Biosynthesis
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.