Glycogen Synthase Kinase:. A. Will only phosphorylate glycogen synthase (GS) after GS has been phosphorylated by casein kinase.
B. Phosphorylates glycogen phosphorylase
C. Phosphorlyates Casein Kinase.
D. Is activated by the insulin signaling pathway.

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.