The Krebs Cycle
- Discovered in 1937 by Sir Hans Krebs.
- In 1953, Krebs and Fritz Albert Lipmann (who discovered Coenzyme A) shared the Nobel Prize for their discoveries.
- The Krebs cycle is a cyclic series of reactions that transfers energy from organic molecules to ATP, NADH, FADH2 and removes carbon atoms as CO2.
- Other names: Citric Acid Cycle, so called because Acetyl-CoA reacts with OAA to produce citrate (hence citric acid cycle). Tricarboxylic Acid Cycle: Citrate has three carboxyl groups.
- Two molecules of acetyl-CoA form for every molecule of glucose. This means the Krebs Cycle occurs twice for each molecule of glucose.
- CoA is eventually recycled.
- Energy is harvested in steps 3, 4, 5, 6, 8.
- NAD+ is reduced to NADH in steps 3, 4, 8.
- Step 5 produces ATP by substrate-level phosphorylation. A phosphate group from the matrix displaces CoA from succinyl-CoA. The phosphate group is transferred to GDP to form GTP. Next, the phosphate group condenses with ADP to form ATP. Free energy is transferred from succinyl-CoA to ATP.
- In step 6 energy is harvested. FAD is reduced to FADH2. The reaction in this step is not exergonic enough to reduce NAD+ to NADH, instead FAD is reduced and stores free energy as FADH2.
- The carbon atoms from the original glucose molecule exit the process as low energy metabolic waste in the form of CO2 in steps 3 and 4.
By the end of the Krebs Cycle...
The final stage in the process is the Electron Transport Chain and Chemiosmosis.
- 6 carbons (originally from the glucose molecule) are released from the cell as waste in the form of CO2.
- 4 ATP (2 from glycolysis, 2 from Krebs).
- 12 reduced coenzymes (2 NADH from glycolysis, 2 NADH from pyruvate oxidation, 6 NADH and 2 FADH2 from Krebs).
- These reduced coenzymes will now move on to stage 4 - the electron transport chain and chemiosmosis, and energy can be transferred to ATP.
The final stage in the process is the Electron Transport Chain and Chemiosmosis.