phosphorylase, glycogen, muscle
The PYGM gene provides instructions for making an enzyme called myophosphorylase. This enzyme breaks down a complex sugar called glycogen. Myophosphorylase is one of three related enzymes called glycogen phosphorylases that break down glycogen in cells. Myophosphorylase is found only in muscle cells, where it breaks down glycogen into a simpler sugar called glucose-1-phosphate. Additional steps convert glucose-1-phosphate into glucose, a simple sugar that is the main energy source for most cells.
Approximately 130 mutations in the PYGM gene have been found to cause glycogen storage disease type V (GSDV). One mutation that is common in North American and European populations is written as Arg50Ter or R50X. This mutation creates a premature stop signal in the instructions for making myophosphorylase, which decreases the production of the enzyme. A shortage of myophosphorylase impairs the normal breakdown of glycogen. Other mutations that cause GSDV may severely reduce enzyme activity or change the way the enzyme folds into a 3-dimensional shape. The defective enzyme is unable to break down glycogen. As a result, muscle cells cannot produce enough energy, so muscles become easily fatigued. Reduced energy production in muscle cells leads to the major features of GSDV.
- glycogen phosphorylase, muscle form