AGPase

ADP-glucose pyrophosphorylase (AGPase) is an enzyme that catalyzes the first committed step in starch biosynthesis by converting glucose-1-phosphate and ATP into ADP-glucose and inorganic pyrophosphate. ADP-glucose then serves as the glucosyl donor for starch synthases to extend starch chains.

In plants, AGPase is located in plastids and exists as a heterotetramer composed of two large subunits (APL) and two small subunits (APS). In contrast, bacterial AGPases are often homotetramers (GlgC). The subunit composition in plants influences catalytic properties and regulation, and multiple gene families encode the different subunits in various tissues.

AGPase activity is allosterically regulated. In plastids, the activator is 3-phosphoglycerate and inorganic phosphate acts as an inhibitor, allowing enzyme activity to reflect the carbon status of photosynthesis. The enzyme can also be regulated by redox status, with thioredoxin-mediated changes modulating activity in response to light.

Because it catalyzes the rate-limiting step of starch biosynthesis, AGPase activity affects overall starch yield and granule structure in both photosynthetic tissues and storage organs. Variation in AGPase subunit genes and expression patterns can alter starch content and properties in crops, making AGPase a target for metabolic engineering and crop improvement programs.

Understanding AGPase function provides insight into plant carbon allocation and starch production, with implications for agriculture and industrial starch applications.