Summary
Highlights
Citrate, with the enzyme aconitase, first releases water to form cis-aconitate, creating a double bond. Then, cis-aconitate takes water back, with the same enzyme, to form isocitrate. This isomerization rearranges the hydroxyl and hydrogen groups.
The video introduces the Tricarboxylic Acid Cycle, also known as the Krebs Cycle or Citric Acid Cycle, as the third step of aerobic respiration. It follows glycolysis and pyruvate oxidation, where glucose converts to pyruvate, then to acetyl-CoA.
Acetyl-CoA combines with oxaloacetate, a four-carbon compound, with the help of citrate synthase and water, to form citrate. Coenzyme A is released during this reaction. The numbering of carbon atoms illustrates how acetyl-CoA and oxaloacetate combine.
Isocitrate is converted to alpha-ketoglutarate by isocitrate dehydrogenase. This reaction releases carbon dioxide and hydrogen, which is picked up by NAD+ to form NADH. This is an oxidative reaction.
Alpha-ketoglutarate is transformed into succinyl-CoA by alpha-ketoglutarate dehydrogenase. This process involves the attachment of coenzyme A, the release of carbon dioxide, and the removal of hydrogen, which reduces NAD+ to NADH.
Succinyl-CoA is converted to succinate by succinyl-CoA synthase. During this reaction, coenzyme A is released, and the energy released is used to convert GDP to GTP, which then transfers a phosphate to ADP to form ATP.
Succinate is converted to fumarate by succinate dehydrogenase. This reaction removes hydrogen from succinate, creating a double bond, and the hydrogen is gained by FAD to form FADH2. This step occurs on the inner mitochondrial membrane.
Fumarate, with the help of fumarase and water, is converted into L-malate. Water adds across the double bond, attaching a hydroxyl group to one carbon and a hydrogen to another.
L-malate is converted back to oxaloacetate by malate dehydrogenase. This is another oxidative reaction where hydrogen is removed from L-malate and transferred to NAD+, forming NADH. The regenerated oxaloacetate is ready for the next acetyl-CoA molecule.
The cycle demonstrates that the two carbons from acetyl-CoA are released as carbon dioxide. For one acetyl-CoA, the cycle generates three NADH, one FADH2, and one ATP. Since one glucose molecule yields two acetyl-CoA, it runs the cycle twice, producing six NADH, two FADH2, and two ATP in total.