The Krebs Cycle, also known as the Citric Acid Cycle, is a metabolic process that takes place within the mitochondrial matrix of eukaryotic cells. Its main purpose is to complete the breakdown, or oxidation, of fuel molecules derived from carbohydrates, fats, and proteins. By oxidizing these fuel components, the cycle generates chemical energy, primarily in the form of high-energy molecules that will later be converted into Adenosine Triphosphate (ATP).
Preparing the Fuel: Pyruvate to Acetyl-CoA
Before the Krebs Cycle can begin, a preparatory step must occur to transform the three-carbon molecule pyruvate into an acceptable fuel source. This conversion takes place as pyruvate is transported from the cell’s cytoplasm into the mitochondrial matrix. The complex enzyme pyruvate dehydrogenase catalyzes this reaction, converting pyruvate into Acetyl-CoA.
One carbon atom is removed from pyruvate and released as carbon dioxide (CO2). The resulting two-carbon acetyl group attaches to Coenzyme A, forming Acetyl-CoA. Crucially, the removal of electrons during this oxidation step reduces one molecule of NAD+ to NADH per pyruvate molecule. Since one glucose molecule yields two pyruvate molecules, this preparatory step generates two molecules of NADH and two molecules of Acetyl-CoA for the cycle.
Direct Energy Output of the Krebs Cycle
The Acetyl-CoA molecule enters the cycle by combining with the four-carbon molecule oxaloacetate to form citrate, a six-carbon compound. Through a series of eight steps, the two carbons from the acetyl group are fully oxidized to carbon dioxide. The starting oxaloacetate molecule is then regenerated to keep the cycle turning.
Only one step in this pathway results in the direct production of a high-energy phosphate compound. This occurs during the conversion of succinyl-CoA to succinate, generating one molecule of Guanosine Triphosphate (GTP) via substrate-level phosphorylation. GTP is energetically equivalent to ATP. Since one glucose molecule yields two Acetyl-CoA molecules, the cycle completes two full turns, resulting in a direct net gain of 2 ATP (via GTP) per glucose molecule.
Calculating the Total ATP Yield from Energy Carriers
The true energetic contribution of the Krebs Cycle lies in the indirect energy generated by high-energy electron carriers, not the small direct ATP yield. For every single turn of the cycle, three molecules of NADH and one molecule of FADH2 are produced. Since two Acetyl-CoA molecules enter the cycle per glucose molecule, the total indirect output from the cycle is six NADH and two FADH2 molecules.
These electron carriers hold the majority of the energy extracted from the original glucose molecule, and they carry their electrons to the Electron Transport Chain (ETC) for oxidative phosphorylation. Current biological models use an approximate conversion rate for these carriers: each NADH is estimated to yield about 2.5 ATP, and each FADH2 is estimated to yield about 1.5 ATP.
The total ATP yield attributable to Acetyl-CoA oxidation and preparation is calculated by adding the direct and indirect outputs. The two NADH from the preparatory step contribute 5 ATP (2 x 2.5 ATP). From the two turns of the Krebs Cycle, the six NADH molecules contribute 15 ATP (6 x 2.5 ATP), and the two FADH2 molecules contribute 3 ATP (2 x 1.5 ATP). Combining these indirect yields with the 2 ATP generated directly within the cycle results in a total of 25 ATP derived from pyruvate oxidation and the Krebs Cycle per glucose molecule.