Glycogenolysis
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Glycogen metabolism Breakdown of glucans |
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Under the glycogenolysis (synonym: glycogen ) is defined as the physiological breakdown of glycogen to glucose-1-phosphate and glucose .
The reverse process is glycogen synthesis .
physiology
Glycogenolysis is used to temporarily compensate for a lack of food glucose. The muscles use the glycogen stored in them, the liver can also make glucose available to other organs through glycogenolysis .
Glycogenolysis is triggered by an increased need for energy in the body and the associated release of the hormones glucagon and adrenaline . The hormone insulin , on the other hand, inhibits glycogenolysis.
biochemistry
Glycogen is a molecule in which glucose molecules are glycosidically linked together like a tree (see schematic illustration). Longer linear chains of glucose molecules (yellow circles in the figure) are formed by alpha-1,4-glycosidic bonds, while the Y-shaped branches (green circles in the figure) are formed by alpha-1,6-glycosidic bonds become.
The hormone glucagon leads to a breakdown of glycogen in the liver cells . For this purpose, cAMP is activated by the liver cell and the enzymes glycogen phosphorylase and the so-called glycogen debranching enzyme are increased.
The linear part of the glycogen with the alpha-1,4-glycosidic bonds is broken down by the glycogen phosphorylase. There the enzyme glycogen phosphorylase catalyzes the binding of free phosphate to the C1 atom of glucose. The glycosidic bond between the glucose molecules is broken down. The product is glucose-1-phosphate , which is then converted ( isomerized ) into an intracellular and usable form, glucose-6-phosphate .
The glycogen phosphorylase can only break down the glycogen up to the fourth glucose molecule before the next branching point. To enable further degradation, the transferase domain of the glycogen debranching enzyme removes three of the four glucose molecules in front of the branching point (light-yellow circles in the figure) and adds them linearly to another side chain (i.e. alpha-1,4- glycosydically bound). The glucose molecules in this side chain can now be split off individually again by the glycogen phosphorylase.
The remaining glucose molecule, which is bound alpha-1,6-glycosidically (blue circle in the figure), is split off from the glucosidase part of the glycogen debranching enzyme. This does not produce glucose-1-phosphate, but free glucose. For example, the branching of the glycogen means that about 90% glucose-1-phosphate is formed during glycogenolysis, 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 s protein is docked. Its α-subunit has bound GDP , which is exchanged for GTP after activation of the GPCR . This releases the α-subunit from the receptor and activates an adenylyl cyclase (AC). The resulting cAMP activates a protein kinase A (PKA), which in turn catalyzes the phosphorylation of a phosphorylase kinase (PPK). The kinase thus stimulated catalytically activates a glycogen phosphorylase (PYG), which catalyzes the breakdown of glycogen to glucose-1-phosphate . Protein kinase A simultaneously phosphorylates a UDP glycogen synthase (GYS), which is thereby inactivated and can no longer catalyze the reverse reaction.
literature
- Jeremy M. Berg, John L. Tymoczko, Lubert Stryer : Biochemistry. 6 edition, Spektrum Akademischer Verlag, Heidelberg 2007. ISBN 978-3-8274-1800-5 .
- Donald Voet, Judith G. Voet: Biochemistry. 3rd edition, John Wiley & Sons, New York 2004. ISBN 0-471-19350-X .
- Bruce Alberts , Alexander Johnson, Peter Walter, Julian Lewis, Martin Raff, Keith Roberts: Molecular Biology of the Cell , 5th Edition, Taylor & Francis 2007, ISBN 978-0815341062 .
Web links
| This text is based in whole or in part on the entry Glykogenolyse in Flexikon , a Wiki of the DocCheck company . The takeover took place on March 5, 2008 under the then valid GNU license for free documentation . |