Krebs cycle (Tricarboxylic Acid Cycle)

Krebs cycle is a series of reactions in which the pyruvate from glycolysis is oxidized to CO2. The Krebs cycle was discovred by Hans Krebs in the I 93th. 1v‘o electron carriers act as hydrogen acceptors. These are

  • Nicotinttinitle adenine tlinucleotide (N AD)
  • Flavin adenine (Hunch:on& (FAD)

There are reduced to NADH and FADH2. These molecules of CO2 are generated from each pyruvate molecule during this phase of the cycle. Some energy is also produced in the form of ATP. Most of the remaining energy is in the form of NADH and FADH,. These two molecules are transferred into the electron transport chain.The Pyruvate  is first changed in Acetyl CoA. The Acetyl CoA then enters into the Krebs cycle. Following reactions occur during Krebs cycle:


. Step 1: Acetyl CoA adds its two-carbon fragment to oxaloacetate. Oxaloacetae is a four-carbon compound. The unstable bond of acetyl CoA is broken attaches to the acetyl group. The product is the six-carbon citrate. This reaction is controlled by enzyme citrate synthase.




  1. Step 2: A molecule of water is removed and another is added back. The net result is the conversion of citrate to its isomer isocitrate. It is controlled by enzyme aconitase.                               •


  1. Step 3: The substrate loses a CO, molecule in the presence of enzyme dehydrogenase. Thus the remaining five carbon compound – is oxidized. It reduces NAD to NADH and a-ketoglutrate is formed.
  2. Step 4: This step is catalyzed by a multienzyme complex. CO, is lost. The remaining four-carbon compound is oxidized and electrons are transferred to NAD to form NADH. The compound is then attached to coenzyme. Thus Succinyl CoA is formed:
  3. Step 5: Substrate level phosphorylation occurs in this step. CoA is replaced by a phosphate group and succinate is formed. The ithosphate group is then transferred to GDP. It forms guanosinetriphosphate (GTP). GTP is similar to ATP.
    1. Step 6: Two hydrogens from succinate are transferred to form FADET. It produces Fumarate.
    2. Step 7: Bonds in the substrate are rearranged by the addition of a’ water molecule and malate is formed.
    3. Steps 8: In this step another molecule , of NADH is produced. It regenerates oxaloacetate. The oxaloacetate is ready to enter into another reaction.

    Products of Krebs cycle

    Krebs cycle gives following products from the oxidation one molecule of acetyl CoA;

    1. . Three molecules of CO,
    1. Three molecules of NADI 4
    2. One molecules of FADH,
    3. One molecule of ATP


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