Glycogen is a rapidly accessible store of glucose used by the body in the absence of carbohydrate intake. It is found in the skeletal muscle and liver. Its stores are usually cleared within 18–24 hours of fasting. Glycogen is a polymer of -D glucose residues linked by alpha 1,4 glycosidic bonds.
Glycogen synthesis occurs in the cytosol and requires ATP and UTP. Glucose 6-phosphate is converted to UDP glucose which acts as the glucosyl residue donor. Fragments of existing glycogen are the primer to which the alpha 1,4 linkages are added using the enzyme glycogen synthase. If there is no primer, a specific protein, glycogenin can act as an acceptor for the glucosyl residues.
Glycogen storage diseases are a group of genetic diseases that result from specific enzyme defects. Either glycogen is formed with abnormal structure or excessive amounts accumulate in specific tissues. Only a few of these inherited glycogen storage diseases are fatal, most of the conditions can be managed with special dietary regimes, or the avoidance of vigorous exercise. Glycogenolysis (or glycogen degradation) requires two stages:
The regulation of glycogen degradation and synthesis is controlled by two main mechanisms. It is important to understand that glycogen phosphorylase and glycogen synthase are controlled by the same hormonal signals, but these signals produce opposing actions for the two enzymes. Only a few of these inherited glycogen storage diseases are fatal, most of the conditions can be managed with special dietary regimes, or the avoidance of vigorous exercise.
The two ‘catabolic’ hormones, glucagon and adrenaline, bind to G-protein linked membrane receptors in liver cells and activate adenyl cyclase. This causes an increase in intracellular cAMP which then activates protein kinase A. The protein kinase in turn causes a cascade of phosphorylation reactions on a range of intracellular enzymes.
These reactions result in the phosphorylation and inactivation of glycogen synthase to inhibit glycogen synthesis, and activation of glycogen phosphorylase to promote glycogen breakdown to glucose. Insulin has the opposite affect to glucagon and adrenaline, and promotes glycogen synthesis and storage.
This phosphorylation/dephosphorylation mechanism of control is known as covalent modification. Another mechanism of control is allosteric activation, whereby glycogen phosphorylase and glycogen synthase respond to levels of certain small metabolites. In the muscle Ca2+ and AMP initiate glycogen degradation. In the liver, glucose inhibits degradation, whereas glucose 6-phosphate activates glycogen synthesis.
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1. Explaining metabolic rate and glucose metabolism
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