Link Reaction and Kreb's Cycle

Link Reaction and Kreb's Cycle

1. Significance of Krebs cycle

   • Breaks down macromolecules into smaller one; pyruvate is broken down into carbon dioxide
   • Produces hydrogen atoms that are carried by NAD to the electron transport chain for oxidative phosphorylation
     • Produces ATP that provides metabolic energy for the cell
   • Regenerates the 4-carbon molecule that combines with acetylcoenzyme A, which would otherwise accumulate
   • It is a source of intermediate compounds used by cells in the manufacture of other important substances such as fatty acids, amino acids and chlorophyll

2. Coenzymes

   • NAD
     • Hydrogen carrier - important for respiration
   • FAD
     • Important in Krebs cycle
   • NADP
     • Important in photosynthesis

3. Link Reaction

   • Pyruvate is oxidised by removing hydrogen
     • The hydrogen is accepted by NAD to form reduced NAD which is later used to produce ATP
   • The 2-carbon molecule (acetyl group) that is formed combines with coenzyme A (CoA) to produce acetylcoenzyme A
   • A carbon molecule is formed from each pyruvate
   • Pyruvate + NAD + CoA --> acetyl CoA + reduced NAD + CO2

4. Krebs Cycle

   • Occurs in the matrix of the mitochondria
   • 2-carbon acetylcoenzyme A from the link reaction combines with a 4-carbon molecule
     • Produces a 6-carbon molecule
   • The 6-carbon molecule loses carbon dioxide and hydrogens
     • Gives a 4-carbon molecule and a single molecule of ATP as a result of substrate-level phosphorylation
   • The 4-carbon molecule can now combine with a new molecule of coenzyme A
     • Begins the cycle again

5. For each molecule of pyruvate, the link reaction and Krebs cycle therefore produce:

   • 1 molecule of ATP
   • 3 molecules of carbon dioxide
   • reduced coenzymes such as NAD and FAD
     • These have the potential to produce ATP molecules and therefore the important products of Krebs cycle