The highly branched structure of glycogen exerts two functions:
first, it increases the solubility of the molecule in the cytosol and
secondly, it provides a multitude of docking sites for glycogen
binding proteins. These include glycogen metabolising enzymes
and regulatory proteins, which bind to glycogen through
carbohydrate binding modules.
Glycogen degradation can occur through two different pathways:
a well-characterised cytosolic pathway and a less-defined lysosomal
pathway. In the cytosol, glycogen breakdown is catalysed by two
enzymes, namely glycogen phosphorylase and glycogen debranching
enzyme (DBE). Glycogen phosphorylase cleaves a-1,4 glycosidic
linkages by the addition of orthophosphate to yield glucose-1-
phosphate (glucose-1P), the free glucose form after glycogen
phosphorylase action. When glycogen phosphorylase reaches the
fourth unit from a branching point, DBE transfers three units to the
end of another glycogen chain and catalyses the hydrolysis of the a-
1,6 glycosidic linkage, yielding free glucose [7,8]. Glucose-1P, by
phosphoglucomutase (PGM)-dependent conversion into glucose-6P,
has three possible fates: (1) it can be metabolised through the
glycolytic pathway and generate ATP, (2) it can enter the pentose
phosphate pathway and be used to fuel anabolic reactions and to
sustain the antioxidant defences of the cell, or (3) in the liver and
other tissues expressing the enzyme glucose 6-phosphatase, it can
be dephosphorylated into free glucose and exit the cell.
Alternatively, glycogen can be transferred to the autophagosomes
and delivered to the lysosomal compartment, where it is
hydrolysed by alpha-acid glycosidase (GAA) in a process called
glycogen autophagy or glycophagy [9]. The role of glycophagy is
well characterised in the liver of newborn animals, where it
provides energy substrates during a period of starvation prior to
the beginning of feeding. In adult animals, little is know on the role
of glycophagy, however, the importance of this pathway is
suggested by the severe consequences of GAA defects in people