NAD.
  • Conversion into acetyl-CoA with Coenzyme A.
• For two pyruvate molecules:
  • Produces two acetyl-CoA, two CO₂, and two reduced NAD.

Important Terms

• Pyruvate: End product of glycolysis.
• Acetyl group: 2-carbon molecule post-decarboxylation.
• Acetyl-CoA: Product that enters the Krebs cycle after linking with Coenzyme A.