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Citric Acid Cycle

This was elucidated by the British biochemist, Hans Krebs, in 1937. This is also known as tricarboxylic acid cycle (TCA cycle) or Krebs cycle. The actual citric acid cycle begins when acetyl-CoA enters into a reaction to form citric acid. The elucidation of this cycle explains how pyruvate is broken down to CO2 and H2O. It also highlights the concept of cycles in metabolism. For this pioneering work, Hans Krebs was awarded the coveted Nobel Prize in 1953.

Citric Acid Cycle or Krebs Cycle

In the first reaction of citric acid cycle, one molecule of acetyl-CoA combines with 4-carbon oxaloacetic acid molecules (OAA) to form 6-carbon citric acid and CoA is released. The reaction is catalyzed by the enzyme citrate synthase. Citrate is then isomerised to isocitrate. It is followed by two successive steps of decarboxylation, leading to the formation of α -ketoglutaric acid and succinyl-CoA. In the remaining steps of citric acid cycle, succinyl CoA is oxidized to OAA, allowing the cycle to continue.

During this cycle, 3 molecules of NAD+ and one molecule of FAD (Flavin Adenine Dinucleotide) are reduced to produce NADH and FADH2, respectively. These reduced electron carriers pass on the hydrogen atoms to oxygen through electron transport system, yielding 11 more ATP molecules or each molecule of pyruvic acid. In addition, one more ATP molecule is generated directly during the cycle, to give a total of 12 ATP molecules per pyruvic acid molecule (3C). As two molecules of pyruvic acid are produced from each molecule of glucose (6C), a total of 24 molecules of ATP are formed during the citric acid cycle.

NADH and FADH2 so produced during the citric acid cycle are linked with the electron transport system and produce ATP by oxidative phosphorylation. The summary equation of this phase of respiration may therefore be written as follows:-


Significance of Citric Acid Cycle

  1. During this pathway many intermediate compounds are formed which are used in the synthesis of biomolecules like amino acids, nucleotides, chlorophyll, cytochromes and fats. For example amino acids are synthesized from pyruvic acid and oxaloaceticacid.
  2. This is the major pathway for the generation of ATP molecules.

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