(a form of posttranslational modification that involves addition
of a phosphate moiety from ATP to an OH amino acid residue in the protein, such as serine or
threonine, mediated by a kinase, and dephosphorylation mediated by a phosphatase) by SPS
kinase and SPS phosphatase. Phosphorylation of the enzyme makes it less active. Glucose-
6-phosphate is an allosteric activator (a molecule that can bind to an enzyme and increase
its activity through enzyme subunit association) of the active form of SPS (dephosphorylated).
Glucose-6-phosphate is an inhibitor of SPS kinase, and inorganic phosphate is an
inhibitor of SPS phosphatase. Thus, under conditions when glucose-6-phosphate/inorganic
phosphate ratio is high, the active form of SPS will dominate, favoring sucrose phosphate
biosynthesis. These regulations are highly complex and may be regulated by the flux of
other sugars in several pathways