How does anabolic synthesis of various polysaccharides occur?
The glycosidic bonds in polysaccharides are formed by coupling the dehydration reactions with nucleotide sugars as intermediates.
For example, a disaccharide is formed by splitting a water molecule from two monosaccharide molecules.
The formation of a glycosidic bond is an endothermic process so it must be coupled biochemically with an energy-producing reaction.
Step 1. Glucose is first phosphorylated in an ATP-driven reaction to glucose-6-phosphate (G6P).
Step 2. The G6P is then converted by phosphoglucomutase to glucose-1-phosphate (G1P).
Step 3. Uridyl transferase transfers a molecule of uridine from UTP (uridine triphosphate) to G1P, yielding UDP-glucose plus pyrophosphate.
Step 4. UDP-glucose is an activated intermediate that then donates its glucose residue to a growing polysaccharide chain in an energetically favourable reaction.
The process is repeated many times to form a polysaccharide.
Thus, chemical energy in the form of UTP drives the synthesis of polysaccharides from simple sugars.