Glycogen Metabolism

Process of glycogen formation from glucose

Cytosol of liver and muscles

Storage of excess glucose to use it later at time of requirement

Glycogen synthase is the rate limiting enzyme

Breakdown of glycogen into glucose in liver during fasting and G6p in muscles during exercise

Glycogen phophorylase is the rate limiting enzyme

Because of the importance of maintaining blood glucose levels, the synthesis and degra- dation of its glycogen storage form are tightly regulated. In the liver, glycogenesis accelerates during well fed state whereas glycogenolysis accelerates during fasting. In skeletal muscle, gly- cogenolysis occurs during active exercise, and glycogenesis begins at rest. Regulation of glycogen synthesis and degradation is accomplished on two levels. First, glycogen synthase and glycogen phosphorylase are hormonally regulated to meet the needs of the body as a whole (Covalent modification). Second, the pathways of glycogen synthesis and degradation are allosterically con- trolled to meet the needs of a particular tissue. (Allosteric modification). Glycogen synthase is activated by insulin ( glut 2) in liver Inhibited by epinephrine and glucagon It is activated in muscle by insulin ( glut 4) and inhibited by epinephrine Glycogen phosphorylase is activated in liver by epinephrine and glucagon and inhibited by insulin It is activated in muscle by epinephrine ca and AMP and inhibited by insulin and ATP

It is genetic characteristized by accumulation of glycogen 1- von gierke : deficiency in glucose 6 phosphatase results in severe hypoglycemia , lactic acidosis, hepatomegaly , hyperlipidimia, hyper uracimia and doll like facies 2- mcardle : deficiency in muscle glycogen phosphorylase results in cramps and weakness in muscle 3- hers : deficiency in hepatic glycogen phosphorylase results in fasting hypoglycemia , hepatomegaly and cirrhosis

1, In liver: • Blood glucose. • Other hexoses: fructose and galactose. • Non-carbohydrate sources: glycerol and lactate. These are converted first to glucose, then to glycogen. • In muscles: Blood glucose only.