Glycogen grams of glucose are present in the blood of humans at all times; in fasted individuals, blood glucose is maintained constant at this level at the expense of glycogen stores in the liver and skeletal muscle. Glycogen stores in skeletal muscle serve as a form of energy storage for the muscle itself; however, the breakdown of muscle glycogen impedes muscle glucose uptake, thereby increasing the amount of blood glucose available for use in other tissues. Liver glycogen stores serve as a store of glucose for use throughout the body, particularly the central nervous system. The human brain consumes approximately 60%
Glycogen synthase protein phosphatase 1 (PP1), which activates glycogen synthase via dephosphorylation. PP1 is targeted to the glycogen pellet by four targeting subunits, G, G, PTG and R6. These regulatory enzymes are regulated by insulin and glucagon signaling pathways. Mutations in the "GYS1" gene are associated with glycogen storage disease type 0. In humans, defects in the tight control of glucose uptake and utilization are also associated with diabetes and hyperglycemia. Patients with type 2 diabetes normally exhibit low glycogen storage levels because of impairments in insulin-stimulated glycogen synthesis and suppression of glycogenolysis. Insulin stimulates glycogen synthase by inhibiting glycogen synthase kinases