The Krebs cycle is the second of three steps in [carbohydrate catabolism]? (the breakdown of carbon-containing compounds), between glycolysis and [oxidative phosphorylation]?. Citrate is both the first and last product of the cycle (Fig. 1), and is regenerated by the condensation of oxaloacetate and acetyl-CoA, the latter being a result of a prior breakdown process, e.g., glycolysis (in glycolysis, glucose (a six-carbon-molecule) is broken down to pyruvate (a three-carbon-molecule), prior to cellular respiration), [protein catabolism]? (Proteins are broken down outside the cells by protease enzymes. The amino acids are brought inside the cells and funnel into glycolysis or the Krebs cycle.) or [fat catabolism]?(Triglycerides/triacylglycerols are hydrolyzed (hydrogen molecules are added) to break them into fatty acids and glycerol. The glycerol is then converted into glyceraldehyde-3-phosphate, which goes through the process known as hydrolysis. Fats that are to be catabolized go through a process known as beta oxidation (the oxidation of beta particles). Beta oxidation breaks down fatty acids into two carbon molecules (Acetyl-CoA). The two carbons are then catabolized in the Krebs cycle.) (Fig. 2).
Molecule | Enzyme | Reaction Type | Reactants/ Coenzymes |
Products/ Coenzymes |
---|---|---|---|---|
I. Citrate | 1. Aconitase | Dehydration | H2O | |
II. cis-Aconitate | 2. Aconitase | Hydration | H2O | |
III. Isocitrate | 3. Isocitrate Dehydrogenase | Oxidation | NAD+ | NADH+H+ |
IV. Oxalosuccinate | 4. Isocitrate Dehydrogenase | Decarboxylation | ||
V. α-Ketoglutarate | 5. α-Ketoglutarate Dehydrogenase | Oxidative Decarboxylation | NAD+ CoA-SH |
NADH+H+ CO2 |
VI. Succinyl-CoA | 6. Succinyl-CoA Synthetase | Hydrolysis | GDP Pi |
GTP CoA-SH |
VII. Succinate | 7. Succinate Dehydrogenase | Oxidation | FAD+ | FADH2 |
VIII. Fumarate | 8. Fumerase | Addition (H2O) | H2O | |
IX. L-Malate | 9. Malate Dehydrogenase | Oxidation | NAD+ | NADH+H+ |
X. Oxaloacetate | 10. Citrate Synthetase | Condensation | ||
XI. Acetyl-CoA |
Two carbons are oxidized to CO2, and the energy from these reactions are stored in ATP(ATP is the "universal energy currency" of the cell), NADH and FADH2. NADH and FADH2 are coenzymes? (Coenzymes are molecules that enable or enhance enzymes.) that store energy and can release it when needed. The total energy gained by the Krebs cycle equals 2 ATP, the total energy gained from all from all four processes in cellular respiration equals 38 ATP, generated from each molecule of glucose.
Figure 2 : Schematic drawing of metabolic pathways associated with the Krebs cycle.