METABOLISM OF CARBOHYDRATES:
SYNTHESIS AND DEGRADATION OF
GLYCOGEN


1. Liver glycogen:
-Forms 8-10% of the wet weight of the liver.
-Maintains blood glucose (especially between meals).
-Liver glycogen is depleted after 12-18 hours fasting.

2. Muscle glycogen:
-Forms 2% of the wet weight of muscle.
-Supplies glucose within muscles during contraction.
-Muscle glycogen is only depleted after prolonged exercise.




Glycogenesis: synthesis of glycogen from glucose.
Glycogenolysis: breakdown of glycogen to glucose-1-phosphate.
Gluconeogenesis: synthesis of glucose or glycogen from noncarbohydrates precursors.

In the well-fed state the glucose after absorption is taken by liver and deposited as a glycogen
Glycogen is a very large, branched polymer of glucose residues that can be broken down to yields glucose molecules when energy is needed
Most glucose residues in glycogen are linked by a-1,4-glycosidic bonds, branches are created by a-1,6-glycosidic bonds

Glycogen serves as a buffer to maintain blood-glucose level.
Stable blood glucose level is especially important for brain where it is the only fuel.
The glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity.
Liver (10 % of weight) and skeletal muscles
(2 %) – two major sites of glycogen storage
Glycogen is stored in
cytosolic granules in muscle and liver cells of vertebrates

Glucose-6-phosphate is the central metabolite in the synthesis and decomposition of glycogen.
In the well-fed state glucose is converted to glucose-6-phosphate, which is the precursor for the glycogen synthesis.
The glucose-6-phosphate derived from the breakdown of glycogen has three fates: (1) glycolysis; (2) pentosephosphate pathway; (3) convertion to free glucose for transport to another organs.

DEGRADATION OF GLYCOGEN
Glycogenolysis - degradation of glycogen
The reaction to release glucose from polysaccharide is not simple hydrolysis as with dietary polysaccharides but cleavage by inorganic phosphate – phosphorolytic cleavage
Phosphorolytic cleavage or phosphorolysis is catalyzed by enzyme glycogen phosphorylase
There are two ends on the molecules of starch or glycogen: a nonreducing end (the end glucose has free hydroxyl group on C4) and a reducing end (the end glucose has an anomeric carbon center (free hydroxyl group on C1)

Glycogen phosphorylase removes glucose residues from the nonreducing ends of glycogen
Acts only on a-1-4 linkages of glycogen polymer
Product is a-D-glucose 1-phosphate (G1P)
Cleavage of a glucose residue from the nonreducing end of glycogen

Structure of glycogen phosphorylase (GP)
• GP is a dimer of identical subunits (97kD each)
• Catalytic sites are in clefts between the two domains of each subunit
• Binding sites for glycogen, allosteric effectors and a phosphorylation site
• Two forms of GP
Phosphorylase a (phosphorylated) active form
Phosphorylase b (dephosphorylated) less active

• GP catalyzes the sequential removal of glucose residues from the nonreducing ends of glycogen
• GP stops 4 residues from an a 1-6 branch point
• Tranferase shifts a block of three residues from one outer branch to the other
• A glycogen-debranching
enzyme or 1,6-
glucosidase hydrolyzes the 1-6-glycosidic bond
• The products are a free glucose-1-phosphate molecule and an elongated unbranched chain

Metabolism of Glucose 1-Phosphate (G1P)
• Phosphoglucomutase catalyzes the conversion of G1P to glucose 6-phosphate (G6P)

• Synthesis and degradation of glycogen require separate enzymatic steps
• Cellular glucose converted to G6P by hexokinase
• Three separate enzymatic steps are required to incorporate one G6P into glycogen
• Glycogen synthase is the major regulatory step

Glucose 1-Phosphate formation
• Phosphoglucomutase catalyzes the conversion of glucose 6-phosphate (G6P) to glucose 1phosphate (G1P).

UDP-glucose is activated form of glucose.
UDP-glucose is synthesized from glucose-
1-phosphate and uridine
triphosphate (UTP) in a reaction catalized by
UDP-glucose pyrophosphorylase

Glycogen synthase adds glucose to the nonreducing end of glycogen

A branching enzyme forms  -1,6-linkages
Glycogen synthase catalyzes only  -1,4linkages.
The branching enzyme is required to form
 -1,6-linkages.
Branching is important because it increases the solubility of glycogen.
Branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase.

• Muscle glycogen is fuel for muscle contraction
• Liver glycogen is mostly converted to glucose for bloodstream transport to other tissues
• Both mobilization and synthesis of glycogen are regulated by hormones
• Insulin, glucagon and epinephrine regulate mammalian glycogen metabolism

Hormones Regulate Glycogen Metabolism
• Insulin is produced by b-cells of the pancreas
(high levels are associated with the fed state)
• Insulin increases rate of glucose transport into muscle, adipose tissue via GluT
4 transporter
• Insulin stimulates glycogen synthesis in the liver via the second messenger phosphatidylinositol
3,4,5-triphosphate (PIP
3
)

• Secreted by the a cells of the pancreas in response to low blood glucose (elevated glucagon is associated with the fasted state)
• Stimulates glycogen degradation to restore blood glucose to steady-state levels
• Only liver cells are rich in glucagon receptors and therefore respond to this hormone

• Released from the adrenal glands in response to sudden energy requirement (“fight or flight”)
• Stimulates the breakdown of glycogen to G1P
(which is converted to G6P)
• Increased G6P levels increase both the rate of glycolysis in muscle and glucose release to the bloodstream from the liver and muscles
• Both liver and muscle cells have receptors to epinephrine

Effects of hormones on glycogen metabolism

Reciprocal Regulation of Glycogen
Phosphorylase and Glycogen Synthase
• Glycogen phosphorylase (GP) and glycogen
synthase (GS) control glycogen metabolism in liver and muscle cells
• GP and GS are reciprocally regulated both covalently and allosterically (when one is active the other is inactive)
• Covalent regulation by phosphorylation (-P) and dephosphorylation (-OH)
• Allosteric regulation by glucose-6-phosphate
(G6P)

COVALENT REGULATION
Active form “a” Inactive form “b”
Glycogen phosphorylase -P -OH
Glycogen synthase -OH -P
ALLOSTERIC REGULATION by
G6P
GP a (active form) - inhibited by
G6P
GS b (inactive form) - activated

Activation of GP and inactivation of GS by
Epinephrine and Glucagone

Activation of GS and inactivation of GP by Insulin


Muscle glycogen can be converted to glucose via indirect pathways:

Cori's cycle: during muscle exercise

Glucose- alanine cycle: during starvation

Glycogen glycolysis gluconeogenesis





When anaerobic conditions occur in active muscle, glycolysis produces lactate.
The lactate moves through the blood stream to the liver, where it is oxidized back to pyruvate.
Gluconeogenesis converts pyruvate to glucose, which is carried back to the muscles.
The Cori cycle is the flow of lactate and glucose between the muscles and the liver.

Glycogen gluconeogenesis transamination