Glycogen Metabolism

The glucose monomers in the main chains are joined by a-1-4
glycosidic linkages, whereas at branch points the linkage is a-1-6.

Glycogen serves as a source of stored glucose mainly in the liver
and skeletal muscle

Glycogenolysis (breakdown of glycogen) in the liver results in the
production of glucose, which is released into the blood, and
eventually utilized by various other tissues

The major site of glucose consumption (75%) is the brain,
through aerobic metabolism

The product of glycogenolysis (glucose-1-phosphate) in
muscle is not available to other tissues, since muscle lacks the crucial enzyme
glucose-6-phosphatase

Animals use the alpha anomer of glucose for
glycogen synthesis

Whereas plants, use the beta anomer of glucose
for cellulose biosynthesis and alpha anomer for starch
synthesis

The Synthesis of glycogen from glucose is carried out by the
enzyme glycogen synthase

Glycogen synthase utilizes an activated glucose
monomer UDP-glucose, to extend the polymer by one
unit

Several rounds of such catalytic condensations result in a large glycogen polymer (amylose,
found in potato starch and glycogen = straight chain polymer of glucose with a-1-4- glycosidic
linkages).

Glucose1-phosphate UTP uridylyl
transferase, using UTP and glucose
1-phosphate catalyzes the formation of
UDP-glucose (UDPG) and
pyrophosphate (PPi).

Pyrophosphatase is a ubiquitous
enzyme, that very rapidly converts
pyrophosphate (PPi) into two inorganic
phosphates (2Pi).

Thus thermodynamically, the forward direction of UDP-glucose synthesis is highly
favored

The energy of the phospho-glycosyl bond of UDP-glucose is utilized by glycogen synthase to catalyze the incorporation of glucose into glycogen.

A protein known as glycogenin has been
shown to perform primer synthesis

Glycogenin autocatalytically attaches
C-1 of a UDP-glucose (UDPG) to its
tyrosine residue

From the first glucosyl residue, 7 more
glucosyl units are sequentially added
using 7 UDPG to form an octamer
attached to glycogenin.

UDP is only involved in polymer formation. It is a byproduct!

The a-1,6 branches in glycogen are synthesized by a( a-1-4)-
(a-1-6)-glucan transferase commonly known as a branching
enzyme.

Branching enzyme transfers a terminal fragment of 6 to 7 glucose
residues (from a polymer of at least 11 glucose residues long) to
an internal glucose residue at the C-6 hydroxyl position to form
a branching point.

Free hydroxyl on C4 C

Free hydroxyl on C-1

The degradation of glycogen
(glycogenolysis) is catalyzed by glycogen
phosphorylase.

Glycogen phosphorylase contains
vitamin B6 (pyridoxal phosphate,
[PLP]) moiety covalently attached to the
lysine residue of the enzyme which
participates in the catalysis.

The PLP phosphate activates inorganic
phosphate by deprotonation at the
active site.

The degradation of the glycogen starts from
the non-reducing end of the glycogen

Glycogen phosphorylase uses inorganic phosphate (phosphorolysis) to produce
glucose1-phosphate.

In contrast to glycogen phosphorylase, a-amylase uses water to hydrolyze glycogen to form glucose during the process of digestion in the mouth (salivary) and stomach (pancreatic).

The advantage of phosphorolytic cleavage is that glucose-1-phosphate is formed without the
need for ATP, and the concentration of inorganic phosphate (Pi) inside the cell is high enough
to drive the reaction in a favorable direction

At four residues before a branching point, phosphorylase
stops cleaving the glycogen.

(1) Debranching enzyme transfers the terminal
trisaccharide onto another branch.
(2) e a-1,6-glucosidase activity
removes the last glucose residue from
the branch by hydrolysis

In theory, glycogenolysis can release one free glucose
molecule/ branch by hydrolysis due to the action of
glucosidase

In muscle, the activity of hexokinase is so high that the free glucose is immediately
phosphorylated into glucose 6-phosphate, which enters the glycolytic pathway

The glucose 1-phosphate produced by the action of
phosphorylase is converted into glucose 6-phosphate
by the enzyme phosphoglucomutase

Phosphoglucomutase contains a phosphorylated
amino acid (Ser) in the active site

Phosphoglucomutase generates glucose 1,6- bisphosphate as a transient intermediate. Phosphate is then transfered forming G6P.

In muscle, the sole purpose of glycogenolysis is to break down glycogen, to form glucose
1-phosphate, which is converted into glucose 6-phosphate for glycolysis

In the liver, the enzyme glucose 6-phosphatase releases free glucose, which enters
the bloodstream for maintaining blood glucose level

Allosteric Regulation: Regulation of enzyme activity by an
effector or a metabolite that binds to the allosteric site

An effector is a compound other than the substrate that
modulates enzyme activity in allosteric regulation.

Glycogen phosphorylase (i.e. phosphorylase-b) is a
homodimeric enzyme that exists in either T or R state

Phosphorylase is capable of binding glycogen when the enzyme is in the R state

-AMP is positive effector
-ATP and Glucose6Phosphate are negative effectors

During basal cellular needs, phosphorylase-b, controlled by metabolites, governs the
glycogen breakdown, which is sufficient to provide the needed energy for various cell types

Phosphorylase is subject to covalent modification by phosphorylation, catalyzed by
phosphorylase kinase.

In resting state Phosphorylase-b, is sufficient to
generate glucose-1-phosphate for entry into glycolysis and production of ATP in liver

During high energy demand, phosphorylase-a is needed
for these entrie

Calcium activates insulin gene expression and its secretion

In response to lowered blood glucose, the a-cells of the
pancreas secrete glucagon (peptide hormone)

Glucagon binds surface receptors located in the liver cell
membrane and causes the activation of the enzyme
adenylate cyclase, which forms the second messenger
cAMP.

1. Glucagon actibates andeylate cylase, which forms second messenger cAMP
2. cAMP binds protein kinase A
3. R subunit modified to active form
4. PKA activates phosphorylase kinase to phosphorylate Ser
4. Activated phosphorylasekinase phosphorylates phosphorylase, turning it into a

Formation of phosphorylase a results is a large increase (cascade effect) in glycogen breakdown

During the “fight or flight response”, epinephrine, is released

Epinephrine binds to its receptor, for example, in muscle cells and activates the glycogenolysis
cascade

In flight or fight, the stored muscle glycogen is immediately released by action of active glycogen
phosphorylase-a in the form of glucose 1-
phosphate, which is immediately
isomerized to glucose 6-phosphate

In flight or fight, the yield/mole of phosphorylated
glucose is 3 net ATP (bc no ATP
used at hexokinase)

Each stored glucosyl unit of glycogen would yield 33 ATP as opposed to the free
glucose which would yield a net 32 ATP

The phosphorylase kinase activity is also affected by calcium ions by activating it

Increased intracellular calcium increase the rate of muscle
contraction, but it also increases glycogenolysis, which provides the muscle cells with the
increased ATP that is also needed for muscle contraction.

Alpha Adrenergic System: The signaling mechanism is mediated by second messenger molecules,
Inositol trisphosphate (IP3) and diacylglycerol (DAG)

The action of epinephrine at a-adrenergic
receptors is coupled through G-proteins that
activate phospholipase C (PLC).

Activation of PLC leads to the hydrolysis of
phosphatidyl inositol 1-4,5-bisphosphate (PIP2)
into inositol trisphosphate (IP3) and
diacylglycerol (DAG).

DAG binds to and activates protein
kinase C (PKC) an enzyme that
phosphorylates numerous substrates,
one of which is glycogen synthase

IP3 binds to receptors on the surface of the
endoplasmic reticulum leading to the release of calcium ions

It actibates PKC, which phosphorylates glycogen synthase (turns it off)

Glycogenolysis is “turned off” by phosphoprotein phosphatase-1 (PP-1).

PP-1 is regulated by phosphoprotein phosphatase inhibitor
(PPI)

Phosphorylation in the case of phosphorylase leads to activation

Phosphorylation in the case of glycogen synthase inactivates it, avoiding the futile
cycle.

The activity of glycogen synthase is regulated by phosphorylation of serine residues in the
subunit proteins

Phosphorylation of glycogen synthase reduces its affinity toward its substrate UDP-glucose

In the non-phosphorylated state, glycogen synthase does not require glucose 6-phosphate as an allosteric activator (I-form of glycogen synthase) Corresponds to glycogen synthase a

In the phosphorylated state, glycogen synthase requires the allosteric activator, glucose
6-phosphate (D-form of glycogen synthase). Corresponds to glycogen synthase b

Phosphorylation of synthase occurs primarily in response to hormonal activation of cAMP-dependent protein kinase A.

One of the major kinases that phosphorylate glycogen synthase is synthase-phosphorylase
kinase; the same enzyme that phosphorylates glycogen phosphorylase

PKA itself, glycogen synthase kinase 3 (GSK-3), and calmodulin dependent protein kinase

When a-adrengenic receptors are stimulates, the DAG and Ca2+ ions released by IP3 activates PKC, which phosphorylates and inactivates glycogen synthase

Phosphorylation causes an increased dependency of glycogen synthase on G6P (allosteric activator)

Dephosphorylation of glycogen synthase is done by protein phosphatase-1 (PP-1)

Insulin the pancreatic hormone exerts an opposing effect to that of glucagon and
epinephrine

Insulin in general is an anabolic (synthesis inducing) hormone, whereas
glucagon is a hormone that induces catabolic degradative) events.

Von Gierke’s Disease: It is attributed to the lack of glucose 6-phosphatase. Mutations of the phosphatase or the translocase make the transfer of liver glucose to the
blood a very limited process.

Remember that muscle tissue is not affected by the glucose 6-phosphate gene mutation(s) since
muscle lacks the enzyme glucose 6-phosphatase.

Pompe’s Disease: Lysosomal a-glucosidase deficiency

McArdles Disease: Skeletal muscle phosphorylase deficiency

Cori’s Disease: Defect in debranching enzyme

Andersen’s Disease: Defect in branching enzyme

Hers Disease: Liver phosphorylase deficiency

In diabetes, insulin/ glucagon ratio is very low, the liver is
breaking down and depleting the storage glycogen to
make further increasing amounts of glucose.

In diabetes, the excess glucose in the blood is converted into
glucitol (sorbitol) through the action of
non-specific dehydrogenases and results in
osmotic imbalances in various tissues,
particularly the eye lens and causes Cataracts
due to the high viscosity of sorbitol.

Type I diabetics due to low levels of insulin exhibit hyperlipidemia in the blood. This is due to a low level of insulin messing up fat storage.

Glycoconjugates are formed by the reactive aldehyde
of glucose that forms linkages with many proteins
either with the amino-terminal residue or with
reactive lysines (i.e. A1C)

The excess glucose from the blood is converted into glucitol
(sorbitol) by non-specific dehydrogenases in some of the tissues
(e.g., retina, kidney, etc.).

Pancreatic tumors can lead to hypoglycemia. Insulin/ glucagon rasion is high.