The metabolic path for Glycogen starts with
alfa-D-Glucose that is phosphorylated by a set of enzymes:
Hexokinase 1 (HXK1) , , , Hexokinase 2 (HXK2) , , , Hexokinase 3 (HXK3) , , , Glucokinase (hexokinase 4)
(HXK4) , , , ,  and alpha-(D)-Glucose-6-phosphate
is formed. Also, alpha-D-Glucose
is isomerically transformed into
beta-D-Glucose by Galactose mutarotase (aldose 1-epimerase)
(GALM) , , .
Beta-D-Glucose undergoes the same transformations that
alpha-D-glucose under the action of the same enzymes to form
beta-(D)-Glucose-6-phosphate which subsequently takes
another stereoisomerisation step to
alpha-(D)-Glucose-6-phosphate. This step is catalyzed by
Glucose phosphate isomerase (GPI) .
Phosphoglucomutase 1 (PGMU) catalyzes the next isomerization
of alpha-(D)-Glucose-6-phosphate leading to formation of
alpha-(D)-Glucose 1-phosphate , . UDP is then attached to
alpha-(D)-Glucose 1-phosphate by UDP-glucose
pyrophosphorylase 1 (UGPA1) or UDP-glucose pyrophosphorylase
2 (UGPA2) to form UDP-D-glucose
, , , .
UDP-D-glucose polymerizes with
(alpha-D-glucosyl-(1-4))n to form
Glycogen. This step is catalyzed by Glucan (1,4-alpha-),
branching enzyme 1 (GLGB) , , , . On the other hand, Glycogen synthase 1 (muscle)
(GYS1) and Glycogen synthase 2 (liver)
(GYS2) can cleave UDP-D-glucose
from Glycogen to release
(alpha-D-glucosyl-(1-4))n , , , , . A number of glycogen phosphorylases
(Phosphorylase, glycogen, liver (PYGL), Phosphorylase,
glycogen, brain (PYGB) and Phosphorylase, glycogen, muscle
(PYGM)) mediate phosphorolysis of Glycogen,
which is regarded as transfer of one sugar residue
(alpha-D-glucose 1-phosphate) from Glycogen
phosphate. As a result Glycogen chain
decreased for one sugar moiety with formation of a reduced product
(alpha-D-glucosyl-(1-4))(,n=3)-alpha-D-glucosyl-(1-6)*(glycogen). , , , , , . This
product is a substrate for Amylo-1, 6-glucosidase, 4-alpha-glucanotransferase - glycogen
debranching enzyme (GDE) that fractionates the polymeric
structure to form two metabolites: the main chain -
alpha-D-glucosyl-(1-6)*(glycogen) and branch chain of
Glycogen - (alpha-D-glucosyl-(1-4))(,n=3) , , , , . GDE
also affects both products of the latter reaction.
Alpha-D-glucosyl-(1-4))(,n=3) can be directly repolymerized
to Glycogen; alpha-D-glucosyl-(1-6)*(glycogen)
can be further hydrolyzed giving
alpha-D-Glucose as a product., , .
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