Text from Keeling (©1998)
The pathway of starch synthesis is known to involve the inter-conversion of sugars, sugar-phosphates and nucleotide-sugars. Shown in the table below are the major enzymes of starch biosynthesis isolated from developing maize endosperm.
|Form 1||Form 2||Form 3||Form 4|
|ADP-glc pyrophosphorylases||Small subunit (Bt2)||Large Subunit (Sh2)||Small subunit (Agp1)||Large subunit (Agp2)|
|Bound starch synthases||GBSSI (Waxy)||-||-||-|
|Branching enzymes||BEI||BEIIa||BEIIb (amylose extender)||-|
|Starch synthases||SSI||SSIIa (Sugary2?)||SSIIb||SSIII (Dull)|
|Debranching enzymes||Isoamylase (Su1)||?? Pullulanase (??)||-||-|
A last step is the production of ADP-glucose by the enzyme ADP-glucose pyrophosphorylase, previously believed to be restricted to the plastid compartment of the cell. Very recently it has been found that pyrophosphorylase may be located in the amyloplast in potato and peas, but may be located in the cytoplasm in corn and barley. Amylopectin and amylose are synthesized in the amyloplast from ADP-glucose, primarily by the enzymes starch synthase and branching enzyme. Some other enzymes such as amylogenin, debranching enzymes, disproportionating enyzmes have been implicated in the process. However, definitive evidence for their role in the starch assembly process awaits further research. Shown below are the various forms of these enzymes in corn endosperm.