Question

Consider the fates of glycogen in both liver and muscle. What physiological conditions will stimulate glycogen degradation in each tissue? What are the signals for these conditions, and how do they stimulate glycogen degradation? What would the major consequence be of the loss of the AMP binding site in glycogen phosphorylase? What would the major consequence be of a mutation of Ser14 to Ala14 in glycogen phosphorylase?

Answer 1

For glycogen degradation, the synchronous activities of glycogen phosphorylase and debranching enzyme are required.Insulin induces the dephosphorylation of glycogen synthase via inhibition of upstream kinases and activation of phosphatases So insulin synergistically stimulates glycogen synthesis in muscle by coordinately increasing intracellular concentrations of G-6-P and by dephosphorylation and activation of glycogen synthase.

In muscle, glycogen is broken down to supply energy (ATP) via glycolysis. Glycogen phosphorylase catalyzes the conversion of stored glycogen to glucose-1- phosphate, which is converted to glucose-6-phosphate. During strenuous muscle activity, skeletal muscle requires large quantities of glucose 6-phosphate. In the liver, the breakdown of glycogen is used to maintain a steady level of blood glucose between meals (glucose 6-phosphate is converted to free glucose).

As far as Prediction of the major consequence of Mutation of Ser 14 to Ala 14 in liver phosphorylase is concerned , Phosphorylase b cannot be converted into the much more active a form. Hence, the mobilization of liver glycogen will be markedly impaired.

Solution for degradation prevention:

Several studies have provided evidence that under certain experimental conditions glucose cycling through glycogen is negligible. In cells that had high glycogen stores and then were treated with a high ratio of glucagon to insulin, glycogen turnover was undetectable due to low synthetic rates. Additionally, in glycogen-depleted cells that were then stimulated with high glucose and insulin, glycogen degradation was markedly suppressed, enabling maximal rates of glycogen repletion