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

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