structure
Last reviewed 01/2018
Glycogen phosphorylase (GP) exists in two forms:
- GPa is the active form that converts glycogen to glucose-1-phosphate; it is phosphorylated
- GPb is the less active form; it is not phosphorylated
Conversion between GPa and GPb depends on the state of phosphorylation:
- GPa phosphatase converts GPa to GPb by removal of phosphate
- phophorylase kinase (PK) phosphorylates GPb to GPa
Phosphorylase kinase can exist in 2 forms:
- PKa is an active form that readily phosphorylates GPb; it is itself activated by phosphorylation
- PKb is the less active form that is dephosphorylated
- PKb has a subunit that is similar to troponin-C; this makes it more active in the presence of calcium after phosphorylation
Further, the phosphorylation of PKb to PKa is stimulated by the enzyme protein kinase. Protein kinase is itself made more active by the presence of the secondary messenger cyclic AMP.