The Krebs cycle, also known as the citric acid cycle, is a series of enzymatic reactions that occur in the mitochondrial matrix of eukaryotic cells. This process is an important part of cellular respiration, which is the process by which cells convert glucose into ATP, the primary source of energy for cells.
The Krebs cycle begins with the conversion of pyruvate, a molecule produced during glycolysis, into acetyl-CoA. This reaction is catalyzed by a complex of enzymes known as the pyruvate dehydrogenase complex, which is located in the mitochondria. During this process, a molecule of CO2 is released, and the resulting acetyl-CoA is used as a substrate for the Krebs cycle.
The Krebs cycle starts with the combination of acetyl-CoA with oxaloacetate to form citrate, which is a 6-carbon molecule. Citrate is then converted into isocitrate through a series of reactions involving the removal and addition of water molecules. Isocitrate is then oxidized to alpha-ketoglutarate, releasing a molecule of CO2 and producing a molecule of NADH in the process.
Alpha-ketoglutarate is then converted into succinyl-CoA, which is a 4-carbon molecule. This reaction releases a molecule of CO2 and produces another molecule of NADH. The succinyl-CoA is then converted into succinate, releasing a molecule of GTP (which is converted to ATP) in the process.
Succinate is then converted into fumarate, producing another molecule of FADH2. Fumarate is then converted into malate, which is a 4-carbon molecule. This reaction produces another molecule of NADH.
Finally, malate is converted back into oxaloacetate, which is the molecule that originally combined with acetyl-CoA to start the cycle. This reaction also produces another molecule of NADH.
Overall, the Krebs cycle produces a total of 3 molecules of NADH, 1 molecule of FADH2, 1 molecule of ATP/GTP, and 2 molecules of CO2 per acetyl-CoA molecule. These energy-rich molecules are then used in the electron transport chain to produce ATP, the main source of cellular energy. The Krebs cycle also produces other important metabolic intermediates, such as amino acids and nucleotides, which are used in various cellular processes.