Now Glycogen synthase A contains several sites where phosphorylation can actually take place and glycogen synthase A also basically responds to a number of different types of protein kinases. But two important protein kinases which basically deactivate glycogen synthase A by phosphorylating it are protein kinase A, PKA and an enzyme known as glycogen synthase kinase or GSK. More specifically, it's glycogen synthase kinase three. So glycogen synthase is affected by several kinases and contains different phosphorylating sites. However, two very important kinases that are able to actually turn off glycogen synthase A are protein kinase A and glycogen synthase kinase. Now remember that protein kinase A is actually stimulated by cyclic Amp.