The fixation of carbon dioxide via photosynthesis is the initiating reaction in saccharide synthesis in nature. Light energy is harvested via chloroplasts and used to provide chemical potential in the form of adenosine triphosphate, and reducing equivalents. The key intermediate, Dribulose 1,5-bisphosphate, fixes carbon dioxide (the dark reaction) yielding products that are ultimately converted to D-glucose via a series of reactions of phosphorylated sugar intermediates. All other naturally occurring sugars are derived from glucose in transformations that involve phosphorylated or nucleotide-linked sugars. Thus, glucose-6- phosphate is converted to fructose-6-phosphate, which in turn is converted to mannose-6-phosphate; fructose- 6-phosphate is also aminated to form 2-deoxy 2-amino glucose-6-phosphate. Galactose is formed by epimerization at C-4 of uridine diphosphoglucose (Fig. 17), fucose by a series of reactions initiating with guanosine diphosphomannose, etc. Thus, the diversity in saccharides seen in the biosphere stems from a single precursor, D-glucose. This is, therefore, the only required dietary saccharide for man.
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