Understanding the Krebs Cycle: Step by Step

Continuing our journey through the Krebs cycle, we now focus on the core reactions where significant energy production occurs.Starting with isocitrate, the enzyme isocitrate dehydrogenase catalyzes its conversion to alpha-ketoglutarate.This first reaction produces NADH and releases carbon dioxide through oxidative decarboxylation.Next, alpha-ketoglutarate is converted to succinyl-CoA by alpha-ketoglutarate dehydrogenase complex.This second decarboxylation also produces NADH and releases another molecule of carbon dioxide.Finally, succinyl-CoA is converted to succinate by succinyl-CoA synthetase, producing GTP through substrate-level phosphorylation.This substrate-level phosphorylation is a direct formation of a high-energy phosphate bond, generating GTP which can be converted to ATP.Now let's complete the Krebs cycle by examining the final conversions that regenerate oxaloacetate.Starting with succinate, the enzyme succinate dehydrogenase removes hydrogen atoms, converting FAD to FADH2 and forming fumarate.Fumarase then catalyzes the addition of water to fumarate, forming malate.Finally, malate dehydrogenase oxidizes malate to oxaloacetate, producing another NADH molecule.Let's summarize the total energy yield from one complete turn of the Krebs cycle.The cycle produces six NADH molecules, two FADH2 molecules, and two ATP molecules, while releasing two carbon dioxide molecules.These energy carriers, particularly NADH and FADH2, then feed into the electron transport chain.This connection allows the Krebs cycle to power the production of most of the cell's ATP through oxidative phosphorylation.