Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Glycolysis as the Central Pathway of Glucose Catabolism
The interconversions of phosphorylase $ and phosphorylase $ are ultimately regulated by hormones

As we saw above, the interconversion of phosphorylase a and phosphorylase b in Muscles is mediated by two Enzymes: phosphorylase a phosphatase and phosphorylase b kinase. Their combined action ultimately determines The ratio of phosphorylase a to phosphorylase b and, consequently, The rate of Glycogen breakdown to yield glucose-1-phosphate. This naturally raises the question: what regulates The activity of phosphorylase phosphatase and phosphorylase kinase in turn?

We will address this crucial question in more detail later (Chapter 25), but for now, suffice it to say that when an Organism is suddenly confronted with a critical situation, The adrenal medulla releases the hormone epinephrine into the Blood, which serves as a molecular signal for The Liver and muscles. In response to this signal, the liver activates its Glycogen phosphorylase, thereby elevating blood glucose levels—that is, supplying fuel to the muscles. This same signal triggers glycogen breakdown in skeletal muscles to produce lactate, thereby boosting the generation of ATP required to cope with the emergency. Through a long cascade of sequential reactions (which we will examine later), epinephrine ultimately stimulates phosphorylase b kinase activity, causing a sharp increase in the phosphorylase a to phosphorylase b ratio. Once the stress subsides and epinephrine is no longer released into the bloodstream, phosphorylase b kinase activity returns to its baseline, lower level; likewise, the phosphorylase a to phosphorylase b ratio normalizes (Chapter 25).

In the liver, glycogen phosphorylase also exists in a and b forms; in principle, the liver enzymes function similarly to their Muscle counterparts, although they differ somewhat in Structure and regulatory properties. Glycogenolysis in the liver serves a different purpose than in muscles, functioning as a source of free blood glucose. The action of liver phosphorylase yields glucose-1-phosphate, which is then converted into glucose-6-phosphate, the immediate precursor of free glucose. The reaction producing blood D-glucose is catalyzed by the enzyme glucose-6-phosphatase:

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Thus, the epinephrine-induced stimulation of phosphorylase a formation from phosphorylase b in the liver leads to an elevated blood glucose concentration, thereby preparing the organism to cope with emergency situations. We will discuss glycogen breakdown, synthesis, and The regulation of these processes in greater detail later (Chapters 20 and 25).