Oct 29 2011

Carbohydrates: More than just calories

Published by at 12:51 pm
Under Glycobiology | Glycomics | Molecular Biology

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). Nevertheless, carbohydrates are important constituents of the human diet, accounting for a high percentage of the calories consumed. Thus some 40 percent of the calorie intake of Americans (and some 50 percent of that of Britons and Israelis) is in the form of carbohydrates: glucose, fructose, lactose (milk sugar, a disaccharide of glucose and galactose), sucrose, and starch.

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 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.

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 N-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. (1)

The name carbohydrate was originally assigned to compounds thought to be hydrates of carbon, that is, to consist of carbon, hydrogen, and oxygen. They are typical hexose monosaccharides, meaning that they have six carbon atoms. However, 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).

The biological roles of carbohydrates are particularly important in the assembly of complex multicellular organs and organisms, which requires interactions between cells and the surrounding matrix. All cells and numerous macromolecules in nature carry an array of covalently attached sugars (monosaccharides) or sugar chains (oligosaccharides and polysaccharides), the latter that are generically referred to as “glycans.” (2)

Localization of glycoconjugates in the intracellular and extracellular compartments.

Because many carbohydrates are on the outer surface of cellular and secreted macromolecules, and are often freestanding entities, they are in a position to modulate or mediate a wide variety of events in cell–cell, cell–matrix, and cell–molecule interactions critical to the development and function of a complex multicellular organism. 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. They can also act as mediators in the interactions between different organisms (for example, between host and a parasite). In addition, simple, rapidly turning over, protein-bound glycans are abundant within the nucleus and cytoplasm, where they can serve as regulatory switches. A more complete paradigm of molecular biology must therefore include glycans, often in covalent combination with other macromolecules, (glycoconjugates) such as glycoproteins and glycolipids. (3) The term glycan may also be used to refer to the carbohydrate portion of a glycoconjugate, such as a glycoprotein, glycolipid, or a proteoglycan.

During the initial phase of the molecular biology revolution of the 1960s and 1970s, studies of glycans lagged far behind those of other major classes of molecules. This was in large part due to their inherent structural complexity and the great difficulty in determining their sequences. Also inhibiting interest was the fact that their biosynthesis could not be directly predicted from a DNA template. In addition, unlike genome products, glycans are highly dynamic and have extraordinarily complex biosynthetic pathways. The development of many new technologies for exploring the structures and functions of glycans has since opened a new frontier of molecular biology. The coming together of the traditional disciplines of carbohydrate chemistry and biochemistry with a modern understanding of the cell and molecular biology of glycans, and in particular, their conjugates with proteins and lipids, is called “glycobiology.” (4)

Analogous to genomics and proteomics, glycomics represents the systematic methodological elucidation of the “glycome” (the totality of glycan structures) of a given cell type or organism. The glycome, a subset of glycobiology, is immense and far more complex than the genome or proteome. In the past decade, over 30 genetic diseases have been identified that alter glycan synthesis and structure, and ultimately the function of nearly all organ systems. Many of the causal mutations affect key biosynthetic enzymes, but more recent discoveries point to defects in chaperones and Golgi-trafficking complexes that impair several glycosylation pathways. As more glycosylation disorders and patients with these disorders are identified, the functions of the glycome are starting to be revealed. (5,6)

  1. Sharon N. Carbohydrates Sci. Am. 245: (5) 90-116. 1980
  2. Varki A and Sharon N. Historical Background and Overview: Varki A, Cummings R, Esko J, Freeze H, Stanley P, Bertozzi C, Hart G, and Etzler M. Essentials of Glycobiology, 2nd edition, Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 2009.
  3. Ibid.
  4. Rademacher TW, Parekh RB, Dwek RA. Glycobiology. Annu Rev Biochem. 1988;57:785-838
  5. Freeze HH. Genetic defects in the human glycome. Nat Rev Genet. 2006 Jul; 7(7):537-51.
  6. Taylor ME and Drickamer. Introduction to Glycobiology. Oxford University Press 2nd Edition 2006

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