Malic Acid

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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.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Malic Acid

 

(also called hydroxysuccinic acid), HOOCCH2CH(OH)COOH, a dibasic hydroxycarboxylic acid. Malic acid takes the form of a colorless, hygroscopic, crystalline compound that is readily soluble in water and ethyl alcohol; it has a melting point of 100°C.

Malic acid was first isolated by K. Scheele, who in 1785 obtained it from unripe apples. The L-form of the acid is found in plants either in the free state or as acid salts; the presence of either the acid or salt makes possible the acid reaction of cell fluid. Fruits rich in malic acid include barberries, raspberries, apples, and the berries of the mountain ash; the vegetative organs of succulents, especially Crassulaceae, also contain considerable amounts of the acid. The tobacco plants Nicotiana rustica and Nicotiana tabacum contain the nicotine salts of the acid.

Malic acid is an intermediate in cell respiration—in the tricarboxylic acid cycle and its variant, the glyoxylate cycle. In plants containing considerable amounts of organic acids (for example, rhubarb, begonia, and dock), free ammonia is rendered harmless by the formation of the ammonium salts of organic acids, including malic acid. Malic acid is used by many microorganisms as an energy substrate or a source of carbon. It is formed as a by-product in various types of fermentation.

Malic acid is used in the production of fruit drinks and candies.

E. P. FEDENKO

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

malic acid

[′mal·ik ′as·əd]
(biochemistry)
COOH·CH2·CHOH·COOH Hydroxysuccinic acid: a dibasic hydroxy acid existing in two optically active isomers and a racemate form; found in apples and many other fruits.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.