Question

How is TCA cycle regulated?

Answer 1

TCA cycle:

1. The TCA cycle, or Tricarboxylic Acid Cycle, is a sequence of chemical events that occur in the cells of all aerobic organisms to release energy stored in the form of ATP by converting Acetyl CoA obtained from carbs, lipids, and proteins.
2. It's also known as the Citric Acid Cycle, and it happens in mitochondria during cellular respiration's second phase. The TCA cycle's processes are catalyzed by soluble enzymes.
3. The TCA cycle is an eight-step process that aids in the decomposition of organic compounds. Glucose, sugars, fatty acids, amino acids, and other macromolecules cannot enter the TCA cycle directly.
4. They're initially broken down into Acetyl CoA, a two-carbon molecule. After entering the TCA cycles, acetyl CoA passes through a series of chemical reactions to produce carbon dioxide and energy. A soluble enzyme catalyzes each step of the process.
5. Pyruvate, which is made from glucose, is oxidized to produce acetyl CoA.
6. As a result, acetyl CoA enters the cycle, triggering a series of reactions.
   1. Acetyl-CoA, a two-carbon molecule, reacts with oxaloacetate, a four-carbon molecule, to create citrate, a six-carbon molecule that releases the CoA group.
   2. In the next stage, citrate is transformed into isocitrate, an isomer of citrate. In this stage, two processes take place at the same time. Citrate loses a water molecule at first, then recovers it back to produce isocitrate.
   3. The oxidation of isocitrate happens in the third stage. With the release of a molecule of carbon dioxide, a five-carbon molecule known as -ketoglutarate is left behind. NAD⁺ is also converted to NADH. The enzyme isocitrate dehydrogenase catalyzes this process.
   4. ɑ-ketoglutarate is oxidized in the fourth step, producing a molecule of carbon dioxide and converting NAD⁺ to NADH. Simultaneously, the remaining four-carbon molecules pick up CoA, generating Succinyl CoA, an unstable chemical. The enzyme -ketoglutarate catalyzes this process.
   5. CoA from succinyl CoA is replaced with the phosphate group. It is subsequently converted to ADP, resulting in the ATP molecule. This process also produces Succinate, a four-carbon compound.
   6. Succinate is oxidized to fumarate in the sixth step. Additionally, two hydrogen atoms are transferred to FAD, resulting in FADH₂. Because the enzyme that catalyzes this reaction is situated in the inner membrane of mitochondria, FADH₂ sends its electrons directly to the electron transport chain (ETC).
   7. With the help of the enzyme Fumarase, a water molecule is added to fumarate, and fumarate is transformed into malate.
   8. The oxidation of malate regenerates oxaloacetate, a four-carbon chemical, and another molecule of NAD⁺ is reduced to NADH in the final stage of the cycle. Malate Dehydrogenase is the enzyme that catalyzes this process.