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Krebs' Cycle Summary

Intermediate Step

This reaction doesn't belong to either glycolysis or Krebs' cycle; it is an 'intermediate' step. Glycolysis ends with the production of two three-carbon pyruvate molecules. These cannot enter Krebs' cycle so they are decarboxylated to two-carbon acetyl-CoA molecules. Two molecules of NADH are also produced. Each acetyl-CoA molecule can now enter the Krebs' cycle.

Krebs' Cycle

This is a series of eight reactions with the last product, oxaloacetate, being the same as the first reactant, thus the term 'cycle'.

1. This step combines a two-carbon with a four-carbon molecule to form the six-carbon citrate molecule and releasing the coenzyme.
2. Isomerization rearranges the atoms within the molecule changing it's structure.
3. Decarboxylation releases carbon dioxide. The reactant is also oxidized and NADH is produced.
4. Decarboxylation releases a second carbon dioxide. The reactant is also oxidized again and NADH is produced. Coenzyme A is attached to the substrate.
5. A phosphate group replaces the coenzyme then it leaves the substrate becoming attached to a nucleotide diphosphate, either ADP or GDP. The triphosphate form is produced.
6. The substrate is oxidized producing FADH₂.
7. The substrate is hydrated.
8. The substrate is oxidized producing NADH and regenerating the oxaloacetate molecule.

Summary of the Summary

In essence, it's as if each two-carbon acetyl-CoA entering the cycle from the intermediate step leaves the cycle as two (2) carbon dioxide molecules. The initial uptake substrate, oxaloacetate, is regenerated. Three (3) molecules of NADH are produced. One (1) molecule of FADH₂ is produced. One (1) high-energy nucleotide molecule (NTP) is produced by substrate-level phosphorylation. Coenzyme A plays several roles in these reactions.

Because the original glucose molecule produces two molecules for the intermediate step the above counts must be doubled. Combining those tallies with the ones from glycolysis we get the following products:

• CO₂ -- none from glycolysis, two from the intermediate step and four from the cycle totals 6.
• ATP -- two from glycolysis, none from the intermediate step and two (NTP) -- count as ATP -- from the cycle totals 4.
• NADH -- two from glycolysis, two from the intermediate step and six from the cycle totals 10.
• FADH₂ -- two from the cycle totals 2.

Continue to the Electron Transport System.