Krebs Cycle Explained with Simple Animations

The Krebs cycle begins with two important molecules: Acetyl-CoA and Oxaloacetate.The enzyme citrate synthase catalyzes their combination to form citrate, a six-carbon molecule.The enzyme aconitase then converts citrate to isocitrate through a series of steps.Isocitrate undergoes oxidation, losing a carbon dioxide molecule and forming α-ketoglutarate.During this process, electrons are transferred, producing NADH in our first energy-harvesting step.This carefully coordinated sequence of enzyme-catalyzed reactions marks the first stage of the Krebs cycle.In this stage of the Krebs cycle, α-Ketoglutarate undergoes a complex transformation.The process begins when α-Ketoglutarate enters the α-Ketoglutarate dehydrogenase complex.NAD+ is reduced to NADH, capturing high-energy electrons from the substrate.During this process, a carbon dioxide molecule is released, and Coenzyme A attaches to form Succinyl-CoA.Next, Succinyl-CoA synthetase catalyzes the conversion of Succinyl-CoA to succinate.This step is coupled to the formation of GTP from GDP and inorganic phosphate.The high-energy thioester bond in Succinyl-CoA drives the formation of GTP, which is equivalent to ATP in energy currency.This stage of the Krebs cycle produces two important energy-carrying molecules: one NADH and one GTP.In the final stage of the Krebs cycle, we begin with succinate.Succinate dehydrogenase removes hydrogens from succinate, converting it to fumarate. This process produces FADH2, an important energy carrier.Next, the enzyme fumarase catalyzes the addition of water to fumarate, forming malate.Finally, malate dehydrogenase removes hydrogens from malate to form oxaloacetate, producing another NADH molecule in the process.This final stage of the Krebs cycle produces two important energy carriers: FADH2 from the oxidation of succinate, and NADH from the oxidation of malate.The key enzymes in this stage have unique properties. Succinate dehydrogenase is unique as it's the only membrane-bound enzyme in the Krebs cycle and also participates in the electron transport chain. Malate dehydrogenase catalyzes a reversible reaction using NAD+ as a cofactor.With oxaloacetate regenerated, the cycle is ready to begin again with a new acetyl-CoA molecule.