What is the role of pyridoxal-5'-phosphate (PLP) in glycogen phosphorylase?

A. phosphate donor for phosphorylation of glycogen synthase

B. electron withdrawing effects stabilize a carbocation intermediate

C. as a general acid-base catalyst to activate inorganic phosphate

D. forms a Schiff base intermediate with the anomeric carbon of glucose

E. autophosphorylation of Ser-14 to activate glycogen phosphorylase