Phosphorylase kinase is activated in response to cAMP (Figure 1). Increasing the concentration of cAMP activates cAMP-dependent protein kinase, which catalyzes the phosphorylation by ATP of inactive phosphorylase kinase b to active phosphorylase kinase a, which in turn, phosphorylates phosphorylase b to phosphorylase a. In the liver, cAMP is formed in response to glucagon, which is secreted in response to falling blood glucose (or exercise). Muscle is insensitive to glucagon; in muscle, the signal for increased cAMP formation is the action of epinephrine, which is secreted in response to fear or fright, when there is a need for increased glycogenolysis to permit rapid muscle activity.
Fig1. Control of glycogen phosphorylase in muscle. The sequence of reactions arranged as a cascade allows amplification of the hormonal signal at each step. (G6P, glucose 6-phosphate; n, number of glucose residues.)
Ca2+ Synchronizes the Activation of Glycogen Phosphorylase With Muscle Contraction
Glycogenolysis in muscle increases several 100-fold at the onset of contraction; the same signal (increased cytosolic Ca2+ ion concentration) is responsible for initiation of both con traction and glycogenolysis. Muscle phosphorylase kinase, which activates glycogen phosphorylase, is a tetramer of four different subunits, α, β, γ, and δ. The α and β subunits contain serine residues that are phosphorylated by cAMP-dependent protein kinase. The δ subunit is identical to the Ca2+-binding protein calmodulin and binds four Ca2+. The binding of Ca2+ activates the catalytic site of the γ subunit even while the enzyme is in the dephosphorylated b state; the phosphorylated a form is only fully activated in the presence of high concentrations of Ca2+.
Glycogenolysis in Liver Can Be cAMP-Independent
In the liver, there is cAMP-independent activation of glycogenolysis in response to stimulation of α1 adrenergic receptors by norepinephrine (in humans the number of α1 receptors is low). This involves mobilization of Ca2+ into the cytosol, followed by the stimulation of a Ca2+/calmodulin-sensitive phosphorylase kinase. cAMP-independent glycogenolysis is also activated by vasopressin, oxytocin, and angiotensin II acting either through calcium or the phosphatidylinositol bisphosphate pathway .
Protein Phosphatase-1 Inactivates Glycogen Phosphorylase
Both phosphorylase a and phosphorylase kinase a are dephosphorylated and inactivated by protein phosphatase-1. Protein phosphatase-1 is inhibited by a protein, inhibitor-1, which is active only after it has been phosphorylated by cAMP-dependent protein kinase. Thus, cAMP controls both the activation and inactivation of phosphorylase (see Figure 1). Insulin reinforces this effect by inhibiting the activation of phosphorylase b. It does this indirectly by increasing uptake of glucose, leading to increased formation of glucose-6-phosphate, which is an inhibitor of phosphorylase kinase.
The Activities of Glycogen Synthase & Phosphorylase Are Reciprocally Regulated
There are different isoenzymes of glycogen synthase in liver, muscle, and brain. Like phosphorylase, glycogen synthase exists in both phosphorylated and nonphosphorylated states, and the effect of phosphorylation is the reverse of that seen in phosphorylase (Figure 2). Activeglycogen synthase ais dephosphorylated and inactiveglycogen synthase bis phosphorylated.
Fig2. Control of glycogen synthase in muscle.(G6P, glucose-6-phosphate; GSK, glycogen synthase kinase; n, number of glucose residues.)
Six different protein kinases act on glycogen synthase, and there are at least nine different serine residues in the enzyme that can be phosphorylated. Two of the protein kinases are Ca2+/ calmodulin dependent (one of these is phosphorylase kinase). Another kinase is cAMP-dependent protein kinase, which allows cAMP-mediated hormone action to inhibit glycogen synthesis synchronously with the activation of glycogenolysis. Insulin also promotes glycogenesis in muscle at the same time as inhibiting glycogenolysis by raising glucose-6-phosphate concentrations, which stimulates the dephosphorylation and activation of glycogen synthase. Dephosphorylation of glycogen synthase b is carried out by protein phosphatase-1, which is under the control of cAMP-dependent protein kinase.
