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In a wide range of organisms, excess glucose is converted into polymeric forms for storage and transport. The principal storage forms of glucose are glycogen in vertebrates and many microorganisms, and starch in plants. In vertebrates, glucose itself is generally transported in the blood, but the transport form in plants is sucrose, or its galactosylated derivatives.
Although the intermediates in glycolysis and gluconeogenesis are sugar phosphates, many of the reactions in which hexoses are transformed or polymerized involve a different type of activating group, a nucleotide bound to the anomeric hydroxyl of the sugar through a phosphate ester linkage. Sugar nucleotides are the substrates for polymerization into disaccharides, glycogen, starch, cellulose, and more complex extracellular polysaccharides. They are also key intermediates in the production of aminohexoses and deoxyhexoses found in some of these polysaccharides. The role of sugar nucleotides (specifically UDP-glucose) in the biosynthesis of glycogen and many other carbohydrate derivatives was discovered by Luis Leloir.
The suitability of sugar nucleotides for biosynthetic reactions stems from several properties:
1. Their formation by the condensation of a nucleoside triphosphate with a hexose phosphate splits one high-energy bond and releases PPi, which is further hydrolyzed by inorganic pyrophosphatase; there is a net cleavage of two high-energy bonds (Fig. 19-11). The resulting large, negative, free-energy change drives the synthetic reaction and reflects a strategy common to many biological polymerization reactions.
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