Biochemistry of Glycogen storage disease Type Ia

Type Ia (Von Gierke's disease) is caused by a deficiency in glucose 6-phosphatase. Glucose-6-phosphatase is the enzyme that removes the phosphoryl group from glucose-6-phosphate (the intermediate of glycogen and glucose) to form glucose which is released from the cell. 

Phosphorylated sugar (sugar having a phosphate group) cannot pass through the cell membrane, only the de-phosphorylated form (sugar without phosphate group) can pass through. During exercise, when energy demands are raised, glycogen stores fail to be mobilized as glucose into the bloodstream to provide the needed energy. The result is severe hypoglycaemia (low blood sugar) and body fatigue. Furthermore, since the glycogen in the liver is not being broken down, it tends to accumulate and will interfere with the proper functioning of the liver cells in which it is stored. Thus, resulting in an enlarged liver.