Citric Acid Cycle

Citric Acid Cycle - Overview of the Citric Acid Cycle

The Citric Acid Cycle is also known as the Krebs
Cycle or Tricarboxylic Acid (TCA) Cycle.
It is a series of chemical reactions that
takes place in the cell that breaks down
food molecules into
carbon dioxide, water, and energy.

Narayanese, wikipedia.org

The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a series of chemical reactions in the cell that breaks down food molecules into carbon dioxide, water, and energy. In plants and animals, these reactions take place in the mitochondria of the cell as part of cellular respiration. Many bacteria perform the citric acid cycle too, though they do not have mitochondria so the reactions take place in the cytoplasm of bacterial cells. Sir Hans Adolf Krebs, a British biochemist, is credited with discovering the cycle. Sir Krebs outlined the steps of the cycle in 1937.

The overall reaction for the citric acid cycle is:

Acetyl-CoA + 3 NAD+ + Q + GDP + Pi + 2 H2O --> CoA-SH + 3 NADH + 3 H+ + QH2 + GTP + 2 CO2

where Q is ubiquinone and Pi is inorganic phosphate

In order for food to enter the citric acid cycle, it must be broken into acetyl groups, (CH3CO). At the start of the citric acid cycle, an acetyl group combines with a four-carbon molecule called oxaloacetate to make a six-carbon compound, citric acid. During the cycle, the citric acid molecule is rearranged and stripped of two of its carbon atoms. Carbon dioxide and 4 electrons are released. At the end of the cycle, a molecule of oxaloacetate remains, which can combine with another acetyl group to being the cycle again.

Substrate --> Products (Enzyme)

Oxaloacetate + Acetyl CoA + H2O --> Citrate + CoA-SH (citrate synthase)

Citrate --> cis-Aconitate + H2O (aconitase)

cis-Aconitate + H2O --> Isocitrate (aconitase)

Isocitrate + NAD+ Oxalosuccinate + NADH + H + (isocitrate dehydrogenase)

Oxalosuccinate á-Ketoglutarate + CO2 (isocitrate dehydrogenase)

α-Ketoglutarate + NAD+ + CoA-SH --> Succinyl-CoA + NADH + H+ + CO2 (α-ketoglutarate dehydrogenase)

Succinyl-CoA + GDP + Pi --> Succinate + CoA-SH + GTP (succinyl-CoA synthetase)

Succinate + ubiquinone (Q) --> Fumarate + ubiquinol (QH2) (succinate dehydrogenase)

Fumarate + H2O --> L-Malate (fumarase)

L-Malate + NAD+ --> Oxaloacetate + NADH + H+ (malate dehydrogenase)