• 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 (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