Phosphoprotein phosphatase

Phosphoprotein phosphatase is an insulin-stimulated enzyme which dephosphorylates certain phosphorylated enzymes.

Overview of control of enzyme activity
The control of enzyme activity involves covalent modifications, including the addition of phosphate ions to serine, threonine and tyrosine residues in the enzyme. This addition is catalysed by protein kinases. The reverse reaction is carried out by phosphatase.

Effects of activation of phosphoprotein phosphatase
Phosphoprotein phosphatase is activated by the hormone insulin, which indicates that there is a high concentration of glucose in the blood. The enzyme then acts to dephosphorylate other enzymes, such as phosphorylase kinase, glycogen phosphorylase and glycogen synthase.

This leads to phosphorylase kinase and glycogen phosphorylase becoming inactive, while glycogen synthase is activated. As a result, glycogen synthesis is increased and glycogenolysis is decreased, and the net effect is for energy to enter and be stored inside the cell.

Mechanism
Phosphorylation involves the transfer of phosphate groups from ATP to the enzyme, the energy for which comes from hydrolysing ATP into ADP or AMP.

However, dephosphorylation releases phosphates into solution as free ions, because attaching them back to ATP would require energy input.