Role of Enzymes and coenzymes, and
The steps involved in the pathway of Glycogenolysis
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Glycogen is a storage form of glucose.
It is a very large, branched polymer of glucose residues that
can be broken down to yield glucose molecules when energy
Most of the glucose residues in glycogen are linked by α-1,4-
Branches at about every tenth residue are created by α-1,6-
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Purpose of Glycogenolysis
The controlled breakdown of glycogen and release of
glucose increase the amount of glucose that is
available between meals. Hence, glycogen serves as a
buffer to maintain blood-glucose levels.
Glycogen's role in maintaining blood glucose levels is
especially important because glucose is virtually
the only fuel used by the brain, except during
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Purpose of Glycogenolysis
The glucose from glycogen is readily mobilized and
is therefore a good source of energy for sudden,
Unlike fatty acids, the released glucose can provide
energy in the absence of oxygen and can thus
supply energy for anaerobic activity.
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Enzymes involved in Glycogenolysis
The efficient breakdown of glycogen requires four
one to degrade glycogen,
two to remodel glycogen so that it remains a
substrate for degradation, and
one to convert the product of glycogen breakdown
into a form suitable for further metabolism.
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Major coenzyme of Glycogenolysis
Pyridoxal phosphate (PLP), a derivative of vitamin
B6, is the major coenzyme involved in the glycogen
serves as prosthetic group for Glycogen
It is held at the active site of Phosphorylase enzyme
by a Schiff base linkage, formed by reaction of the
aldehyde group of PLP with the ε-amino group of a
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Glycogen degradation is not just the
reverse of glycogenesis
Glucose-6-P –> Glucose-1-P
Depolymerization- Removal of
glucose as glucose-1-P
Conversion of Glucose-1-P to
Conversion of Glucose-6-P to
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Specific steps of Glycogenolysis
Step-1- Depolymerization (Release of Glucose-1-P from
Coenzyme– Pyridoxal phosphate
Reaction involved :
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Step-1- Reaction catalyzed by
Phosphorylase catalyzes the sequential removal of
glucosyl residues from the nonreducing ends of the
glycogen molecule (the ends with a free 4-OH group.
Orthophosphate splits the glycosidic linkage between
C-1 of the terminal residue and C-4 of the adjacent
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Advantages of Phosphoroyltic cleavage
The phosphoroylytic cleavage of glycogen is energetically
advantageous because the released sugar is already
In contrast, a hydrolytic cleavage would yield glucose, which
would then have to be phosphorylated at the expense of the
hydrolysis of a molecule of ATP to enter the glycolytic pathway.
An additional advantage of phosphoroylytic cleavage for muscle
cells is that glucose 1-phosphate, negatively charged under
physiological conditions, cannot diffuse out of the cell.
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Problem with Phosphorylase
The α-1,6-glycosidic bonds at the branch points are
not susceptible to cleavage by phosphorylase.
Glycogen phosphorylase stops cleaving α -1,4
linkages when it reaches a terminal residue four
residues away from a branch point.
Because about 1 in 10 residues is branched, glycogen
degradation by the phosphorylase alone would come
to a halt after the release of six glucose molecules per
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Step-2- Remodeling and Debranching
enzyme with two enzyme
Both these enzymes
remodel the glycogen for
continued degradation by
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Role of Transferase
Transferase shifts a block of three glucosyl
residues from one outer branch to the other.
This transfer exposes a single glucose residue
joined by an α-1,6-glycosidic linkage.
Debranching enzyme, hydrolyzes the α -1, 6-
glycosidic bond, resulting in the release of a
free glucose molecule.
The transferase and α-1,6-glucosidase convert
the branched structure into a linear one,
which paves the way for further cleavage by
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Phosphorylase versus debranching
Glucose-1-P is released as
an outcome of reaction
Free glucose is released
by the action of
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Step-3- Conversion of Glucose-1-P to
Phosphoglucomutase converts glucose 1-phosphate into glucose 6-
phosphate in a reversible reaction.
The catalytic site of an active mutase molecule contains a
phosphorylated serine residue.
The phosphoryl group is transferred from the serine residue to the
C-6 hydroxyl group of glucose 1-phosphate to form glucose 1,6-
The C-1 phosphoryl group of this intermediate is then shuttled to
the same serine residue, resulting in the formation of glucose 6-
phosphate and the regeneration of the phosphoenzyme.
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Reaction catalyzed by Phosphoglucomutase
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Step-4- Fate of Glucose-6-P
Glucose 6-phosphate derived from
(1) be used as a fuel for anaerobic or
aerobic metabolism as in, for instance,
(2) be converted into free glucose in the
liver and subsequently released into the
(3) be processed by the pentose
phosphate pathway to generate NADPH or
ribose in a variety of tissues.
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The fate is different in liver and muscle
The liver contains a hydrolytic enzyme, glucose 6-phosphatase,
which cleaves the phosphoryl group to form free glucose and
Glucose 6-phosphatase is absent from most other tissues.
Consequently, glucose 6-phosphate is retained for the
generation of ATP.
In contrast, glucose is not a major fuel for the liver. The liver
releases glucose into the blood during muscular activity and
between meals to be taken up primarily by the brain and
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Glycogenesis versus Glycogenolysis
• Glycogenolysis and
Glycogenesis are not
the just the reverse
of each other.
• The reaction
and coenzymes are
all different and,
• both the ways are
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Regulation of glycogen metabolism
To be continued in the next section…
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