Phosphorylase, glycogen degradation

The glycogen phosphorylase reaction degrades glycogen to produce limit dextrins, which are further degraded by debranching enzyme, as already described. 

Since the change in rate of glycogen degradation, catalysed by glycogen phosphorylase, depends on a balance between the two activities, which constimte a cycle, the relationship is also known as an interconversion cycle. By convention, the active form is the a form and the inactive form is the b form (Figure 3.12). 

Fig. 7.18. Regulation of glycogen metabolism in muscle. Phosphorylase kinase stands at the center of regulation of glycogen metabolism. Phosphorylase kinase may exist in an active, phosphorylated form and an inactive, unphosphorylated form. Phosphorylation of phosphorylase kinase is triggered by hormonal signals (e.g. adrenahne) and takes place via an activation of protein kinase A in the cAMP pathway. In the absence of hormonal stimulation, phosphorylase kinase can also be activated by an increase in cytosolic Ca. The active phosphorylase kinase stimulates glycogen degradation and inhibits glycogen synthesis, in that, on the one side, it activates glycogen phosphorylase by phosphorylation, and on the other side, it inactivates glycogen synthase by phosphorylation.

During muscle contraction, l Caz+ is released from the sarcoplasmic reticulum. The Caz+ binds to the calmodulin subunit of phosphorylase kinase, activating it without phosphorylation. Phosphorylase kinase can then activate glycogen phosphorylase, causing glycogen degradation. 

Correct answer = B. Epinephrine and glucagon both cause increased glycogen degradation in the liver. Therefore, glycogen phosphorylase activity is increased, whereas glycogen synthase activity is decreased. Both cAMP-dependent protein kinase and its substrate, phosphorylase kinase, are also activated. 

Glycogen degradation requires two enzymes glycogen phosphorylase and glycogen-debranching enzyme. 

During muscle contraction, Ca2+ ions released from the sarcoplasmic reticulum partially activate dephosphorylated phosphorylase kinase and this in turn in turn activates phosphorylase, stimulating glycogen degradation. 

Glycogen degradation requires phosphorylated (active) phosphorylase kinase to maintain the production of phosphorylated (active) phosphorylase a the a-l,6-glucosidase is required to remove branch molecules from partially degraded glycogen, and phosphorylated (active) inhibitor protein is required to inactivate phosphoprotein phosphatase. 

The exact mechanism by which the AKH/RPCHs activate fat body phosphorylase and trehalose synthesis remains uncertain. In P. americana, CC extracts with hypertrehalosemic activity neither stimulate trehalose-6-P synthase for trehalose synthesis nor increase fat body trehalose permeability (28). In all insect species tested, phosphorylase activation and glycogen degradation occur in response to CC extracts or AKH GH peptides (2.3.29-32). Therefore, all the data suggest... 

---