12 Link reaction & Krebs cycle

• Pyruvate is produced during glycolysis in the cytoplasm
• Pyruvate is then actively transported into the matrix of the mitochondria

Main steps

• Pyruvate is oxidised into acetate
  • 3-carbon pyruvate loses a carbon dioxide molecule and two hydrogens
  • The hydrogens are accepted by NAD which forms reduced NAD which can later be used during oxidative phosphorylation
• The two-carbon acetate combines with coenzyme A (CoA) - this produces acetylcoenzyme A

Pyruvate + NAD + CoA --> acetyl CoA + reduced NAD + CO2

• Series of oxidation-reduction reactions
• 1) The 2-carbon acetylcoenzyme A combines with 4-carbon oxaloacetate making a 6-carbon molecule
• 2) In a series of reactions, the 6-carbon molecule loses 2 carbon dioxide molecules and some hydrogen to produce oxaloacetate and a single ATP molecule (substrate-level phosphorylation)

• Coenzymes
  • not actually enzymes
  • they are molecules that enzymes need to function
  • they carry hydrogen atoms from one molecule to another
  • NAD, FAD, NADP
• Significance
  • breaks down macromolecules - pyruvate into carbon dioxide
  • produces reduced coenzymes
  • regenerates oxaloacetate
  • it is a source of intermediate compounds such as fatty acids, amino acids and chlorophyll