Glycogenolysis is the breakdown ofglycogen (n) toglucose-1-phosphate and glycogen (n-1). Glycogen branches arecatabolized by the sequential removal of glucose monomers viaphosphorolysis, by the enzymeglycogen phosphorylase.[1]
In the muscles, glycogenolysis begins due to the binding ofcAMP tophosphorylase kinase, converting the latter to its active form so it can convert phosphorylase b tophosphorylase a, which is responsible for catalyzing the breakdown of glycogen.[2]
The overall reaction for the breakdown of glycogen to glucose-1-phosphate is:[1]
Here,glycogen phosphorylase cleaves the bond linking a terminal glucoseresidue to a glycogen branch bysubstitution of aphosphoryl group for the α[1→4] linkage.[1]
Glucose-1-phosphate is converted toglucose-6-phosphate (which often ends up inglycolysis) by the enzymephosphoglucomutase.[1]
Glucose residues are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with a α[1→6] linkage.Glycogen debranching enzyme then transfers three of the remaining four glucose units to the end of another glycogen branch. This exposes the α[1→6] branching point, which ishydrolysed byα[1→6] glucosidase, removing the final glucose residue of the branch as a molecule of glucose and eliminating the branch. This is the only case in which a glycogen metabolite is not glucose-1-phosphate. The glucose is subsequently phosphorylated to glucose-6-phosphate byhexokinase.[1]
Glycogenolysis takes place in the cells of themuscle andliver tissues in response to hormonal and neural signals. In particular, glycogenolysis plays an important role in thefight-or-flight response and the regulation of glucose levels in the blood.
Inmyocytes (muscle cells), glycogen degradation serves to provide an immediate source of glucose-6-phosphate forglycolysis, to provide energy for muscle contraction. Glucose-6-phosphate can not pass through the cell membrane, and is therefore used solely by the myocytes that produce it.
Inhepatocytes (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 glucose-6-phosphate is removed by the enzymeglucose-6-phosphatase, which is not present in myocytes, and the free glucose exits the cell viaGLUT2 facilitated diffusion channels in the hepatocyte cell membrane.
Glycogenolysis is regulated hormonally in response to blood sugar levels byglucagon andinsulin, and stimulated byepinephrine during thefight-or-flight response. Insulin potently inhibits glycogenolysis.[4]
In myocytes, glycogen degradation may also be stimulated by neural signals;[5] glycogenolysis is regulated by epinephrine and calcium released by thesarcoplasmic reticulum.[3]
Glucagon has no effect on muscle glycogenolysis.[3]
Calcium binds withcalmodulin and the complex activates phosphorylase kinase.[3]
Parenteral (intravenous) administration of glucagon is a common human medical intervention indiabetic emergencies when sugar cannot be given orally. It can also be administeredintramuscularly.
Metabolism map | ||
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 Majormetabolic pathways inmetro-style map. Click any text (name of pathway or metabolites) to link to the corresponding article. Single lines: pathways common to most lifeforms. Double lines: pathways not in humans (occurs in e.g. plants, fungi, prokaryotes).  Orange nodes:carbohydrate metabolism. Violet nodes:photosynthesis. Red nodes:cellular respiration. Pink nodes:cell signaling. Blue nodes:amino acid metabolism. Grey nodes:vitamin andcofactor metabolism. Brown nodes:nucleotide andprotein metabolism. Green nodes:lipid metabolism. | ||
