The Eight Essential Monosaccharides

FIGURE 21.8 Structures ofthe eight monosaccharides essential to humans. 

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Figure 25.10 An overview of biosynthetic pathways for the eight essential monosaccharides.

Galactose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from UDP-glucose by galactose 4-epimerase, where UDP = uridylyl diphosphate (a ribonucleotide diphosphate Section 28.1). The enzyme requires NAD+ for activity (Section 17.7), but it is not a stoichiometric reactant. and NADH is not a final reaction product. Propose a mechanism. 

Mannose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized as its 6-phosphate derivative from fructose 6-phosphate. No enzyme cofactor is required. Propose a mechanism. 

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