Glycogen degradation

Regulation of glycogen synthesis and degradation is essentially the same in the liver and muscle, but there are a couple of wrinkles. Glycogen degradation is also activated in muscle in response to the rise in intracellular calcium levels that accompanies contraction. This is achieved by... 

Starvation ( ] glucagon i insulin) Glycogen exhaustion Diabetes ( f glucagon [ insulin) Glycogen degradation Excitement ( ] epinephrine). Glycogen degradation... 

In muscle, all the glucose 6-phosphate arising from glycogen degradation (via glucose 1-phosphate) either goes down glycolysis or enters the HMP pathway. 

GLYCOGEN DEGRADATION FATTY ACID OXIDATION PROTEIN DEGRADATION... 

Figure 6.24 First steps in glycogen degradation (compare with Figure 6.39)...

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). 

This mechanism explains how a decrease in the glucose concentration stimulates glycogen degradation in the liver it is an intriguing mechanism as follows. 

The active form of vitamin Be, pyridoxai phosphate, is the most important coenzyme in the amino acid metabolism (see p. 106). Almost all conversion reactions involving amino acids require pyridoxal phosphate, including transaminations, decarboxylations, dehydrogenations, etc. Glycogen phosphory-lase, the enzyme for glycogen degradation, also contains pyridoxal phosphate as a cofactor. Vitamin Be deficiency is rare. 

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.

---