Glycogenolysis

Under glycogenolysis (synonym: glycogen ) refers to the physiological degradation of glycogen to glucose -1-phosphate and glucose.

The reverse process is the synthesis of glycogen.

Physiology

The glycogenolysis is the temporary compensation does not eat glucose. The muscles use the glycogen stored in them himself, the liver can provide through glycogenolysis also other organs glucose disposal.

Trigger for the glycogenolysis is an increased energy requirement of the body, and the associated distribution of the hormones glucagon and adrenaline. The hormone insulin, however, inhibits glycogenolysis.

Biochemistry

Glycogen is a molecule in the tree-like glucose molecules are linked glycosidically to one another (see schematic diagram). This long linear chains of glucose molecules (yellow circles in the figure) formed by alpha-1 ,4- glycosidic bonds, whereas the Y-shaped Verzweigstellen (green circles in the figure) formed by alpha-1 ,6- glycosidic bonds be.

The hormone Glucagon causes the liver cell to a breakdown of glycogen. These activated by the liver cell cAMP and produces increased the enzymes glycogen phosphorylase and the so-called glycogen debranching enzyme.

The linear portion of glycogen with the alpha-1 ,4-glucosidic bonds is reduced by the glycogen phosphorylase. There, the enzyme glycogen phosphorylase catalyzes the binding free phosphate at the C1 atom of the glucose. The glycosidic bond between the glucose molecules is split by it. As the product is formed glucose -1-phosphate, which is then still on the intracellular and usable form, glucose- 6-phosphate converted ( isomerized ) is.

The glycogen phosphorylase, however, can only degrade until the fourth glucose molecule before the next branching point, the glycogen. In order to facilitate the further reduction, removes the Transferasedomäne of glycogen debranching enzyme, three of the four glucose molecules before the branch point (light - yellow circles in the figure) and adds it to another side chain linear ( ie alpha-1, 4 - glykosydisch bound ) to. The glucose molecules of this side chain can be individually removed by the glycogen phosphorylase again.

The remaining glucose molecule is alpha-1 ,6- glycosidically bound (blue circle in the figure), is cleaved from the Glucosidaseteil the glycogen debranching enzyme. In this case, there is no glucose-1- phosphate, but free glucose. Thus, the branching of glycogen causes glycogenolysis in about 90% of glucose-1- phosphate occurs, and only about 10% free glucose.

Regulation of glycogen metabolism

Adrenaline ( muscle) or glucagon (liver) activate a G-protein coupled receptor (GPCR), to which a G - protein is docked. The α - subunit has bound GDP, which is replaced after activation of the GPCR with GTP. Characterized the α - subunit is released from the receptor and activates the adenylyl cyclase (AC). CAMP formed thereby activates a protein kinase A (PKA), which in turn catalyzes the phosphorylation of phosphorylase kinase (PPK ). The resulting catalytically active kinase stimulated glycogen phosphorylase a ( PYG), which catalyzes the breakdown of glycogen to glucose -1-phosphate. A protein kinase phosphorylates a UDP simultaneously glycogen synthase ( GYS ), which is thereby inactivated and the reverse reaction can no longer catalyze.