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INTERCONVERSIONS AND ACTIVATION OF DIETARY SUGARS
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Glucose is a precursor of all sugars in the body
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Figure 25.6 Interconversions of glucoseView drug information, mannose, galactose, and their nucleotide sugars. Fuc, fucose.
Humans have a requirement for some essential amino acidsView drug information and fatty acids, but all the sugars in glycoconjugates can be synthesized from d-glucose. Figure 25.6 shows the reactions that occur in animal cells to convert glucoseView drug information to mannose or galactose. The latter is achieved by epimerization of the nucleoside diphosphate sugar, uridine diphosphate-glucose (UDP-Glc) (see Fig. 25.8), to UDP-galactose (UDP-Gal) by UDP-Gal 4-epimerase, providing a source of UDP-Gal for glycoconjugate biosynthesis. Glucose-6-phosphate (Glc-6-P), which is an early intermediate in that pathway, may also be converted to fructose-6-phosphate (Fru-6-P) by the glycolytic enzyme, phosphoglucose isomerase, and Fru-6-P can then be isomerized to mannose-6-phosphate (Man-6-P) by phosphomannose isomerase. Man-6-P is then converted by a mutase to mannose-1-phosphate (Man-1-P), which reacts with guanosine triphosphate (GTP) in a pyrophosphorylase reaction to form guanosine diphosphate-mannose (GDP-Man), the activated form of mannose. Mannose also occurs in the diet, although in small amounts; it is phosphorylated by hexokinase and enters metabolism through phosphomannose isomerase.
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