Question

What is glycogenolysis? Please explain all about it.

Answer 1

hey dude,


Glycogenolysis is the biochemical breakdown of glycogen to glucose

Glycogenolysis takes place in the cells of muscle and liver tissues in response to hormonal and neural signals.

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 glucose-6-phosphate) is synthesized directly from glucose or as the end product of gluconeogenesis.

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Answer 2

Glycogenolysis, or glycogen breakdown, releases glucose when it is needed.
In the liver, glycogen is a glucose reserve for the maintenance of normal blood glucose levels, and its breakdown occurs primarily:

in the fasted state, e.g. during the nocturnal fast;between meals;during a high intensity physical activity.

In hepatocytes, glycogenolysis is stimulated by glucagon and adrenalin, inhibited by insulin, and subject to negative allosteric regulation by glucose as well (see below).
In the muscle, glycogen is an energy source for muscular activity; therefore, glycogenbreakdown occurs during contraction, and only in muscles involved in the activity.
In muscle cells, glycogenolysis is stimulated by adrenaline, and regulated by positive and negative allosteric effectors, AMP and calcium ion (Ca2+), and ATP and glucose 6-phosphate, respectively (see below).



The steps of glycogenolysis

Glycogenolysis begins by the action of glycogen phosphorylase (EC 2.4.1.1), a homodimer that for its activity requires the presence of pyridoxal-5-phosphate, a derivative of pyridoxine or vitamin B6. The enzyme catalyzes the phosphorolytic cleavage of α-(1,4) glycosidic bond, releasing glucose molecules one at a time from the non-reducing ends, that is, the ends with a free 4’-OH group, of the external branches. This reaction, which does not consume ATP but an orthophosphate, yields glucose 1-phosphate.

Glycogen(n glucose residues) + Pi → Glucose 1-Phosphate + Glycogen(n-1 glucose residues)

Note: in the small intestine, pancreatic α-amylase (EC 3.2.1.1) catalyzes the  hydrolytic cleavage of the α-(1,4) glycosidic bonds of the starch, and yields glucose molecules.

In vivo, glycogen phosphorylase catalyzes an irreversible phosphorolysis, a particularly advantageous reaction for skeletal muscle and heart (see below). The irreversibility of the reaction is ensured by the ratio [Pi]/[glucose 1-phosphate], which is usually greater than 100. Conversely, the reaction is easily reversible in vitro.
Glycogen phosphorylase acts repetitively on the non-reducing ends of branches, coming to a halt when the glucose unit that is 4 residues away from the branch point is reached: this is the outer limit of the limit dextrin.
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