carbohydrate metabolism

(biochemistry) The sum of the biochemical and physiological processes involved in the breakdown and synthesis of simple sugars, oligosaccharides, and polysaccharides and in the transport of sugar across cell membranes.

Many aspects of biochemistry and physiology have to do with the breakdown and synthesis of simple sugars, oligosaccharides, and polysaccharides, and with the transport of sugars across cell membranes and tissues. The breakdown or dissimilation of simple sugars, particularly glucose, is one of the principal sources of energy for living organisms. The dissimilation may be anaerobic, as in fermentations, or aerobic, that is, respiratory. In both types of metabolism, the breakdown is accompanied by the formation of energy-rich bonds, chiefly the pyrophosphate bond of the coenzyme adenosine triphosphate (ATP), which serves as a coupling agent between different metabolic processes. In higher animals, glucose is the carbohydrate constituent of blood, which carries it to the tissues of the body. In higher plants, the disaccharide sucrose is often stored and transported by the tissues. Certain polysaccharides, especially starch and glycogen, are stored as endogenous food reserves in the cells of plants, animals, and microorganisms. Others, such as cellulose, chitin, and bacterial polysaccharides, serve as structural components of cell walls. As constituents of plant and animal tissues, various carbohydrates become available to those organisms which depend on other living or dead organisms for their source of nutrients. Hence, all naturally occurring carbohydrates can be dissimilated by some animals or microorganisms. See also Adenosine triphosphate (ATP); Carbohydrate; Cellulose; Chitin; Glycogen; Starch.

Certain carbohydrates cannot be used as nutrients by humans. For example, cellulose cannot be digested by humans or other mammals and is a useful food only for those, such as the ruminants, that harbor cellulose-decomposing microorganisms in their digestive tracts. The principal dietary carbohydrates available to humans are the simple sugars glucose and fructose, the disaccharides sucrose and lactose, and the polysaccharides glycogen and starch. Lactose is the carbohydrate constituent of milk and hence one of the main sources of food during infancy. The disaccharides and polysaccharides that cannot be absorbed directly from the intestine are first digested and hydrolyzed by enzymes, glycosidases, secreted into the alimentary canal. See also Fructose; Lactose.

The simple sugars reach the intestine or are produced there through the digestion of oligosaccharides. They are absorbed by the intestinal mucosa and transported across the tissue into the bloodstream. This process involves the accumulation of sugar against a concentration gradient and requires active metabolism of the mucosal tissue as a source of energy. The sugars are absorbed from the blood by the liver and are stored there as glycogen. The liver glycogen serves as a constant source of glucose in the bloodstream. The mechanisms of transport of sugars across cell membranes and tissues are not yet understood, but they appear to be highly specific for different sugars and to depend on enzymelike components of the cells.

The degradation of monosaccharides may follow one of several types of metabolic pathways. In the phosphorylative pathways, the sugar is first converted to a phosphate ester (phosphorylated) in a reaction with ATP. The phosphorylated sugar is then split into smaller units, either before or after oxidation. In the nonphosphorylative pathways, the sugar is usually oxidized to the corresponding aldonic acid. This may subsequently be broken down either with or without phosphorylation of the intermediate products. Among the principal intermediates in carbohydrate metabolism are glyceraldehyde-3-phosphate and pyruvic acid. The end products of metabolism depend on the organism and, to some extent, on the environmental conditions. Besides cell material the products may include carbon dioxide (CO₂), alcohols, organic acids, and hydrogen gas. In the so-called complete oxidations, CO₂ is the only excreted end product. In incomplete oxidations, characteristic of the vinegar bacteria and of certain fungi, oxidized end products such as gluconic, ketogluconic, citric, or fumaric acids may accumulate. Organic end products are invariably found in fermentations. The amount of biosynthesis and mechanical work that an organism can do at the expense of a given amount of sugar is many times greater in respiration than in fermentation. See also Fermentation; Respiration.

The principal phosphorylative pathway involved in fermentations is known as the glycolytic, hexose diphosphate, or Embden-Meyerhof pathway (see illustration). This sequence of reactions is the basis of the lactic acid fermentation of mammalian muscle and of the alcoholic fermentation of yeast. For every molecule of glucose fermented through the glycolytic sequence, two molecules of ATP are used for phosphorylation, while four are produced. Thus, fermentation results in the net gain of two energy-rich phosphate bonds as ATP at the expense of inorganic phosphate esterified. The excess ATP is converted back to ADP and inorganic phosphate through coupled reactions useful to the organism, such as the mechanical work done by the contraction of muscle or biosynthetic reactions associated with growth. See also Adenosine diphosphate (ADP); Nicotinamide adenine dinucleotide (NAD).

Glycolysis in lactic acid fermentation.

The oxidative or respiratory metabolism of sugars differs in several respects from fermentative dissimilation. First, the oxidative steps, that is, the reoxidation of NADH, are linked to the reduction of molecular oxygen. Second, the pyruvic acid produced through glycolytic or other mechanisms is further oxidized, usually to CO₂ and H₂O. Third, in most aerobic organisms, alternative pathways either supplement or completely replace the glycolytic sequence of reactions for the oxidation of sugars. Where pyruvic acid appears as a metabolic intermediate, it is generally oxidatively decarboxylated to yield CO₂ and the two-carbon acetyl fragment which combines with coenzyme A. The acetyl group is then further oxidized via the Krebs cycle. The principal alternative pathways by which sugars are dissimilated involve the oxidation of glucose-6-phosphate to the lactone of 6-phosphogluconic acid and are known as the hexose monophosphate pathways. See also Citric acid cycle.

The metabolism of simple sugars other than glucose usually involves the conversion of the sugar to one of the intermediates of the phosphorylative pathways described for glucose metabolism. For example, fructose may be phosphorylated to fructose-6-phosphate, which can then be degraded via the glycolytic pathway or converted to glucose-6-phosphate and oxidized through the hexose monophosphate pathway.

The dissimilation and biosynthesis of the oligosaccharides are effected through the enzymatic cleavage or formation of glycosidic bonds between simple monosaccharide constituents of the complex carbohydrates. The principal types of enzyme which split or synthesize glycosidic bonds are the hydrolases or glycosidases, phosphorylases, and transglycosylases. The enzymes are generally highly specific with respect to the glycosidic portion, or moiety, and the type of linkage of the substrates which they attack. The essential function of all three types of enzymes is the transfer of the glycosyl moiety of the substrate to an appropriate glycosyl acceptor. The phosphorylases catalyze the reversible phosphorolysis of certain disaccharides, polysaccharides, and nucleosides by transferring the glycosyl moieties to inorganic phosphate. The breakdown of glycogen and starch by the enzymes known as amylophophorylases is an example of biologically important phosphorolytic reactions.

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