Steps of Krebs Cycle

Krebs cycle was postulated by Hans Adolph Krebs in 1937. This represents efficient way to cells for producing energy during degradation of energy-rich molecules. Electrons are removed from intermediate metabolic products during Krebs cycle. These electrons reduce coenzyme molecules nicotinamide adenine dinucleotide [NAD+] and flavin mononucleotide [FAD] to NADH to FADH2. Subsequently, these coenzymes are oxidized in electron transport chain. In this process, enzyme series transfers electrons of FADH2 and NADH to oxygen. This is the last electron acceptor of cellular respiration in eukaryotes.

Transition Phase:

Krebs cycle is also knwon as tricarboxylic acid cycle or citric acid cycle. In transitional phase, pyruvic acid is converted to acetyl coenzyme A (acetyl CoA). Transitional phase is a three step process.


  1. From pyruvic acid a carbon is removed and released as carbon dioxide, that is expelled out by lungs.
  2. Oxidation occurs by removing hydrogen atoms, after decarboxylation. This is collected by NAD+ .
  3. Acetic produced in the step-2 combines with coenzyme A for producing acetyl coenzyme A.

After transitional phase, acetyl coenzyme A enters into Krebs Cycle to break completely. Krebs cycle is an 8 step process. Citric acid atoms are rearranged as the steps progress. Hence, produces several intermediate products. Many of these intermediate products are keto acids.

Krebs Cycle


Steps of Krebs Cycle

  1. Coenzyme A transfers 2-carbon acetyl groups from it to 4-carbon compound oxaloacetate. This forms 6-carbon molecule citrate.
  2. Formation of isomeric form takes place by rearranging citrate.
  3. 6-carbon isocitrate is oxidized and 1-carbon dioxide molecule is moved for producing alpha-ketoglutarate is a 5-carbon molecule. During oxidation process, NAD+ is reduced to NADH + H+.
  4. Alpha-ketogluterate is oxidized and carbon dioxide is removed. Coenzyme A is added to this to form 4-carbon compound succinyl-CoA. During oxidation process, NAD+ is reduced to NADH + H+.
  5. From succinyl- CoA, coenzyme is removed for producing succinate. From guanosine diphosphate (GDP) and P by substrate-level phosphorylation, guanosine triphosphate (GTP) is formed from the released energy. Later GTP may be used for making ATP.
  6. Succinate is oxidized to fumarate. During oxidation, FAD is reduced to form FADH2.
  7. Malate is formed by adding water to fumarate.
  8. Oxaloacetate is produced by adding malate, initial compound of Krebs cycle. During oxidation process, NAD+ is reduced to NADH + H+.

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