Carbohydrate polymers Glycogen

Glycogen phosphorylase, an enzyme involved in the metabolism of the carbohydrate polymer glycogen, catalyzes the reaction ... 

In mammalian cells, glucose is the most abundant carbohydrate energy source. It is metabolized in all cells as a glycolytic fuel and is stored in liver and muscle as the polymer glycogen. But certain cells have the enzymes to catalyze the synthesis of glucose under certain conditions. The requirements are (1) the availability of specific carbon skeletons (carbon backbone structures of various types), (2) energy, in the form of ATP, necessary to accomplish the sequence of reactions, and (3) the enzymes to catalyze reactions of the sequence. 

Very early reports on these systems described them as polycondensates, consisting of broad molar-mass distributions with randomly branched topologies. The methods of synthesis included Friedel-Crafts coupling of benzyl alcohols [108] and the polymerization of 2,5,6-tribromophenol involving aryl ether formation [109], In addition, hyperbranched natural carbohydrate polymers, such as amylopectin, dextrin, and glycogen have been extensively studied [73-75]. 

Numerous analogs of carbohydrate polymers (i.e., amylose, glycogen) have been prepared from modified monosaccharide 1-phosphates with phosphorylase (Fig. 13-11 shows the natural substrates) l159 162l. 

Although the carbohydrate glucose or its polymer glycogen is regarded as the fundamental fuel or nutrient, other carbohydrates may be involved, even nitrogen-containing amines, or amino acids. Thus, for example, there is the role of glutamine,... 

Matheson N.K., Caldwell R.A., a(l-4) Glucan chain disposition in models of a(l )(l-6) glucans Comparison with structural data for mammalian glycogen and waxy amylopectin, Carbohydr. Polym., 40,1999, 191-209. 

Carbohydrates in general may have a free OH-stretch absorption near 6940 cm (1440 nm). This band has been reported in crystalline sucrose, for example, and has been assigned specifically to the C4 hydroxyl within a crystalline matrix. Trott et al. discuss four different OH first overtone bands in carbohydrates in different solvent systems, using a monomer (glucose) and its polymer (glycogen) as models. 

From that point on, the sjmthesis of glucose is easy, being simply the reverse of glycolysis. Only the phosphorylation reactions of hexose by ATP are not reversible, but phosphate can be spht off the hexose diphosphate by specific phosphatases. Fiudihermore, the synthesis of carbohydrates does not end with free glucose but usually with the polymer glycogen. The polymerization, however, proceeds through phosphorylated intermediates. 

Glycogen is the major storage carbohydrate in animals, corresponding to starch in plants it is a branched polymer of a-D-glucose. It occurs mainly in liver (up to 6%) and muscle, where it rarely exceeds 1%. However, because of its greater mass, muscle contains about three to four times as much glycogen as does liver (Table 18—1). 

Carbohydrates are classified based upon the products formed when they are hydrolyzed. Monosaccharides are simple sugars that cannot be broken down into simpler sugars upon hydrolysis. Examples of monosaccharides are glucose, ribose, deoxyribose, and fructose. Disaccharides contain two monosaccharide units and yield two monosaccharides upon hydrolysis. Examples of disaccharides are lactose, maltose, and sucrose. Polysaccharides are polymers of monosaccharide units and yield many individual monosaccharides upon hydrolysis. Examples of polysaccharides are starch, glycogen, and cellulose. 

Carbohydrates mainly occur in food in the form of polymers (starches and glycogen). They are cleaved by pancreatic amylase into oligosaccharides and are then hydrolyzed by glycosidases, which are located on the surface of the intestinal epithelium, to yield monosaccharides. Glucose and galactose are taken up into the enterocytes by secondary active cotransport with Na"" ions (see p. 220). In addition, monosaccharides also have passive transport systems in the intestine. 

The complex carbohydrate glycogen, a polymer made of glucose monomer units, is found in animal tissue. 

Carbohydrates form the major structural components of the cell walls. The most common form is cellulose which makes up over 30 per cent of the dry weight of wood. Other structural forms are hemicellulose (a mixed polymer of hexose and pentose sugars), pectins and chitin. Apart from contributing to the structure, some polymers also act as energy storage materials in living systems. Glycogen and starch form the major carbohydrate stores of animals and plants, respectively. Carbohydrate structure, like that of nucleic acids and proteins, is complex, and various levels of structure can be identified. 

Biological Functions of Oligosaccharides and Polysaccharides. Polysaccharides serve two important biological roles. Glycogen and starch are polymers of glucose units linked in a(l —> 4) linkages that serve as carbohydrate reserves for animals, bacteria, and plants. Because these polymers are readily converted to intermediates for pathways that yield metabolic energy, they can also be... 

All bacteria which have been adequately studied have proved to possess polysaccharides, and, in most instances, these are much more complex than the D-glucose polymers which have attracted the lion s share of attention from carbohydrate chemists. More than twenty different monosaccharide units are known to occur in bacterial polysaccharides, and five or more different kinds of unit may be present in the same polymer. Considering that some of the finer structural details of starch and glycogen remain to be elucidated, it is not surprising that little of the structure of the complex heteropolysaccharides has yet been unravelled. There have also been difficulties in preparing sufficient purified material for chemical work and in... 

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