| 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 glucose , mannose, galactose, and their nucleotide sugars. Fuc, fucose. |
| Humans have a requirement for some essential amino acids 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 glucose 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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