C O N T E N T S
Carbohydrates are sugars or (like starch and cellulose) chains of sugars. To most people sugar is the common household foodstuff, which to the chemist is sucrose. Chemically the molecule of sucrose consists of two monosaccharides, or simple sugars, glucose and fructose, that are hooked together; it is thus a disaccharide. More than 20 different monosaccharides have been found in nature, all of which are chemically related to glucose or fructose. As a rule they are black crystalline solids tha dissolve readily in water. Some of then have not been obtained in amounts sufficient for testing their sweetness, but the are still called sugars, as are the monosaccharides that are found to be not sweet.
Glucose is the best-known monosaccharide?; indeed, it has probably been investigated more thoroughly than any other organic compound. It was undoubtedly known to the ancients because of its occurrence in granulated honey and wine must. References to grape sugar, which is glucose, are to be found in Moorish writings of the 12th century.
In 1747 the German pharmacist Andreas Marggraf, whose isolation of pure sucrose from sugar beets is an example of the chemical art of the time at its best, wrote of isolating from raisins "eine Art Zucker" (a type of sugar) different from cane sugar; it was what is now called glucose. The action of acids on starch was shown to produce a sweet syrup from which a crystalline sugar was isolated by Constantine Kirchoff in 1811. Later workers established that the sugar in grapes is identical with the sugar found in honey, in the urine of diabetics and in acid hydrolysates of starch and cellulose. The French chemist Jean Baptiste Andre Dumas gave it the name glucose in 1838. The structure of glucose and of several other monosaccharides, including fructose, galactose? and mannose, was established by about 1900, mainly by the brilliant work of the German chemist Emil Fischer, who thereby laid the foundations of carbohydrate chemistry.
Water of Carbon
The name carbohydrate was originally assigned to compounds thought to be hydrates of carbon, that is, to consist of carbon, hydrogen and oxygen in the general formula C,(H20),. Indeed, glucose and other simple sugars such as galactose, mannose and fructose do have the general formula C61-11206. They are typical hexose monosaccharides, meaning that they have six carbon atoms. With the accumulation of more data the definition has been modified and broadened to encompass numerous compounds with little or no resemblance to the original "water of carbon." Carbohydrates now include polyhydroxy aldehydes, ketones, alcohols, acids and amines, their simple derivatives and the products formed by the condensation of these different compounds through glycosidic linkages (essentially oxygen bridges) into oligomers (oligosaccharides?) and polymers (polysaccharides?). (1)
Much of the current interest in carbohydrates is focused on such substances as glycoproteins and glycolipids?, complex carbohydrates in which sugars are linked respectively to proteins and lipids. They are termed glycoconjugates. It should also be noted that in the excitement about nucleic acids a simple fact is being forgotten: they too are complex carbohydrates, since monosaccharides are among their major constituents (ribose in RNA and deoxyribose in DNA).
Carbohydrates are the most abundant group of biological compounds on the earth, and the most abundant carbohydrate is cellulose, a polymer of glucose; it is the major structural material of plants. Another abundant carbohydrate is chitin, a polymer of acetylglucosamine; it is the major organic component of the exoskeleton of arthropods such as insects, crabs and lobsters which make up the largest class of organisms, comprising some 900,000 species (more than are found in all other families and classes together). It has been estimated that millions of tons of chitin are formed yearly by a single species of crab!
Carbohydrates are the fuel of life, being the main source of energy for living organisms and the central pathway of energy storage and supply for most cells. They are the major products through which the energy of the sun is harnessed and converted into a form that can be utilized by living organisms. According to rough estimates, more than 100 billion tons of carbohydrates are formed each year on the earth from carbon dioxide and water by the process of photosynthesis. Polymers of glucose, such as the starches and the glycogens, are the mediums for the storage of energy in plants and animals respectively. Coal, peat and petroleum were probably formed from carbohydrates by microbiological and chemical processes.
Carbohydrates comprise only about 1 percent of the human body; proteins comprise 15 percent, fatty substances 15 percent and inorganic substances 5 percent (the rest being water).
The four major classes of compounds essential to life are nucleic acids, proteins, lipids and carbohydrates. over the past 30 years the first three classes have received much attention from chemists and biologists, whereas during most of that time the carbohydrates were largely neglected, partly in the belief that their chemistry and biology had been fully worked out. In the past decade, however, research on carbohydrates has been revived and is now expanding rapidly. As a result of many new developments carbohydrate research is today broad and diverse.
A common theme behind many of the recent findings, which is also a powerful driving force in carbohydrate research, is the realization that monosaccharides (the basic units of carbohydrates) can serve, as nucleotides and amino acids do, as code words in the molecular language of life, so that the specificity of many natural compounds is written in monosaccharides.(2)
Blood group antigens as structural carbohydrates
The ABO blood-group system was first described by Karl Landsteiner of the Rockefeller Institute in 1900, but it was not until 1953 that Walter Morgan and Winifred Watkins of the Lister Institute demonstrated that the specificity of the major blood types is determined by sugars. For example, the difference between the blood types A and B lies in a single sugar unit that sticks out from the end of a carbohydrate chain of a glycoprotein or glycolipid on the surface of the red blood cell. In blood type A the determinant is acetylgalactosamine, in blood type B it is galactose. The two monosaccharides differ by only a small group of atoms, but that little difference is sometimes a matter of life and death, since using the wrong type of blood in a transfusion can have fatal results.(3)
The blood group antigens have been dismissed by some researchers as merely 'icing on the cake' of glycoprotein structures. The fact that there are no lethal mutations and individuals have been described lacking ABO, H and Lewis antigens seems to lend weight to the argument. Research suggests that these antigens do indeed have function and argues that blood group antigens play important roles in modulation of protein activity, infection and cancer.((4)
1. Paulson. J.C. 1989. Glycoproteins: What are the sugar side chains for? Trends Biochem. Sci. 14: 272-276.
2. Sharon, N, and Lis, H . Carbohyrates in cell recognition; Scientific American, January, p. 74.
3. Sharon N. 1980. Carbohydrates Sci. Am. 245: (5) 90-116. (PubMed)
4. Greenwell P.Blood group antigens: molecules seeking a function? Glycoconj J. 1997 Feb;14(2):159-73.
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