A. Will only phosphorylate glycogen synthase (GS) after GS has been phosphorylated by casein kinase.
Explanation:
Glycogen synthase kinase (GSK) is a type of enzyme capable of phosphorylating serine and threonine residues on the glycogen synthase (GS). The insulin hormone activates the synthesis of glycogen by dephosphorylating GS, but it is a mechanism that depends on the previous phosphorylation of GS by Casein Kinase (CK). GSK is also known to phosphorylate the insulin receptor (IRS), and thus controlling glucose metabolism.
