Link Reaction & Krebs Cycle (Edexcel International A Level Biology): Revision Note

Link Reaction & Krebs Cycle

The Link reaction

• The end product of glycolysis is pyruvate

• Pyruvate contains a substantial amount of chemical energy that can be further utilised in respiration to produce more ATP

• The enzymes and coenzymes that are required for the link reaction are found in the mitochondrial matrix

• When oxygen is available pyruvate will enter the mitochondrial matrix and aerobic respiration will continue

• Pyruvate moves across the double membrane of the mitochondria via active transport

  • It requires a transport protein and a small amount of ATP

• Once in the mitochondrial matrix pyruvate takes part in the link reaction

Pyruvate enters the mitochondrial matrix from the cytosol (cytoplasm) by active transport

• The link reaction takes place in the matrix of the mitochondria

• It is referred to as the link reaction because it links glycolysis to the Krebs cycle

• The steps are:

  • Pyruvate is oxidised (hydrogen is removed) by enzymes to produce acetate, CH₃CO(O) (also known as acetic acid)

  • Pyruvate is also decarboxylated (carbon is removed) in the form of carbon dioxide

  • Reduction of NAD to NADH or reduce NAD by collecting hydrogen from pyruvate

  • Acetate combines with coenzyme A to form acetyl coenzyme A (acetyl CoA)

• No ATP is produced during the link reaction

• It produces:

  • Acetyl coA

  • Carbon dioxide (CO₂)

  • Reduced NAD (NADH)

pyruvate + NAD + CoA → acetyl CoA + carbon dioxide + reduced NAD

The link reaction occurs in the mitochondrial matrix. It dehydrogenates and decarboxylates the three-carbon pyruvate to produce the two-carbon acetyl CoA that can enter the Krebs Cycle

• Every molecule of glucose produces two pyruvate molecules

• The link reaction and the Krebs cycle will therefore occur twice for every molecule of glucose

• Thus, each molecule of glucose will produce:

  • Two molecules of acetyl CoA

  • Two molecules of CO₂

  • Two molecules of reduced NAD

The Krebs Cycle

• The Krebs cycle (sometimes called the citric acid cycle) consists of a series of enzyme-controlled reactions

• 2 carbon (2C) Acetyl CoA enters the circular pathway from the link reaction in glucose metabolism

  • Acetyl CoA formed from fatty acids (after the breakdown of lipids) and amino acids enters directly into the Krebs Cycle from other metabolic pathways

• 4 carbon (4C) oxaloacetate accepts the 2C acetyl fragment from acetyl CoA to form the 6 carbon (6C) citrate

  • Coenzyme A is released in this reaction to be reused in the next link reaction

• Citrate is then converted back to oxaloacetate through a series of oxidation-reduction (redox) reactions

The Krebs Cycle uses acetyl CoA from the link reaction and the regeneration of oxaloacetate to produce reduced NAD, reduced FAD and ATP

Regeneration of oxaloacetate

• Oxaloacetate is regenerated in the Krebs cycle through a series of redox reactions

• Decarboxylation of citrate

  • Releasing 2 CO₂ as waste gas

• Oxidation (dehydrogenation) of citrate

  • Releasing H atoms that reduce coenzymes NAD and FAD

  • These will be used during oxidative phosphorylation

  • 3 NAD and 1 FAD → 3NADH + H⁺ and 1 FADH₂

• Substrate linked phosphorylation

  • A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP to supply energy

• Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required per glucose molecule

• Therefore, at the end of two cycles, the products are:

  • Two ATP

  • Six NADH (reduced NAD)

  • Two FADH₂ (reduced FAD)

  • Four CO₂