Krebs cycle
Krishna. Wikimedia Commons (Public Domain)

Entries

Chemistry: Foundations and Applications BiologyWorld of Microbiology and ImmunologyThe Columbia Encyclopedia, 6th ed. Further reading

NON JS

Krebs Cycle

Krebs Cycle


The Krebs cycle is a series of enzymatic reactions that catalyzes the aerobic metabolism of fuel molecules to carbon dioxide and water, thereby generating energy for the production of adenosine triphosphate (ATP) molecules. The Krebs cycle is so named because much of its elucidation was the work of the British biochemist Hans Krebs. Many types of fuel molecules can be drawn into and utilized by the cycle, including acetyl coenzyme A (acetyl CoA), derived from glycolysis or fatty acid oxidation . Some amino acids are metabolized via the enzymatic reactions of the Krebs cycle. In eukaryotic cells , all but one of the enzymes catalyzing the reactions of the Krebs cycle are found in the mitochondrial matrixes.

The sequence of events known as the Krebs cycle is indeed a cycle; oxaloacetate is both the first reactant and the final product of the metabolic pathway (creating a loop). Because the Krebs cycle is responsible for the ultimate oxidation of metabolic intermediates produced during the metabolism of fats, proteins, and carbohydrates, it is the central mechanism for metabolism in the cell. In the first reaction of the cycle, acetyl CoA condenses with oxaloacetate to form citric acid. Acetyl CoA utilized in this way by the cycle has been produced either via the oxidation of fatty acids, the breakdown of certain amino acids, or the oxidative decarboxylation of pyruvate (a product of glycolysis). The citric acid produced by the condensation of acetyl CoA and oxaloacetate is a tricarboxylic acid containing three carboxylate groups. (Hence, the Krebs cycle is also referred to as the citric acid cycle or tricarboxylic acid cycle.)

After citrate has been formed, the cycle machinery continues through seven distinct enzyme-catalyzed reactions that produce, in order, isocitrate, α -ketoglutarate, succinyl coenzyme A, succinate, fumarate, malate, and

oxaloacetate. The freshly produced oxaloacetate, in turn, reacts with yet another molecule of acetyl CoA, and the cycle begins again. Each turn of the Krebs cycle produces two molecules of carbon dioxide, one guanosine triphosphate molecule (GTP), and enough electrons to generate three molecules of NADH and one molecule of FADH2.

The Krebs cycle is present in virtually all eukaryotic cells that contain mitochondria, but functions only as part of aerobic metabolism (when oxygen is available). This oxygen requirement is owing to the close relationship between the mitochondrial electron transport chain and the Krebs cycle. In the Krebs cycle, four oxidationreduction reactions occur. A high energy phosphate bond in the form of GTP is also generated. (This high energy phosphate bond is later transferred to adenosine diphosphate [ADP] to form adenosine triphosphate [ATP].) As the enzymes of the Krebs cycle oxidize fuel molecules to carbon dioxide, the coenzymes NAD+, FAD, and coenzyme Q (also known as ubiquinone) are reduced. In order for the cycle to continue, these reduced coenzymes must become reoxidized by transferring their electrons to oxygen, thus producing water. Therefore, the final acceptor of the electrons produced by the oxidation of fuel molecules as part of the Krebs cycle is oxygen. In the absence of oxygen, the Krebs cycle is inhibited.

The citric acid cycle is an amphibolic pathway, meaning that it can be used for both the synthesis and degradation of biomolecules. Besides acetyl CoA (generated from glucose , fatty acids, or ketogenic amino acids), other biomolecules are metabolized by the cycle. Several amino acids are degraded to become what are intermediates of the cycle. Likewise, odd-chain fatty acids are metabolized to form succinyl coenzyme A, another intermediate of the cycle. Krebs cycle intermediates are also used by many organisms for the synthesis of other important biomolecules. Some organisms use the Krebs cycle intermediates α -ketoglutarate and oxaloacetate in the synthesis of several amino acids. Succinyl coenzyme A is utilized in the synthesis of porphyrin rings, used in heme manufacture and chlorophyll biosynthesis . Oxaloacetate and malate are utilized in the synthesis of glucose, in a process known as gluconeogenesis.

see also Glycolysis; Krebs, Hans Adolf.

Robert Noiva

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W. H. Freeman.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

Noiva, Robert. "Krebs Cycle." Chemistry: Foundations and Applications. 2004. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

Noiva, Robert. "Krebs Cycle." Chemistry: Foundations and Applications. 2004. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1G2-3400900280.html

Noiva, Robert. "Krebs Cycle." Chemistry: Foundations and Applications. 2004. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3400900280.html

Krebs Cycle

Krebs Cycle

When glucose is converted to pyruvate during glycolysis , two adenosine triphosphates (ATPs ) are formed, but most of the energy in the original glucose remains in pyruvate. In most aerobic cells, the pyruvate formed by glycolysis is further degraded in a pathway called the Krebs cycle (also called the tricarboxylic acid cycle or citric acid cycle). In the Krebs cycle, the carbon of pyruvate is fully oxidized to carbon dioxide in a series of oxidationreduction reactions. During these reactions, much of the energy in the original pyruvate is carried as high-energy electrons by the electron shuttles NADH and FADH2. These electrons will ultimately be passed to the electron transport chain, where their energy will be used to synthesize ATP by oxidative phosphorylation . Much more ATP is made by the Krebs cycle and oxidative phosphorylation than by glycolysis alone.

In eukaryotic cells , pyruvate is transported to the mitochondrial matrix , where the Krebs cycle takes place. Before entering the Krebs cycle, the three-carbon pyruvate is oxidized to a two-carbon acetate molecule and carbon dioxide, producing one molecule of NADH. The acetate joins to a molecule of coenzyme A to form acetyl coenzyme A, which carries the acetyl group to the Krebs cycle. The acetate enters the cycle by combining with OAA (oxaloacetic acid) to form citric acid. At this point, two of the original three carbon atoms in pyruvate have been incorporated into citric acid and one has been oxidized to carbon dioxide, and one molecule of NADH has been produced.

As the reactions of the Krebs cycle continue, the two acetyl carbons are successively oxidized to carbon dioxide, forming two molecules of NADH and one of FADH2, which will provide electrons to the electron transport chain to form ATP. In addition, one guanosine triphosphate (GTP) is formed directly by substrate-level phosphorylation , or transfer of a phosphate directly from the reacting molecules. (The GTP eventually transfers its phosphate to form ATP.) The final unoxidized product of the entire cycle is OAA, which can accept another acetyl group to start the cycle again.

The Krebs cycle occupies a central position in cellular metabolism . It can break down the pyruvate produced in glycolysis, but these two pathways do not form an isolated system in cells. Both are linked to other processes in many ways. Acetyl coenzyme A is produced by other means, notably by fatty-acid oxidation, and the Krebs cycle will oxidize this acetyl coenzyme A as readily as that produced from pyruvate.

Similarly, other substances are fed into the Krebs cycle at this and other points, either to be consumed as fuel or to be transformed for other cellular needs. For example, amino acids can be consumed by entering the Krebs cycle at several points. Conversely, several amino acids can be synthesized from intermediates of the Krebs cycle. Thus the Krebs cycle can serve either to degrade amino acids, releasing energy in the process, or to supply precursor molecules for amino acid synthesis. Which of these activities prevails depends on the needs of the cell at any particular time.

see also Glycolysis and Fermentation; Metabolism, Cellular; Mitochondrion; Oxidative Phosphorylation

David W. Tapley

Bibliography

Bodner, G. M. "The Tricarboxylic Acid (TCA), Citric Acid or Krebs Cycle." Journal of Chemical Education 63 (1986): 673677.

Hinkle, P. C., and R. E. McCarty. "How Cells Make ATP." Scientific American 238 (March 1978).

Racker, E. "The Membrane of the Mitochondrion." Scientific American 218 (February 1968).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

Tapley, David W.. "Krebs Cycle." Biology. 2002. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

Tapley, David W.. "Krebs Cycle." Biology. 2002. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1G2-3400700247.html

Tapley, David W.. "Krebs Cycle." Biology. 2002. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3400700247.html

Krebs Cycle

Krebs cycle

The Krebs cycle is a set of biochemical reactions that occur in the mitochondria. The Krebs cycle is the final common pathway for the oxidation of food molecules such as sugars and fatty acids. It is also the source of intermediates in biosynthetic pathways, providing carbon skeletons for the synthesis of amino acids, nucleotides, and other key molecules in the cell. The Krebs cycle is also known as the citric acid cycle, and the tricarboxylic acid cycle. The Krebs cycle is a cycle because, during its course, it regenerates one of its key reactants.

To enter the Krebs cycle, a food molecule must first be broken into two-carbon fragments known as acetyl groups, which are then joined to the carrier molecule coenzyme A (the A stands for acetylation). Coenzyme A is composed of the RNA nucleotide adenine diphosphate, linked to a pantothenate, linked to a mercaptoethylamine unit, with a terminal S-H.Dehydration of this linkage with the OH of an acetate group produces acetyl CoA. This reaction is catalyzed by pyruvate dehydrogenase complex, a large multienzyme complex.

The acetyl CoA linkage is weak, and it is easily and irreversibly hydrolyzed when Acetyl CoA reacts with the four-carbon compound oxaloacetate. Oxaloacetate plus the acetyl group form the six-carbon citric acid, or citrate. (Citric acid contains three carboxylic acid groups, hence the alternate names for the Krebs cycle.)

Following this initiating reaction, the citric acid undergoes a series of transformations. These result in the formation of three molecules of the high-energy hydrogen carrier NADH (nicotinamide adenine dinucleotide), 1 molecule of another hydrogen carrier FADH2 (flavin adenine dinucleotide), 1 molecule of high-energy GTP (guanine triphosphate), and 2 molecules of carbon dioxide, a waste product. The oxaloacetate is regenerated, and the cycle is ready to begin again. NADH and FADH2 are used in the final stages of cellular respiration to generate large amounts of ATP.

As a central metabolic pathway in the cell, the rate of the Krebs cycle must be tightly controlled to prevent too much, or too little, formation of products. This regulation occurs through inhibition or activation of several of the enzymes involved. Most notably, the activity of pyruvate dehydrogenase is inhibited by its products, acetyl CoA and NADH, as well as by GTP. This enzyme can also be inhibited by enzymatic addition of a phosphate group, which occurs more readily when ATP levels are high. Each of these actions serves to slow down the Krebs cycle when energy levels are high in the cell. It is important to note that the Krebs cycle is also halted when the cell is low on oxygen, even though no oxygen is consumed in it. Oxygen is needed further along in cell respiration though, to regenerate NAD+ and FAD. Without these, the cycle cannot continue, and pyruvic acid is converted in the cytosol to lactic acid by the fermentation pathway.

The Krebs cycle is also a source for precursors for biosynthesis of a number of cell molecules. For instance, the synthetic pathway for amino acids can begin with either oxaloacetate or alpha-ketoglutarate, while the production of porphyrins, used in hemoglobin and other proteins, begins with succinyl CoA. Molecules withdrawn from the cycle for biosynthesis must be replenished. Oxaloacetate, for instance, can be formed from pyruvate, carbon dioxide, and water, with the use of one ATP molecule.

See also Mitochondria and cellular energy

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Krebs Cycle." World of Microbiology and Immunology. 2003. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

"Krebs Cycle." World of Microbiology and Immunology. 2003. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1G2-3409800335.html

"Krebs Cycle." World of Microbiology and Immunology. 2003. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3409800335.html

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.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Krebs cycle." The Columbia Encyclopedia, 6th ed.. 2016. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

"Krebs cycle." The Columbia Encyclopedia, 6th ed.. 2016. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1E1-Krebscyc.html

"Krebs cycle." The Columbia Encyclopedia, 6th ed.. 2016. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-Krebscyc.html

Krebs cycle

Krebs cycle (citric acid cycle; tricarboxylic acid cycle; TCA cycle) A cyclical series of biochemical reactions that is fundamental to the metabolism of aerobic organisms, i.e. animals, plants, and many microorganisms (see illustration). The enzymes of the Krebs cycle are located in the mitochondria and are in close association with the components of the electron transport chain. The two-carbon acetyl coenzyme A (acetyl CoA) reacts with the four-carbon oxaloacetate to form the six-carbon citrate. In a series of seven reactions, this is reconverted to oxaloacetate and produces two molecules of carbon dioxide. Most importantly, the cycle generates one molecule of guanosine triphosphate (GTP – equivalent to 1 ATP) and reduces three molecules of the coenzyme NAD to NADH and one molecule of the coenzyme FAD to FADH2. NADH and FADH2 are then oxidized by the electron transport chain to generate three and two molecules of ATP respectively (depending on the values of their respective P/O ratios). This gives a net yield of 12 molecules of ATP per molecule of acetyl CoA.

Acetyl CoA can be derived from carbohydrates (via glycolysis), fats, or certain amino acids. (Other amino acids may enter the cycle at different stages.) Thus the Krebs cycle is the central ‘crossroads’ in the complex system of metabolic pathways and is involved not only in degradation and energy production but also in the synthesis of biomolecules. It is named after its principal discoverer, Hans Krebs.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Krebs cycle." A Dictionary of Biology. 2004. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

"Krebs cycle." A Dictionary of Biology. 2004. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1O6-Krebscycle.html

"Krebs cycle." A Dictionary of Biology. 2004. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O6-Krebscycle.html

Krebs Cycle

KREBS CYCLE

The Krebs cycle is a series of biochemical changes that occur during the metabolism of nutrients, facilitating the storage of energy for further use. It is named after Hans Adolph Krebs (19001981), the biochemist who identified it. The alternative, and more descriptive, name is the tricarboxylic, or citric acid, cycle. The fundamental process involves oxidizing acetate molecules to carbon dioxide (CO2) and water with transfer of the metabolic energy to "high energy" bonds for later use by the body. In the process, acetate is attached biochemically to a dicarboxylic acid to produce citric acidsthe tricarboxylic acid from which the cycle derives its name. The citric acid then goes through a number of biochemical steps to oxidize the two carbons from acetate, and to regenerate the dicarboxylic acid to which the acetate was originally attached.

George A. Bray

(see also: Energy; Nutrition )

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

Bray, George A.. "Krebs Cycle." Encyclopedia of Public Health. 2002. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

Bray, George A.. "Krebs Cycle." Encyclopedia of Public Health. 2002. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1G2-3404000477.html

Bray, George A.. "Krebs Cycle." Encyclopedia of Public Health. 2002. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3404000477.html

Krebs cycle

Krebs cycle (citric acid or tricarboxylic acid cycle) Biochemical pathway by which most eukaryote organisms, including animals and plants, obtain much of their energy by oxidizing foodstuffs. Occurring in the mitochondria of cells, the Krebs cycle comprises a number of complex chemical reactions, many of which release energy, in association with a process called the electron transport system, as adenosine triphosphate (ATP) becomes adenosine diphosphate (ADP). ATP provides chemical energy for metabolic reactions. The Krebs cycle is an essential part of the process of cell respiration and metabolism. It is named after British biochemist Sir Hans Krebs. See also mitochondrion

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Krebs cycle." World Encyclopedia. 2005. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

"Krebs cycle." World Encyclopedia. 2005. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1O142-Krebscycle.html

"Krebs cycle." World Encyclopedia. 2005. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-Krebscycle.html

Krebs cycle

Krebs cycle (citric acid cycle) (krebz) n. a complex cycle of enzyme-catalysed reactions, occurring within the cells of all living animals, in which acetate is broken down to produce energy in the form of ATP and carbon dioxide. The cycle is the final step in the oxidation of carbohydrates, fats, and proteins. [ Sir H. A. Krebs (1900–81), German-born biochemist]

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Krebs cycle." A Dictionary of Nursing. 2008. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

"Krebs cycle." A Dictionary of Nursing. 2008. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1O62-Krebscycle.html

"Krebs cycle." A Dictionary of Nursing. 2008. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O62-Krebscycle.html

Krebs cycle

Krebs' cycle Or citric acid cycle, a central pathway for the metabolism of fats, carbohydrates, and amino acids.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

DAVID A. BENDER. "Krebs cycle." A Dictionary of Food and Nutrition. 2005. Encyclopedia.com. 29 Jul. 2016 <http://www.encyclopedia.com>.

DAVID A. BENDER. "Krebs cycle." A Dictionary of Food and Nutrition. 2005. Encyclopedia.com. (July 29, 2016). http://www.encyclopedia.com/doc/1O39-Krebscycle.html

DAVID A. BENDER. "Krebs cycle." A Dictionary of Food and Nutrition. 2005. Retrieved July 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O39-Krebscycle.html

Facts and information from other sites