Table of Contents
Definition
Animals store glucose as glycogen, which is broken down in a process called glycogenolysis. Glycogenolysis is a metabolic process that converts glycogen from the muscles and liver to its monosaccharide form, glucose. Glycogen is a glucose polysaccharide stored in the muscles and liver. The body utilises this to produce ATP (adenosine triphosphate), an organic substance that provides energy to drive various activities in living cells.
Low levels of ATP within live cells trigger the glycogenolysis process. When the cells detect a low level of ATP, the liver and muscles liberate glycogen and break it down into glucose or simple sugars, which are then used to produce ATP.
Thus, glycogen (n) is broken down into glucose-1-phosphate and glycogen (n-1) during glycogenolysis.
Location
Glycogenolysis occurs in the cytoplasm of cells in the liver, muscles, and adipose tissue.
The liver breaks down the glycogen to maintain the glucose level in the blood. The muscle cells break down the glycogen to conserve the energy required for the contraction of muscles.
Steps or Mechanisms
- Glycogen phosphorylase and phosphorylase kinase, activated by phosphorylation, are the two main regulating enzymes of glycogenolysis. These will primarily be expressed in the brain, muscles, and liver.
- Adenyl cyclase and cAMP activity in the muscle triggers the beginning of glycogenolysis. After phosphorylase kinase is bound by cAMP and transformed into its active state, phosphorylase b is changed into phosphorylase a, which ultimately catalyses glycogen degradation.
- Glycogenolysis can occur either in the lysosomes or in the cytosol. The cytosolic enzyme glycogen phosphorylase uses inorganic phosphate to cleave α-1,4 bonds to catalyse the production of glucose-1-phosphate from the terminals of glycogen branches.
- The enzyme phosphoglucomutase converts glucose-1-phosphate into glucose-6-phosphate, which frequently ends in glycolysis.
- Acid α-glucosidase, an enzyme in the lysosome, uses an autophagy-dependent mechanism to break down lysosomal glycogen. This mechanism acts as an instant source of energy during the newborn stage.
- When glycogen phosphorylase enzyme reaches a branch point that is four glucose residues away from it, it transfers one of the branches to another chain, generating a new α-1,4 bond and leaving one glucose unit at the branch site later hydrolysed by α-1,6-glucosidase to produce free glucose. This occurs because the phosphorylase enzyme can only cleave until it is four units from the branch point.
- The general reaction for the conversion of glycogen to glucose-1-phosphate is:
Glycogen(n residues) + Pi ⇌ Glycogen(n-1 residues) + Glucose-1-phosphate
Enzymes
The important enzymes involved in the process of glycogenolysis include glycogen phosphorylase, phosphorylase kinase, and phosphoglucomutase.
In the muscle cells, adenyl cyclase and cAMP bind to and activate the enzyme phosphorylase kinase and transform phosphorylase b into phosphorylase a, which catalyses the glycogen breakdown.
Glycogen is broken down into glucose-1-phosphate and glucose-6-phosphate in the cytosol by glycogen phosphorylase.
Phosphoglucomutase converts glucose-1-phosphate (reversibly) into glucose-6-phosphate in the liver, kidney, and intestines.
Functions and Significance
The liver and muscle tissue cells undergo glycogenolysis in response to neurological and hormonal impulses. Glycogenolysis, in particular, is crucial for controlling blood glucose levels and the fight-or-flight response.
Glycogen breakdown in the muscle cells (myocytes) serves as an instant resource of glucose-6-phosphate for the process of glycolysis, which produces energy for muscle contraction.
The process of glycogenolysis differs in liver cells or hepatocytes. The liver does not immediately utilise the glucose that is created during glycogenolysis in the liver. Instead, glucose travels via the bloodstream to be utilised by other cells.
Glycogen is a form of energy storage in animals similar to starch, and a form of energy storage in plants that may be degraded when a plant requires energy.
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