Precursors[ edit ] Catabolism of proteinogenic amino acids. Amino acids are classified according to the abilities of their products to enter gluconeogenesis:  Ketogenic amino acids do not. These products may still be used for ketogenesis or lipid synthesis. Some amino acids are catabolized into both glucogenic and ketogenic products. In humans the main gluconeogenic precursors are lactate , glycerol which is a part of the triacylglycerol molecule , alanine and glutamine.
|Published (Last):||3 November 2012|
|PDF File Size:||11.24 Mb|
|ePub File Size:||2.8 Mb|
|Price:||Free* [*Free Regsitration Required]|
See Article History Glycogenolysis, process by which glycogen , the primary carbohydrate stored in the liver and muscle cells of animals, is broken down into glucose to provide immediate energy and to maintain blood glucose levels during fasting.
Glycogenolysis occurs primarily in the liver and is stimulated by the hormones glucagon and epinephrine adrenaline. Various enzyme defects can prevent the release of energy by the normal breakdown of glycogen in muscles. Enzyme defects that can give rise to other carbohydrate diseases include galactokinase A1 ; galactose 1-phosphate UDP transferase A2 ; fructokinase B ; aldolase C ; fructose 1,6-diphosphatase deficiency D ; pyruvate dehydrogenase complex E ; and pyruvate carboxylase F.
When blood glucose levels fall, as during fasting, there is an increase in glucagon secretion from the pancreas. That increase is accompanied by a concomitant decrease in insulin secretion, because the actions of insulin, which are aimed at increasing the storage of glucose in the form of glycogen in cells, oppose the actions of glucagon.
Following secretion, glucagon travels to the liver, where it stimulates glycogenolysis. The vast majority of glucose that is released from glycogen comes from glucosephosphate , which is formed when the enzyme glycogen phosphorylase catalyzes the breakdown of the glycogen polymer. In the liver, kidneys , and intestines , glucosephosphate is converted reversibly to glucosephosphate by the enzyme phosphoglucomutase.
Those tissues also house the enzyme glucosephosphatase, which converts glucosephosphate into free glucose that is secreted into the blood, thereby restoring blood glucose levels to normal. Glucosephosphate is also taken up by muscle cells, where it enters glycolysis the set of reactions that breaks down glucose to capture and store energy in the form of adenosine triphosphate , or ATP.
Small amounts of free glucose also are produced during glycogenolysis through the activity of glycogen debranching enzyme, which completes the breakdown of glycogen by accessing branching points in the polymer. Epinephrine, similar to glucagon, stimulates glycogenolysis in the liver, resulting in the raising of the level of blood glucose. However, that process is generally initiated by the fight-or-flight response , as opposed to the physiological drop in blood glucose levels that stimulates glucagon secretion.
Compare glycogenesis. Get exclusive access to content from our First Edition with your subscription. Subscribe today Various rare inherited diseases of glycogen storage produce abnormalities in glycogenolysis. For example, glycogen storage disease type V McArdle disease results in a lack of glycogen phosphorylase, which impairs glycogen breakdown and prevents muscles from meeting the energy demands of exercise.
Glycogen storage disease type III Cori, or Forbes, disease is caused by mutations in a gene involved in the production of glycogen debranching enzyme. The disease results in cellular accumulation of abnormal, incompletely broken down glycogen molecules, leading to tissue damage, particularly in the liver and muscles.
This article was most recently revised and updated by Kara Rogers , Senior Editor. Learn More in these related Britannica articles:.
¿Qué son las glucogenosis?
Glucosephosphate is converted into glucosephosphate by the action of phosphoglucomutase , passing through the obligatory intermediate glucose-1,6-bisphosphate. Pyrophosphate is formed, which is later hydrolysed by pyrophosphatase into two phosphate molecules. The enzyme glycogenin is needed to create initial short glycogen chains, which are then lengthened and branched by the other enzymes of glycogenesis. Glycogenin , a homodimer, has a tyrosine residue on each subunit that serves as the anchor for the reducing end of glycogen. Metabolism of common monosaccharides , including glycolysis , gluconeogenesis , glycogenesis and glycogenolysis Control and regulations[ edit ] Glycogenesis responds to hormonal control. One of the main forms of control is the varied phosphorylation of glycogen synthase and glycogen phosphorylase.