Final answer:
Glycogen Phosphorylase Kinase (GPK) is active in a fasted state to facilitate the breakdown of glycogen into glucose, keeping blood glucose levels stable when dietary glucose is not available.
Step-by-step explanation:
Glycogen Phosphorylase Kinase (GPK) is on in a fasted state. During fasting, the body needs to mobilize stored energy sources to maintain normal glucose levels in the blood. One such source is glycogen, a storage form of glucose mostly found in the liver and muscles. The breakdown of glycogen, known as glycogenolysis, is catalyzed by glycogen phosphorylase, which is activated when phosphorylated by GPK. In a fasted state, hormones like glucagon and adrenaline (epinephrine) are elevated, which trigger a signaling cascade resulting in the activation of GPK. This, in turn, stimulates glycogen phosphorylase to break down glycogen into glucose-1-phosphate (G-1-P), which is then converted to glucose-6-phosphate (G-6-P) and eventually to free glucose that is released into the bloodstream to maintain energy supply during the fast.