Phosphoprotein Phosphatase I is an enzyme of glycogen regulatory pathways that acts to reverse the effects of the kinase cascade.

Phosphoprotein phosphatase counters the effects of the kinase cascade by removing phosphates from proteins (Figure 16.11 and Figure 16.12). Thus, it converts the phosphorylated glycogen phosphorylase a (more active) to the dephosphorylated glycogen phosphorylase b (less active) and also converts phosphorylated glycogen synthase D (less active) to the more active glycogen synthase I form (dephosphorylated). The activity of phosphoprotein phosphatase I is controlled by phosphoprotein phosphatase inhibitor.