Carbohydrates are classified into four categories according to their chemical structure and degree of polymerization: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
Monosaccharides are the simplest form of carbohydrate and cannot be further hydrolyzed into smaller subunits. According to their chain length, the monosaccharides can be divided into several categories, the more nutritionally important being the pentoses, with skeletons containing five carbon atoms (e.g., ribose), and the hexoses, with skeletons containing six carbon atoms (e.g., glucose).
The presence of asymmetrical carbon atoms in monosaccharides with different functional groups attached gives rise to optical activity, meaning that if polarized light is passed through a solution of these compounds, the plane of light will be rotated to the left (levorotatory or l form) or to the right (dextrorotatory or d form). As a result, mirror-imaged structures of the same compound exist and are called stereoisomers. Monosaccharides of the d form are more nutritionally important because the metabolic and digestive enzymes are specific for the d stereoisomers.
Monosaccharides demonstrate another type of stereoisomerism due to their formation of cyclic structures. The pentoses form furanose (five-membered rings), and the hexoses form pyranose (six-membered rings). Cyclization can produce two stereoisomers (the a and fi configurations), and generally monosaccharide solutions contain an equilibrium mixture of these two forms. Figure 1 illustrates D-glucose in its pyranose form in the a and fi configurations. The isomerization produces compounds with different properties and has major metabolic importance, because of enzyme specificity for particular stereoisomers.
Glucose is the most abundant monosaccharide, and is a major cell fuel in the human body, and can be found unbound in body tissues and fluids. Glucose is the building block of several polysacchar-ides. Galactose and fructose are also used as cell
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