IB Biology Topic C1.2.11: Oxidation and Decarboxylation of Pyruvate Notes

The Link Reaction Connects Glycolysis to the Krebs Cycle

1. Glycolysis in the cytoplasm breaks glucose down into pyruvate (3C).
2. The Krebs cycle in the mitochondrial matrix needs acetyl groups (2C).
3. There is a carbon mismatch, because pyruvate has 3 carbons but the Krebs cycle uses 2-carbon acetyl groups.
4. The link reaction bridges this gap in the mitochondrial matrix by converting pyruvate (3C) into a 2-carbon acetyl group.
5. The acetyl group then joins coenzyme A to form acetyl-CoA.

How the Link Reaction Works Step by Step

1. Step 1: Decarboxylation (removing carbon)
   1. One carbon atom is removed from pyruvate (3C) as carbon dioxide (CO₂).
   2. This leaves a 2-carbon molecule.
2. Step 2: Oxidation (removing hydrogen)
   1. Hydrogen is removed from the 2-carbon molecule.
   2. This hydrogen is accepted by NAD to form reduced NAD (NADH).
3. Step 3: Acetyl group joins coenzyme A
   1. The remaining 2-carbon acetyl group joins coenzyme A (CoA) to form acetyl-CoA.
   2. Coenzyme A carries the acetyl group into the Krebs cycle.

Lipids Also Feed Acetyl Groups Into the Cycle

1. Carbohydrates reach acetyl-CoA through glycolysis, then pyruvate, then the link reaction.
2. Fatty acids form acetyl-CoA directly through beta-oxidation, bypassing glycolysis and the link reaction.
3. Both pathways converge at acetyl-CoA, which enters the Krebs cycle.