Allosteric control and covalent modification:

Phosphorylase exists in two interchangeable forms that are; active phosphorylase a and a commonly   inactive   phosphorylase   b. A Phosphorylase   b is a dimer   and   is converted into phosphorylase a through phosphorylation of a one serine residue on each subunit through the enzyme phosphorylase kinase. The procedure can be reversed and phosphorylase inactivated through removal of the phosphate group through protein phosphatase I shown in the figure.

Figure: Regulation of (a) glycogen phosphorylase  activity and (b) glycogen synthase activity by phosphorylation (covalent modification).

In skeletal muscle and high concentrations  of AMP can activate phosphorylase  b (through acting at an allosteric site) but this is opposed via the concentrations  of ATP and glucose 6-phosphate  establish in resting muscle so which in this condition phosphorylase b is indeed inactive. Because most of the phosphorylase in resting muscle is phosphorylase b and significant glycogen degradation does not occur under these conditions.  Moreover, during exercise, the concentrations of ATP and glucose 6- phosphate   fall and the concentration   of AMP increase.  Therefore phosphorylase b becomes activated and this leads to the rapid degradation of glycogen to yield energy as needed.  Phosphorylase a is unaffected through ATP and AMP or glucose 6- phosphate and so remains active under all conditions.

In liver, phosphorylase b is not activated through AMP and is thus always inactive.  Unlike muscle, thus, glycogen degradation in liver is not responsive to the energy status of the cell. Rather, phosphorylase a is deactivated through glucose.  This fits with the variant role of glycogen storage in liver which is namely to maintain blood levels of glucose. Therefore as glucose levels increased, glycogen degradation through liver phosphorylase  a is shut off and degradation begins again only as the glucose level falls.

Glycogen synthase is also regulated through covalent modification and allosteric interactions.  The enzyme live as an active glycogen synthase a and a commonly inactive glycogen synthase b. Moreover, in compare to phosphorylase it is the active form of glycogen synthase (synthase a) which is dephosphorylated while the inactive synthase b form is the phosphorylated form described in the figure.

A high concentration of glucose 6-phosphate can activate glycogen synthase b. In during muscle contraction glucose 6-phosphate levels are low and thus glycogen synthase b is inhibited. This is at the period of time when phosphorylase b is most active. Therefore glycogen degradation happens and glycogen synthesis is inhibited, avoiding the operation of a futile cycle. When the muscle revisit to the resting state and ATP and glucose 6-phosphate levels increased, phosphorylase b is inhibited and turning off glycogen degradation, while glycogen synthase is activated to rebuild glycogen reserves. The synthase a structure is active irrespective of the concentration of glucose 6- phosphate.