The Krebs Cycle

- Also known as the tricarboxylic acid cycle (TCA cycle), or the citric acid cycle

- Discovered in 1937 by Sir Hans Krebs, a biochemist working at the University of Sheffield in England

- Eight-step process, each step catalyzed by a specific enzyme; cyclic because oxaloacetate is the product of the last step as well as the reactant in the first step

- Overall chemical equation for the Krebs cycle: oxaloacetate + acetyl-CoA + ADP + Pi + 3NAD⁺ + FAD → CoA + ATP + 3NADH + 3H⁺ + FADH2 + 2CO2 + oxaloacetate


- By the end of the Krebs cycle, the original glucose molecule is entirely consumed. The 6 carbon atoms of the original glucose molecule leave the process as 6 low-energy CO2 molecules. The energy of the glucose is preserved and stored in the form of 4 ATP molecules (2 from glycolysis, 2 from Krebs cycle), and 12 reduced coenzymes (2 NADH from glycolysis, 2 NADH from pyruvate oxidation, 6 NADH from the Krebs cycle, and 2 FADH2 from the Krebs cycle). Most of the free energy stored in NADH and FADH2 will eventually be transferred to ATP through processes called electron transport and chemiosmosis.


Steps of the Krebs Cycle:

1. The acetyl group (2-C) of acetyl-CoA condenses with oxaloacetate (4-C) to form citrate (6-C).
2. Citrate (6-C) is rearranged to isocitrate (6-C).
3. Isocitrate (6-C) converted to a-ketoglutarate (5-C) by losing a CO2 and two hydrogen atoms that reduce NAD⁺ to NADH.
4. a-ketoglutarate (5-C) is converted to succinyl-CoA (4-C). A CO2 is removed, coenzyme A is added, and two hydrogen atoms reduce NAD⁺ to NADH.
5. Succinyl CoA (4-C) is converted to succinate (4-C). ATP is formed by substrate level phosphorylation, and coenzyme A is released.
6. Succinate (4-C) is converted to fumarate (4-C). 2 hydrogen atoms reduce FAD to  FADH2.
7. Fumarate (4-C) is converted to malate (4-C) by the addition of H2O.
8. Malate (4-C) is converted to oxaloacetate (4-C). 2 hydrogen atoms reduce NAD⁺ to NADH. The cycle repeats.

Key Features of the Krebs Cycle:

- Since two acetyl-CoA molecules are formed from one glucose molecule, the cycle occurs twice for each molecule of glucose processed.


- In step 1, acetyl-CoA enters the cycle and reacts with a molecule of oxaloacetate (OAA) to produce a molecule of citrate. In this reaction, OAA (4-C) is converted into citrate (6-C) by adding the 2-C acetyl group of acetyl-CoA, releasing CoA, which is recycled. Also, OAA has 2 carboxyl groups and citrate has 3 carboxyl groups.


- Energy is harvested in steps 3, 4, 5, 6, and 8.


- In steps 3, 4, and 8, NAD⁺ is reduced to NADH.


- In step 5, ATP is formed by substrate-level phosphorylation. A phosphate from the mitochondrial matrix displaces CoA from succinyl-CoA. The phosphate is then transferred to guanosine diphosphate (GDP) to form guanosine triphosphate (GTP). Next, phosphate condenses with ADP, forming ATP. Overall, free energy is transferred from succinyl-CoA to ATP.


- Energy is harvested in step 6. However, reaction is not exergonic enough to reduce NAD⁺ to NADH. Instead, free energy is stored by reducing FAD to FADH2, a step closely related to the electron transport chain in mitochondria.


- Last 4 carbon atoms of the original glucose leave as CO2 in steps 3 and 4 (two Krebs cycles to process 1 glucose). The CO2 molecules diffuse out of the mitochondrion and the cell as metabolic waste.