Krebs cycle

Krebs cycle, series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolism of glucose and other simple sugars. The breakdown of glucose to carbon dioxide and water is a complex set of chemical interconversions called carbohydrate catabolism, and the Krebs cycle is the second of three major stages in the process, occurring between glycolysis and oxidative phosphorylation. This cycle, also known as the citric acid cycle, was named in recognition of the German chemist Hans Krebs, whose research into the cellular utilization of glucose contributed greatly to the modern understanding of this aspect of metabolism. The name citric acid cycle is derived from the first product generated by the sequence of conversions, i.e., citric acid. The reactions are seen to comprise a cycle inasmuch as citric acid is both the first product and the final reactant, being regenerated at the conclusion of one complete set of chemical rearrangements. Citric acid is a so-called tricarboxylic acid, containing three carboxyl groups (COOH). Hence the Krebs cycle is sometimes referred to as the tricarboxylic acid (TCA) cycle. The Krebs cycle begins with the condensation of one molecule of a compound called oxaloacetic acid and one molecule of acetyl CoA (a derivative of coenzyme A; see coenzyme). The acetyl portion of acetyl CoA is derived from pyruvic acid, which is produced by the degradation of glucose in glycolysis. After condensation, the oxaloacetic acid and acetyl CoA react to produce citric acid, which serves as a substrate for seven distinct enzyme-catalyzed reactions that occur in sequence and proceed with the formation of seven intermediate compounds, including succinic acid, fumaric acid, and malic acid. Malic acid is converted to oxaloacetic acid, which, in turn, reacts with yet another molecule of acetyl CoA, thus producing citric acid, and the cycle begins again. Each turn of the citric acid cycle produces, simultaneously, two molecules of carbon dioxide and eight atoms of hydrogen as byproducts. The carbon dioxide generated is an ultimate end product of glucose breakdown and is removed from the cell by the blood. The hydrogen atoms are donated as hydride ions to the system of electron transport molecules, which allow for oxidative phosphorylation. In most higher plants, in certain microorganisms, such as the bacterium Escherichia coli, and in the algae, the citric acid cycle is modified to a form called the glyoxylate cycle, so named because of the prominent intermediate, glyoxylic acid.

or Krebs cycle or citric-acid cycle

Last stage of the chemical processes by which living cells obtain energy from foodstuffs. Described by Hans Adolf Krebs in 1937, the reactions of the cycle have been shown in animals, plants, microorganisms, and fungi, and it is thus a feature of cell chemistry shared by all types of life. It is a complex series of reactions beginning and ending with the compound oxaloacetate. In addition to re-forming oxaloacetate, the cycle produces carbon dioxide and the energy-rich compound ATP. The enzymes that catalyze each step are located in mitochondria in animals, in chloroplasts in plants, and in the cell membrane in microorganisms. The hydrogen atoms and electrons that are removed from intermediate compounds formed during the cycle are channeled ultimately to oxygen in animal cells or to carbon dioxide in plant cells.

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The reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle) is a sequence of chemical reactions that are used by some bacteria to produce carbon compounds from carbon dioxide and water.

The reaction is basically the Citric acid cycle run in reverse. Where the Krebs cycle takes complex carbon molecules in the form of sugars and oxidizes them to CO₂ and water, the reverse cycle takes CO₂ and water to make carbon compounds. This process is used by some bacteria to synthesise carbon compounds, sometimes using hydrogen or sulphates as electron donors. In this process it can be seen as an alternative to the far more common photosynthesis production of organic molecules.

The reaction is one of the possible candidates for prebiotic early earth conditions and so is of interest in the origin of life research. It has been found that some of the steps can be catalysed by minerals.

See also

• Carbon fixation

References

External links

• Prebiotic Soup: Ugly, Yet Simmering with Metabolic Potential