Polysaccharides glycogen

In a second class of regulatory enzymes the active and inactive forms are inter-converted by covalent modifications of their structures by enzymes. The classic example of this type of control is the use of glycogen phosphorylase from animal tissues to catalyse the breakdown of the polysaccharide glycogen yielding glucose-1-phosphate, as illustrated in Fig. 5.37. 

Structure of the storage polysaccharides glycogen and starch. The main chain is a(l,4)-linked. Side chains are connected to the main chain by a(I,6) linkages. 

When fructose-6-phosphate is generated by gluconeogenesis or photosynthesis (see chapter 15), an equimolar amount of glucose-1-phosphate is usually removed from the hexose monophosphate pool by conversion to storage polysaccharide (glycogen in animals and many kinds of microorganisms starch in green plants). 

Polysaccharide glycogen (animals) and starch (plants) produced for energy storage from glucose... 

When an animal eats more carbohydrate than it uses up, it stores the excess some as the polysaccharide glycogen (Sec. 35.9), but most of it as fats. Fats, we know (Sec. 33.2), are triacylglycerols, esters derived (in most cases) from long straight-chain carboxylic acids containing an even number of carbon atoms. These even numbers, we said, are a natural consequence of the way fats are synthesized in biological systems. 

See also Phosphorolysis, Glycogen phosphorylase. Figure 13.18, Kinase Cascade, Figure 13.16, Figure 13.17, Polysaccharides, Glycogen Breakdown, Hydrolysis vs Phosphorolysis, Glycogen Breakdown Regulation... 

Early structural studies (63) on starch indicated an a-(1 4)-linked glucan with some a-(1 6) branches, but it was not realized until considerably later (64) that starch is composed of two polymers, a fully linear component termed amylose and a branched component in which a-(1 6) branches lead to a tree-like structure (amylopectin). The chemistry and technology of starch is detailed in two major treatises (65,66). The animal storage polysaccharide glycogen has a structure similar to that of amylopectin, but is more highly branched (67). 

Figure 7.18 A simplified representation of the branched polysaccharide glycogen (branches every 8-12 glucose units). Amylopectin is much less densely branched (branches every 24—30 glucose units). Each small hexagon represents a single glucose unit.

In contrast to the neutral polysaccharides, the carbohydrate skeleton of heparin and mucopolysaccharides is based on an amino sugar uronic acid repeating unit. An important biological difference is that the neutral polysaccharides (glycogen, starch) are metabolic stores, the typical mucopolysaccharides (chondroitin sulfuric acids, keratin sulfate) are important structural materials of connective tissue, and the naturally occurring heparins and heparinoids are trace substances and appear to be associated with special cells. 

Histological analysis on all the smdied tissues using hematoxylin and eosin as well as periodic Schiff stain indicated that GA-administered lean rats on normal diet had more intense magenta coloration compared to the control rats indicating increased deposition of polysaccharide, glycogen in the cells. Similar observatimis were seen in tissues from rats on high-sucrose or high-fat diet with GA [35, 36]. 

Phosphorylases have been described for the polysaccharides, glycogen and amylose, and for sucrose. Phosphorylases from both muscle and plants are able to cleave the 1, 4 -qj linkages of glycogen and starch to yield glucose-1-phosphate. Sucrose phosphorylase is a separate enzyme obtained from bacteria which produces glucose-l-phosphate and fructose from sucrose. Phosphorolysis has also been involved in the cleavage of the pentosidic... 

Glucose molecules are bound in starch by the easily hydrolyzed a bonds. The same type of bond can also be seen in the animal reserve polysaccharide glycogen. This is in contrast to many structural polysaccharides such as chitin, cellulose and peptidoglycan, which are bound by P-bonds and are much more resistant to hydrolysis. A starch branching enzyme introduces 1,6-a glycosidic bonds between these chains, creating the branched amylopectin (Figure 5.28). 

Carbohydrates.—About 60 to 80 per cent, of the solids of the human diet are carbohydrates, represented by four sugars (glucose, fructose, lactose, and sucrose) and three polysaccharides, glycogen, starch, and cellulose. 

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