Final answer:
Glycogen phosphorylase is stimulated by hormones such as adrenaline and glucagon, which increase cyclic AMP levels, thereby activating protein kinase A that in turn phosphorylates and activates the enzyme. Glycogen synthase is inhibited by the same pathway, preventing the wasteful cycle of glycogen synthesis during times when energy needs to be quickly mobilized. Insulin has the opposite effect, promoting glycogen synthesis and inhibiting glycogenolysis.
Step-by-step explanation:
What stimulates glycogen phosphorylase and inhibits glycogen synthase are mainly hormones involved in energy regulation, particularly during stress responses. For instance, adrenaline (also known as epinephrine) activates the ß-adrenergic receptors in muscle cells, leading to an increase in cyclic AMP (cAMP). cAMP then activates protein kinase A (PKA), which phosphorylates and activates glycogen phosphorylase, stimulating glycogenolysis - the breakdown of glycogen into glucose-1-phosphate. This process liberates glucose units for immediate energy release during fight-or-flight responses. Concurrently, the phosphorylation of glycogen synthase by PKA results in the inhibition of this enzyme, which prevents the formation of glycogen from glucose, ensuring a readily available pool of glucose for energy use. In the liver, a similar process occurs with glucagon elevating cAMP levels under hypoglycemic conditions, thereby encouraging glycogen breakdown and inhibiting its synthesis.
Insulin, on the other hand, achieves the opposite effect; it inhibits glycogenolysis and promotes glycogenesis (the formation of glycogen from glucose) under conditions where blood glucose levels are high. This hormone thereby aids in the storage of excess glucose within the liver and muscle tissues. Additionally, insulin stimulates glycolysis and inhibits gluconeogenesis, further managing blood glucose levels.