Citric Acid Cycle Steps

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Citric Acid Cycle

Citric Acid Cycle diagram
The citric acid cycle is a series of chemical reactions that aid in releasing the stored energy in the foods we eat. Image Courtesy Evelyn Bailey

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is the second stage of cellular respiration. This cycle is catalyzed by several enzymes and is named in honor of the British scientist Hans Krebs who identified the series of steps involved in the citric acid cycle. The usable energy found in the carbohydrates, proteins, and fats we eat is released mainly through the citric acid cycle. Although the citric acid cycle does not use oxygen directly, it works only when oxygen is present.

Citric Acid Cycle Steps

The first phase of cellular respiration called glycolysis takes place in the cytosol of the cell's cytoplasm. The citric acid cycle, however, occurs in the matrix of cell mitochondria. Prior to the beginning of the citric acid cycle, pyruvic acid generated in glycolysis crosses the mitochondrial membrane and is used to form acetyl coenzyme A (acetyl CoA). Acetyl CoA is then used in the first step of the citric acid cycle. Each step in the cycle is catalyzed by a specific enzyme.

  • Step 1 - The two-carbon acetyl group of acetyl CoA is added to the four-carbon oxaloacetate to form the six-carbon citrate. The conjugate acid of citrate is citric acid, hence the name citric acid cycle. Oxaloacetate is regenerated at the end of the cycle so that the cycle may continue. Enzyme: citrate synthase.
  • Step 2 - Citrate loses a molecule of water and another is added. In the process, citric acid is converted to its isomer isocitrate. Enzyme: aconitase.
  • Step 3 - Isocitrate loses a molecule of carbon dioxide (CO2) and is oxidized forming the five-carbon alpha ketoglutarate. Nicotinamide adenine dinucleotide (NAD+) is reduced to NADH + H+ in the process. Enzyme: isocitrate dehydrogenase.
  • Step 4 - Alpha ketoglutarate is converted to the 4-carbon succinyl CoA. A molecule of CO2 is removed and NAD+ is reduced to NADH + H+ in the process. Enzyme: alpha ketoglutarate dehydrogenase.
  • Step 5 - CoA is removed from the succinyl CoA molecule and is replaced by a phosphate group. The phosphate group is then removed and attached to guanosine diphosphate (GDP) thereby forming guanosine triphosphate (GTP). Like ATP, GTP is an energy-yielding molecule and is used to generate ATP when it donates a phosphate group to ADP. The final product from the removal of CoA from succinyl CoA is succinate. Enzyme: succinyl-CoA synthetase.
  • Step 6 - Succinate is oxidized and fumarate is formed. Flavin adenine dinucleotide (FAD) is reduced and forms FADH2 in the process. Enzyme: succinate dehydrogenase.
  • Step 7 - A water molecule is added and bonds between the carbons in fumarate are rearranged forming malate. Enzyme: fumarase.
  • Step 8 - Malate is oxidized forming oxaloacetate, the beginning substrate in the cycle. NAD+ is reduced to NADH + H+ in the process. Enzyme: malate dehydrogenase.

Citric Acid Cycle Summary

In eukaryotic cells, the citric acid cycle uses one molecule of acetyl CoA to generate 1 ATP, 3 NADH, 1 FADH2, 2 CO2, and 3 H+. Since two acetyl CoA molecules are generated from the two pyruvic acid molecules produced in glycolysis, the total number of these molecules yielded in the citric acid cycle is doubled to 2 ATP, 6 NADH, 2 FADH2, 4 CO2, and 6 H+. Two additional NADH molecules are also generated in the conversion of pyruvic acid to acetyl CoA prior to the start of the cycle. The NADH and FADH2 molecules produced in the citric acid cycle are passed along to the final phase of cellular respiration called the electron transport chain. Here NADH and FADH2 undergo oxidative phosphorylation to generate more ATP.
Sources:

  • Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 17, The Citric Acid Cycle. Available from: http://www.ncbi.nlm.nih.gov/books/NBK21163/
     
  • The Citric Acid Cycle. BioCarta. Updated March 2001. (http://www.biocarta.com/pathfiles/krebpathway.asp)