5.2.4 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
• 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 by enzymes to produce acetate, CH₃CO(O)-  and carbon dioxide, requiring the reduction of NAD to NADH
  • Combination with coenzyme A to form acetyl coenzyme A (acetyl CoA)

• 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.

Role of coenzyme A

• A coenzyme is a molecule that helps an enzyme carry out its function but is not used in the reaction itself
• Coenzyme A consists of a nucleoside (ribose and adenine) and a vitamin
• In the link reaction, CoA binds to the remainder of the pyruvate molecule (acetyl group 2C) to form acetyl CoA
• It then supplies the acetyl group to the Krebs cycle where it is used to continue aerobic respiration
• This is the stage that brings part of the carbohydrate (or lipid/amino acid) into the further stages of respiration and links the initial stage of respiration in the cytoplasm to the later stages in the mitochondria