Krebs Cycle: What it is and How it Happens (scheme)

The Krebs cycle, or citric acid cycle, is a sequence of chemical reactions that take place in the mitochondria of eukaryotic cells as part of cellular respiration. It is also called the tricarboxylic acid cycle, because citric acid has three carboxylic groups in its structure.

This cycle consists of 8 steps. It begins with the reaction of 4-carbon oxaloacetate with activated acetate in the form of acetyl-CoA to form citrate or citric acid, a six-carbon molecule. In the following steps, the citrate loses electrons and two carbon dioxide molecules, transforming itself back into oxaloacetate, closing the cycle.

The acetyl-CoA that enters the citric acid cycle can come from glycolysis, glucose being the raw material for this process.

The main function of the citric acid cycle It is to capture the electrons that are released from the molecules when they oxidize (they lose electrons). These electrons are captured by carrier molecules and then transformed into adenosine triphosphate ATP, the energy molecule that the cell uses to carry out its functions.

Krebs cycle scheme

Scheme of the Krebs cycle. Acetyl-CoA: acetyl coenzyme A; NADH: reduced nicotinamide adenine dinucleotide; FADHtwo: reduced flavin adenine dinucleotide; GTP: guanosine triphosphate; COtwo: carbon dioxide

Products of the Krebs cycle

Each Krebs cycle produces:

  • 3 NADH (reduced nicotinamide adenine dinucleotide)
  • 1 GTP (guanosine triphosphate)
  • 1 FADHtwo (reduced flavin adenine dinucleotide)
  • 2 molecules of carbon dioxide.

Although ATP (adenosine triphosphate) is not directly formed in the Krebs cycle, GTP can be converted to ATP. In addition, NADH and FADHtwo that are formed in the cycle, transfer their electrons to the electron transport chain in the mitochondria which, by oxidative phosphorylation, leads to the production of ATP.

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See also:

  • glycolysis
  • Calvin Cycle.

Steps of the Krebs cycle

krebs cycle

1. Citrate formation: Acetyl-coenzyme A (acetyl-CoA) combines with oxaloacetate to form citrate and release coenzyme A. The enzyme that catalyzes this reaction is citrate synthase.

2. Formation of isocitrate: citrate is transformed into isocitrate, by action of the enzyme aconitase.

3. Oxidation of isocitrate to α-ketoglutaramate: isocitrate, with six carbon atoms, loses one carbon as carbon dioxide COtwo and a pair of electrons, to become α-ketoglutarato, with five carbons. Electrons are captured by a NAD+ (oxidized nicotinamide adenine dinucleotide) and is converted to NADH (reduced nicotinamide adenine dinucleotide). The enzyme is isocitrate dehydrogenase.

4. Oxidation of α-ketoglutaramate to succinyl-CoA and COtwo: the molecule of α-ketoglutarato of five carbons is oxidized obtaining succinyl-CoA (four carbon atoms), with release of COtwo. a molecule of NAD+ is reduced to NADH. The enzyme involved in this reaction is α-ketoglutarate dehydrogenase.

5. Conversion of succinyl-CoA to succinate: Succinyl-CoA is transformed into succinate when it releases the CoA group to form GTP (guanosine triphosphate) from GDP (guanosine diphosphate) and inorganic phosphate. The enzyme that catalyzes this reaction is succinyl-CoA synthetase.

6. Oxidation of succinate to fumarate: succinate loses two electrons to form fumarate. The electrons in this reaction are captured by the oxidized flavin adenine dinucleotide (FAD) which is reduced to FADH.two. The enzyme involved is succinate dehydrogenase.

7. Hydration of fumarate to malate: the fumarate gains a molecule of water and is transformed into malate, by the action of the enzyme fumarase.

8. Oxidation of malate to oxaloacetate: the last step of the Krebs cycle regenerates oxaloacetate, by the action of malate dehydrogenase. In this reaction, malate is oxidized and loses two electrons to NAD.+forming NADH.

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See also:

  • Molecule.
  • Glucose.
  • Mitochondria.
  • cellular respiration
  • types of breathing
  • mitochondria function


Alberts, B., Johnson, A., Lewis, J, Raff, M., Roberts, K., Walter, P. (2008) Molecular Biology of the Cell 5th Ed. Garland Science. UK.

Nelson, DL, Cox, MM, Hoskins, AA (2021) Lehninger Principles of Biochemistry. 8th ed. Macmillan Learning. Boston.