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
Stores of readily available glucose to supply
the tissues with an oxidizable energy source
are found principally in the liver, as glycogen.
Glycogen is a polymer of glucose residues
linked by α-(1,4)- and α-(1,6)-glycosidic
bonds. A second major source of stored
glucose is the glycogen of skeletal muscle.
However, muscle glycogen is not generally
available to other tissues, because muscle
lacks the enzyme glucose-6-phosphatase.

The major site of daily glucose consumption
(75%) is the brain via aerobic pathways. Most of
the remainder of is utilized by erythrocytes,
skeletal muscle, and heart muscle. The body
obtains glucose either directly from the diet or
from amino acids and lactate via
gluconeogenesis. Glucose obtained from these
two primary sources either remains soluble in the
body fluids or is stored in a polymeric form,
glycogen. Glycogen is considered the principal
storage form of glucose and is found mainly in
liver and muscle, with kidney and intestines
adding minor storage sites.

With up to 10% of its weight as glycogen, the
liver has the highest specific content of any
body tissue. Muscle has a much lower amount
of glycogen per unit mass of tissue, but since
the total mass of muscle is so much greater
than that of liver, total glycogen stored in
muscle is about twice that of liver. Stores of
glycogen in the liver are considered the main
buffer of blood glucose levels.

Glycogen homeostasis involves the concerted
regulation of the rate of glycogen synthesis
(glycogenesis) and the rate of glycogen
breakdown (glycogenolysis). Theses two
processes are reciprocally regulated such that
hormones that stimulate glycogenolysis (e.g.
glucagon, cortisol, epinephrine, norepinephrine)
simultaneously inhibit glycogenesis. Conversely,
insulin, which directs the body to store excess
carbon for future use, stimulates glycogenesis
while simultaneously inhibiting glycogenolysis.

Degradation of stored glycogen, termed
glycogenolysis, occurs through the action of
glycogen phosphorylase. There are three
distinct human genes encoding proteins with
glycogen phosphorylase activity. One gene
(PYGL) expresses the hepatic form of the
enzyme, a second (PYGM) expresses the
muscle form, and the third (PYGB) expresses
the brain form.

The action of phosphorylase is to
phosphorolytically remove single glucose
residues from α-(1,4)-linkages within the
glycogen molecules. The product of this
reaction is glucose-1-phosphate. The
advantage of the reaction proceeding through
a phosphorolytic step is that:


1. The glucose is removed from glycogen in
an activated state, i.e. phosphorylated and
this occurs without ATP hydrolysis.
2. The concentration of Pi in the cell is high
enough to drive the equilibrium of the
reaction in the favorable direction since the
free energy change of the standard state
reaction is positive.

The glucose-1-phosphate produced by the
action of phosphorylase is converted to glucose6-phosphate by phosphoglucomutase
(phosphohexose mutase): this enzyme, like
phosphoglycerate mutase of glycolysis, contains
a phosphorylated amino acid in the active site (in
the case of phosphoglucomutase it is a Ser
residue). The enzyme phosphate is transferred to
C-6 of glucose-1-phosphate generating
glucose-1,6-phosphate as an intermediate.

phosphate on C-1 is then transferred to the
enzyme regenerating active enzyme and
glucose-6-phosphate is the released product.
There are four different phosphoglucomutase
genes in humans identified as PGM1, PGM2,
PGM3, and PGM5.

The protein encoded by the PGM5 gene is called
phosphoglucomutase-like protein 5. The PGM1
gene is expressed in most tissues, whereas PGM2
expression predominates in red blood cells. The
PGM1 gene is located on chromsome 1p31 and is
composed of 13 exons that generates three
alternatively spliced mRNAs and three isoforms
of this enzyme. Mutations in the PGM1 gene are
associated with the congenital disorder of
glycosylation, CDG1T (once referred to as
glycogen storage disease type 14, GSD14).

The phosphorylase mediated release of
glucose from glycogen yields a charged
glucose residue without the need for
hydrolysis of ATP. An additional necessity of
releasing phosphorylated glucose from
glycogen ensures that the glucose residues
do not freely diffuse from the cell. In the case
of muscle cells this is acutely apparent since
the purpose in glycogenolysis in muscle cells
is to generate substrate for glycolysis.

The conversion of glucose-6-phosphate to
glucose, which occurs in the liver, kidney and
intestine, by the action of glucose-6phosphatase does not occur in skeletal
muscle as these cells lack this enzyme.
Therefore, any glucose released from
glycogen stores of muscle will be oxidized in
the glycolytic pathway. In the liver the action
of glucose-6-phosphatase allows
glycogenolysis to generate free glucose for
maintaining blood glucose levels.

Glycogen phosphorylase cannot remove glucose
residues from the branch points (α-1,6 linkages) in
glycogen. The activity of phosphorylase ceases
approximately four glucose residues from the branch
point. The removal of the these branch point glucose
residues requires the action of glycogen debranching
enzyme (GDE). The official name of GDE is amylo1,6-glucosidase, 4-α-glucanotransferase (gene
symbol: AGL) which contains 2 activities:
glucotransferase and glucosidase. The AGL gene is
located on chromosome 1p21 and is composed of 36
exons that generates three alternative spliced isoform
of the enzyme. Isoform 1 contains 1532 amino acids,
isoform 2 contains 1515 amino acids, and isoform 3
contains 1516 amino acids.

The transferase activity of debranching enzyme removes
the terminal three glucose residues of one branch and
attaches them to a free C-4 end of a second branch. The
glucose in α-(1,6)-linkage at the branch is then removed
by the action of glucosidase. This glucose residue is
uncharged since the glucosidase-catalyzed reaction is not
phosphorylytic. This means that theoretically
glycogenolysis occurring in skeletal muscle could generate
free glucose which could enter the blood stream. However,
the activity of hexokinase in muscle is so high that any
free glucose is immediately phosphorylated and enters the
glycolytic pathway. Indeed, the precise reason for the
temporary appearance of the free glucose from glycogen is
the need of the skeletal muscle cell to generate energy
from glucose oxidation, thereby, precluding any chance of
the glucose entering the blood.

For de novo glycogen synthesis to proceed the first
glucose residue is attached to a protein known as
glycogenin. Glycogenin has the unusual property of
catalyzing its own glycosylation, attaching C-1 of a UDPglucose to a tyrosine residue on the enzyme. The attached
glucose then serves as the primer required by glycogen
synthase to attach additional glucose molecules via the
mechanism described below. There are two glycogenin
genes in humans identified as GYG1 and GYG2. The GYG1
gene is located on chromosome 3q24-q25.1 and is
composed of 8 exons that generate three splice variant
mRNA. These three mRNAs produce three glycogenin-1
isoforms identified as isoform 1 (350 amino acids),
isoform 2 (333 amino acids), and isoform 3 (279 amino
acids).

The GYG1 gene is predominantly expressed
in muscle but is also expressed in many other
tissues as well. Mutations in the GYG1 gene
are associated the recently (2010)
characterized glycogen storage disease
identified as type 15 (GSD15). The GYG2 gene
is located on chromosome Xp22.3 and is
composed of 14 exons that multiple splice
variant mRNAs leading to the generation of
multiple glycogenin-2 isoforms. The GYG2
gene is predominantly expressed in the liver.

Synthesis of glycogen from glucose is carried out
by the enzyme glycogen synthase (GS). This
enzyme utilizes UDP-glucose as one substrate
and the non-reducing end of glycogen as
another. The activation of glucose to be used for
glycogen synthesis is carried out by the enzyme
UDP-glucose pyrophosphorylase. This enzyme
exchanges the phosphate on C-1 of glucose-1phosphate for UDP. The energy of the phosphoglycosyl bond of UDP-glucose is utilized by
glycogen synthase to catalyze the incorporation
of glucose into glycogen. UDP is subsequently
released from the enzyme.

There are two distinct glycogen synthase
enzymes in humans. One is expressed in skeletal
muscle the other in the liver. The muscle enzyme
is encoded by the GYS1 gene and the liver
enzyme is encoded by the GYS2 gene. The GYS1
gene is located on chromosome 19q13.3 and is
composed of 16 exons that produce two splice
variant encoding two isoforms of the muscle
enzyme. Isoform 1 is composed of 737 amino
acids and isoform 2 is composed of 673 amino
acids. The GYS2 gene is located on chromosome
12p12.2 and is composed of 20 exons that
produce a protein of 703 amino acids.

Beginning with free glucose, several reactions are
required to initiate and then produce glycogen
polymers. Glucose is first phosphorylated by
hexokinases or glucokinase to glucose-6-phosphate
(G6P). G6P is then converted to glucose-1-phosphate
(G1P) via the action of phosphoglucomutase (PGM).
G1P is then "activated" for glycogen synthesis via the
addition of uridine nucleotide catalyzed by G1P
uridyltransferase. The resultant UDP-glucose can
then be used as a substrate for the self-glucosylating
reaction of glycogenin, or if pre-exisiting glycogen
polymers exist, the UDP-glucose is utilized as the
substrate for glycogen synthase.

The α-1,6 branches in glucose are produced
by amylo-(1,4 to 1,6)-transglucosidase, also
termed the glycogen branching enzyme (gene
symbol: GBE1). This enzyme transfers a
terminal fragment of 6-7 glucose residues
(from a polymer at least 11 glucose residues
long) to an internal glucose residue at the C6 hydroxyl position. The GBE1 gene is located
on chromosome 3p12.3 and is composed of
16 exons that encode a protein of 702 amino
acids.

Functional glycogen phosphorylase is a homodimeric
enzyme that exist in two distinct conformational states: a
T (for tense, less active) and R (for relaxed, more active)
state. Phosphorylase is capable of binding to glycogen
when the enzyme is in the R state. This conformation is
enhanced by binding of AMP (allosteric activator) and
inhibited by binding of ATP or glucose-6-phosphate
(allosteric inhibitors). The enzyme is also subject to
covalent modification by phosphorylation as a means of
regulating its activity. The relative activity of the unmodified phosphorylase enzyme (given the name
phosphorylase-b) is sufficient to generate enough
glucose-1-phosphate for entry into glycolysis for the
production of sufficient ATP to maintain the normal
resting activity of the cell. This is true in both liver and
muscle cells.

PKA is cAMP-dependent protein kinase. PPI-1 is
phosphoprotein phosphatase-1 inhibitor. Green arrows
denote positive effects on any enzyme is indicated. Red Tlines indicate inhibitory actions. Briefly, phosphorylase b is
phosphorylated, and rendered highly active, by
phosphorylase kinase, PHK (glycogen synthase-glycogen
phosphorylase kinase, GS/GP kinase). Phosphorylase
kinase is itself phosphorylated, leading to increased
activity, by PKA (itself activated through receptormediated mechanisms). PKA also phosphorylates PPI-1
leading to an inhibition of phosphate removal allowing the
activated enzymes to remain so longer. Calcium ions can
activate phosphorylase kinase even in the absence of the
enzyme being phosphorylated. This allows neuromuscular
stimulation by acetylcholine to lead to increased
glycogenolysis in the absence of receptor stimulation.

In response to lowered blood glucose the α cells
of the pancreas secrete glucagon which binds to
cell surface receptors that are predominantly
found on hepatocytes. Glucagon receptors are
only found on one other cell type, white
adipocytes, but at significantly lower levels than
those seen on heptocytes. Because of this
distribution of receptors, it is easy to understand
why liver cells are the primary target for the
action of glucagon. The glucagon receptor is a
Gα-coupled GPCR. The response of cells to the
binding of glucagon to its cell surface receptor is,
therefore, the activation of the enzyme adenylate
cyclase.

Activation of adenylate cyclase leads to a
large increase in the formation of cAMP which
then binds to, and activates the enzyme
cAMP-dependent protein kinase, PKA (see
Figure below). Binding of cAMP to the
regulatory subunits of PKA leads to the
release and subsequent activation of the
catalytic subunits. The catalytic subunits then
phosphorylate a number of proteins on serine
and threonine residues.

Pathways involved in the regulation of
glycogen synthase by epinephrine:
Epinephrine (or norepinephrine) activation of
α1-adrenergic receptors results in the
activation of PLCβ. See the text for details of
the regulatory mechanisms. PKC is protein
kinase C. PLCβ is phospholipase Cβ. The
substrate for PLCβ is phosphatidylinositol4,5-bisphosphate (PIP2) and the products are
IP3 and DAG.




The net effects of the various
phosphorylations of glycogen synthase result
in:
1. decreased affinity of synthase for UDPglucose.
2. decreased affinity of synthase for glucose6-phosphate.
3. increased affinity of synthase for ATP and
Pi.

Reconversion of glycogen synthase-b to
glycogen synthase-a requires dephosphorylation.
This is carried out predominately by the
serine/threonine phosphatase described earlier,
PP1. This, of course is the same phosphatase
involved in the dephosphorylation of glycogen
phosphorylase described above. Although
another serine/threonine phosphatase, namely
protein phosphatase-2A (PP-2A), has been
shown to dephosphorylate glycogen synthase in
vitro, its role in vivo is significantly less than that
of PP1.

The activity of PP1 is also affected by insulin.
The pancreatic hormone exerts an opposing
effect to that of glucagon and epinephrine.
This should appear obvious since the role of
insulin is to increase the uptake of glucose
from the blood.

Since glycogen molecules can become enormously
large, an inability to degrade glycogen can cause cells
to become pathologically engorged; it can also lead
to the functional loss of glycogen as a source of cell
energy and as a blood glucose buffer. Although
glycogen storage diseases are quite rare, their effects
can be most dramatic. The debilitating effect of many
glycogen storage diseases depends on the severity of
the mutation causing the deficiency. In addition,
although the glycogen storage diseases are attributed
to specific enzyme deficiencies, other events can
cause the same characteristic symptoms. For
example, Type I glycogen storage disease (von Gierke
disease) is attributed to lack of glucose-6phosphatase.

However, this enzyme is localized on the
cisternal surface of the endoplasmic reticulum
(ER); in order to gain access to the phosphatase,
glucose-6-phosphate must pass through a
specific translocase in the ER membrane (see
Figure below). Mutation of either the
phosphatase or the translocase makes transfer of
liver glycogen to the blood a very limited
process. Thus, mutation of either gene leads to
symptoms associated with von Gierke disease,
which occurs at a rate of about 1 in 200,000
people.

The metabolic consequences of the hepatic
glucose-6-phosphate deficiency of von Gierke
disease extend well beyond just the obvious
hypoglycemia that results from the deficiency in
liver being able to deliver free glucose to the
blood. The inability to release the phosphate
from glucose-6-phopsphate results in diversion
into glycolysis and production of pyruvate as well
as increased diversion onto the pentose
phosphate pathway. The production of excess
pyruvate, at levels above of the capacity of the
TCA cycle to completely oxidize it, results in its
reduction to lactate resulting in lactic acidemia.

In addition, some of the pyruvate is transaminated to
alanine leading to hyperalaninemia. Some of the pyruvate
will be oxidized to acetyl-CoA which can't be fully
oxidized in the TCA cycle and so the acetyl-CoA will end
up in the cytosol where it will serve as a substrate for
triglyceride and cholesterol synthesis resulting in
hyperlipidemia. The oxidation of glucose-6-phophate via
the pentose phosphate pathway leads to increased
production of ribose-5-phosphate which then activates
the de novo synthesis of the purine nucleotides. In excess
of the need, these purine nucleotides will ultimately be
catabolized to uric acid resulting in hyperuricemia and
consequent symptoms of gout. The interrelationships of
these metabolic pathways is diagrammed in the Figure
below.

In the absence of glucose-6-phosphatase activity
free glucose cannot be release from the liver
contibuting to severe fasting hypoglycemia. In
addition the increased glucose-6-phosphate levels
lead to increased pentose phosphate pathway (PPP)
activity as well as increased glycolysis to pyruvate.
The incresased levels of pyruvate lead to increased
lactate produciton via lactate dehydrogenase (LDH)
and alanine via alanine transaminase (ALT). In
addition, the increased pyruvate is oxidized via the
pyruvate dehydrogenase complex (PDHc) leading to
increased production of acetyl-CoA which is, in turn,
used for the synthesis of fatty acids and cholesterol.

The excess glycolysis also results in
increased production of glycerol-3phosphate (G3P) from DHAP via the action of
glycerol-3-phosphate dehydrogenase (GPD1).
Increased G3P and fatty acids leads to
increased triglyceride synthesis which, in
conjunction with the increased cholesterol,
leads to hyperlipidemia as well as fatty
infiltration in hepatocytes contributing to
hepatomegaly and cirrhosis.

The glycogen storage diseases are divided into
two primary categories: those that result
principally from defects in liver glycogen
homeostasis and those that represent defects in
muscle glycogen homeostasis. The liver glycogen
storage diseases result in hepatomegaly and
hypoglycemia or cirrhosis, whereas the muscle
glycogen storage diseases result in skeletal and
cardiac myopathies and/or energy impairment.
The most notable muscle glycogen storage
disease is Pompe disease (type II GSD).

Several glycogenoses are the result of
deficiencies in enzymes of glycolysis whose
symptoms and signs are similar to those seen
in McArdle disease (type V GSD). These
include deficiencies in muscle
phosphoglycerate kinase and muscle
pyruvate kinase as well as deficiencies in
fructose 1,6-bisphosphatase, lactate
dehydrogenase and phosphoglycerate
mutase.