Glycogen phosphorylase V, McArdle's Disease). More than 65 mutations in the PYGM gene that lead to McArdle disease have been identified to date. Symptoms of McArdle disease include muscle weakness, myalgia, and lack of endurance, all stemming from low glucose levels in muscle tissue. Mutations in the liver isoform of glycogen phosphorylase (PYGL) are associated with Hers' Disease (glycogen storage disease type VI). Hers' disease is often associated with mild symptoms normally limited to hypoglycemia, and is sometimes difficult to diagnose due to residual enzyme activity. The brain isoform of glycogen phosphorylase (PYGB) has been proposed as a biomarker for gastric cancer.
Glycogen phosphorylase Glycogen phosphorylase Glycogen phosphorylase is one of the phosphorylase enzymes (). Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond. Glycogen phosphorylase is also studied as a model protein regulated by both reversible phosphorylation and allosteric effects. Glycogen phosphorylase breaks up glycogen into glucose subunits (see also figure below): (α-1,4 glycogen chain) + Pi ⇌ (α-1,4 glycogen chain) + α-D-glucose-1-phosphate. Glycogen is left with one fewer glucose molecule, and the free glucose molecule is in the form of glucose-1-phosphate. In order to be used for metabolism, it must be converted to