Synthesis

The pathways of starch, cellulose and hemicellulose biosynthesis have a common metabolic origin: namely that they both utilize nucleotide sugars as the source of carbohydrate for glucan polymerization.  These pathways are known to involve the inter-conversion of sugars, sugar-phosphates and nucleotide-sugars.

One key step in the pathway of starch synthesis is the production of ADP-glucose by the enzyme ADP-glucose pyrophosphorylase (AGPase). This enzyme was originally believed to be restricted to the plastid compartment of the cell. Recently it has been found that pyrophosphorylase is located in the amyloplast in dicot plants such as potato and peas, but is located in the cytoplasm in monocot plants such as corn, barley and other cereals as well as being located in the amyloplast. Amylopectin and amylose are synthesized in the amyloplast from ADP-glucose, primarily by the enzymes starch synthase and branching enzyme and debranching enzymes. Some other enzymes such as starch phosphorylase, amylogenin (“starch glycogenin”), disproportionating enyzmes have been implicated in the process. However, definitive evidence for their role in the starch assembly process awaits further research.