Glycogenolysis, process by which glycogen, the primary carbohydrate stored in the liver and muscle cells of animals, is broken down into glucose to provide. Ultimate Guide on – Glycogenesis and its Cycle, steps, ppt, pdf, animation, diagrams and covering Difference between Glycogenesis and Glycogenolysis . Glycogen synthesis. • Glycogenolysis. • Pentose phosphate pathway. • Metabolism of other hexoses. Carbohydrate Metabolism.
|Published (Last):||16 September 2012|
|PDF File Size:||3.83 Mb|
|ePub File Size:||14.27 Mb|
|Price:||Free* [*Free Regsitration Required]|
Glycogenesis – Wikipedia
Glycogenolysis is the biochemical breakdown of glycogen to glucose whereas glycogenesis is the opposite, the formation of glycogen from glucose. Glycogenolysis takes place in the cells of muscle and liver tissues in response to hormonal and neural signals. In particular, glycogenolysis plays an important role in the adrenaline-induced fight-or-flight response and the regulation of glucose levels in the blood.
The reverse process, glycogenesis, the formation of glycogen from glucose, occurs in liver and muscle cells when glucose and ATP are present in relatively high amounts. In the synthesis of glycogen, one ATP is required for every glucose unit incorporated into the polymeric branched structure of glycogen. The glucose in the form of glucosephosphate is synthesized directly from glucose or as the end product of gluconeogenesis.
Structure of glycogen Figure 1. Glycogen structure Click for enlarged view. Schematic two-dimensional cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units.
The entire globular granule may contain around 30, glhcogenesis units. Glycogen is a multi-branched polysaccharide of glucose that serves as an energy store primarily in muscle and liver. It is stored in the form of granules in the cytoplasm of the cell and is the main storage form of glucose in the body.
Glycogen is an energy reserve that can be glyckgenesis mobilized to meet a sudden need for glucose. The significance of the multi-branched structure is that multiple glucose units, rather than a single glucose can be mobilized from any glycogen molecule when glycogenolysis is tlycogenesis.
The structure of glycogen is summarized in Figure 1 . A second enzyme, phosphoglucomutase, converts the glucosephosphate to glucosephosphate see Figure 2.
Glycogen involves two types of glycosidic linkage: In muscle, but not liver cells, the glucose is subsequently gllycogenolysis to glucosephosphate by a fifth enzyme hexokinase and enters the glycolytic pathway . In liver cells the main purpose of the breakdown of glycogen is for the release of glucose into the bloodstream for uptake by other cells.
The phosphate group of glucosephosphate is removed by the enzyme glucosephosphatase and the free glucose exits the cell via the lgycogenolysis localized GLUT2 glucose transporter.
In muscle, glycogenolysis serves to provide an immediate source of glucosephosphate for glycolysis to provide energy for muscle contraction but not for other body tissues. Muscle cells lack the enzyme glucosephosphatase and glycogenolysie cannot convert glucosephosphate which cannot be transported across the cell membrane to glucose. The regulation of glycogenolysis involves phosphorylation cascades as summarized in Figure 2.
Cyclic AMP binding to the regulatory subunit of protein kinase A causes the catalytic subunit of PKA to dissociate from the regulatory subunit enabling it to phosphorylate other proteins including glycogen phosphorylase b.
Details of the structural changes associated with the activation of glycogen phosphorylase can be found Figure 2. Enzyme reactions involved in glycogenolysis and glycogenesis and their regulation Click for enlarged view and see text for details.
Insulin inhibits glycogenolysis by activating protein phosphatase 1 PP1 and the enzyme phosphodiesterase. Both contribute to the inactivation of glycogen phosphorylase by reducing its phosphorylated state. Activated PP1 directly glycogenwsis glycogen phosphorylase a, reforming the inactive glycogen phosphorylase b, whereas phosphodiesterase converts cAMP to AMP, thus inactivating PKA and its ability to phosphorylate activate glycogen phosphorylase.
A detailed description of the process can be found in any Biochemistry textbook . As summarized in Figure 2, glucose is converted into glucosephosphate by the action of glucokinase liver or hexokinase muscle. Glucosephosphate is then converted into glucosephosphate by the action of the enzyme phosphoglucomutase, passing through an obligatory intermediate step of glucose-1,6-bisphosphate.
Next the glucosephosphate is converted into UDP-glucose by the action of uridyl transferase also called UDP-glucose pyrophosphorylase. One molecule of UTP is used in this step and one molecule of pyrophosphate is formed, which is hydrolyzed by pyrophosphatase glycogenolyxis 2 molecules of inorganic phosphate Pi.
UDP-glucose molecules are incorporated into the growing glycogen chain by glyogenolysis enzyme glycogen synthase, which must act on a pre-existing primer. Initially this is the small protein glycogenin, but once initiated the primer is the growing glycogen chain.
The mechanism for joining glucose units is that glycogen synthase binds to UDP-glucose, causing it to break down into an oxonium ion, which can readily add to the 4-hydroxyl group of a glucosyl residue on the 4 end of the glycogen chain.
After every 10 to 14 glucose units a side branch with an additional chain of glucose units occurs. The side chain attaches at carbon atom 6 of a glucose unit, and the linkage is termed an alpha-1,6 glycosidic bond. To form this connection a separate enzyme known as a branching enzyme is used.
Branching enzyme systematic name: As summarized in Figure 2, there is a reciprocal relationship between glycogen synthesis glycogenesis and glycogen breakdown glycogenolysis and factors that enhance one inhibit the other. One of the main forms of control is the varied phosphorylation of glycogen synthase and glycogen phosphorylase by protein kinase A PKA. Phosphorylated glycogen synthase is inactive in contrast to glycogen phosphorylase which is activated following phosphorylation.
As discussed above, conditions such as low glucose levels or stress that promote the activation of PKA as a result of released adrenaline or glucagon binding to their G-protein coupled receptors, promote the process of energy generation through glycogen breakdown and inhibit the process of glycogen synthesis. Similarly calcium ions inhibit glycogen synthase indirectly through their activation of PKA. Finally glycogenesis is enhanced by elevated levels of ATP which act as an allosteric inhibitor of glycogen phosphorylase .
Nobel lecture December Critical Reviews Biochemistry and Molecular Biology 24 1: Please use the following format when referencing this article: Glycogenolysis and glycogenesis [internet]. Last modified on 13 August at Diabetologia publishes original clinical and experimental research within the field of diabetes. Diapedia is created by Driebit, Amsterdam.
Home News About Get involved Log in. Glycogenolysis and glycogenesis Creator: Regulation of glycogenolysis The regulation of glycogenolysis involves phosphorylation cascades as summarized in Figure 2.
Enzymes involved in glycogenesis A detailed description of the process can be found in any Biochemistry textbook . Regulation of glycogenesis As summarized in Figure 2, there is a reciprocal relationship between glycogen synthesis glycogenesis and glycogen breakdown glycogenolysis and factors that enhance one inhibit the other. Comments Nobody has commented on this article. Consider making a donation to support us!