Glycogenolysis

Source: Wikipedia, the free encyclopedia.
Not to be confused with Glycolysis, Glycogenesis, or Gluconeogenesis.
Glycogen
Glucose
Glucose-6-phosphate

Glycogenolysis is the breakdown of

glucose-1-phosphate and glycogen (n-1). Glycogen branches are catabolized by the sequential removal of glucose monomers via phosphorolysis, by the enzyme glycogen phosphorylase.[1]

Mechanism

In the muscles, glycogenolysis begins due to the binding of

phosphorylase 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]

glycogen(n residues) + Pi ⇌ glycogen(n-1 residues) + glucose-1-phosphate

Here,

phosphoryl group for the α[1→4] linkage.[1]

Glucose-1-phosphate is converted to

glucose-6-phosphate (which often ends up in glycolysis) by the enzyme phosphoglucomutase.[1]

Glucose residues are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with a α[1→6] linkage.

α[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 by hexokinase.[1]

Enzymes

  1. Glycogen phosphorylase with Pyridoxal phosphate as prosthetic group
  2. Alpha-1,4 → alpha-1,4 glucan transferase
  3. Alpha-1,6-glucosidase
  4. Phosphoglucomutase
  5. Glucose-6-phosphatase (absent in muscles)[3]

Function

Glycogenolysis takes place in the cells of the muscle and liver tissues in response to hormonal and neural signals. In particular, glycogenolysis plays an important role in the fight-or-flight response and the regulation of glucose levels in the blood.

In

myocytes (muscle cells), glycogen degradation serves to provide an immediate source of glucose-6-phosphate for glycolysis
, to provide energy for muscle contraction. Glucose-6-phosphate can not cross cell membrane of myocyte because of that muscle is called such a selfish organ who produce glycogen, store it and use it for its own purpose.

In

glucose-6-phosphatase, which is not present in myocytes, and the free glucose exits the cell via GLUT2
facilitated diffusion channels in the hepatocyte cell membrane.

Regulation

Glycogenolysis is regulated hormonally in response to blood sugar levels by

epinephrine during the fight-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 the sarcoplasmic reticulum.[3]

Glucagon has no effect on muscle glycogenolysis.[3]

Calcium binds with calmodulin and the complex activates phosphorylase kinase.[3]

Clinical significance

Parenteral (

intramuscularly
.

Pathology

Main article: Glycogen storage disease

See also

References

  1. ^
    OCLC 191854286
    .
  2. PMID 32119304
    .
  3. ^
    ISBN 978-93-5465-648-4
    .
  4. PMID 31377934
    .
  5. ISBN 978-1-4292-0314-2
    .

External links

Wikimedia Commons has media related to Glycogenolysis.
Scholia has a topic profile for Glycogenolysis.
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