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Glycogen breakdown, Lecture notes of Biochemistry

St. George's UniversityBiochemistry

Biochemistry Glycogen metabolism

Typology: Lecture notes

2015/2016

Uploaded on 10/11/2016

Dinielle.White
Dinielle.White 🇬🇩

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Glycogen Breakdown:
• In muscle, the need for ATP results in the conversion of glycogen to glucose-6-phosphate for
entry into glycolysis.
• Glycogen's branched structure is important: it permits glycogen's rapid degradation through the
simultaneous release of the glucose units at the end of every branch.
• Plants store glucose as starch; animals store glucose as glycogen.
• Glycogen is considered a quick energy for the body because it is metabolized rapidly, it can be
metabolized anaerobically, and glycogen can be used to alter blood glucose level.
• Stored body fats produce higher quality energy than glycogen does.
• Liver and muscle are two main storage areas for glycogen.
Enzymes Required for Glycogen breakdown:
Glycogen phosphorylase:
• This is a dimer that catalyzes the controlling step in glycogen breakdown. The phosphorylase
reaction results in the cleavage of the C1-O1 bond from a nonreducing terminal glucosyl unit of
glycogen, yielding G1P.
Phosphoglucomutase:
• When G1P is formed, it is converted to glucose-6-phosphate either for entry into glycolysis in
muscle or hydrolysis to glucose in liver.
Glycogen debranching enzyme:
• This enzyme acts as an alpha (1->4) transglycosylase (glycosyl transferase) by transferring an
alpha (1->4)-linked trisaccharide unit from a limit branch of glycogen to the nonreducing end of
another branch.
Glycogen Synthesis:
• As obvious from the cause of McAdrle's disease, glycogen synthesis and breakdown occur in
separate pathways.
Enzymes of Glycogen Synthesis:
UDP-glucose pyrophosphorylase:
• This enzyme catalyzes the reaction of UTP and G1P to yield UDP-Glucose and release PPi. The
formation of PPi releases free energy that can be used nucleoside triphosphate hydrolysis to
drive an otherwise endergonic reaction to completion.
Glycogen synthase:
• This enzyme transfers the C4-OH group on one of glycogen's nonreducing ends to form alpha
(1->4)-glycosidic bond. Note: For each molecule of G1P that is converted to glycogen and then
regenerated, one molecule of UTP is hydrolyzed to UDP and Pi. The cyclic synthesis and
breakdown of glycogen is not a perpetual motion machine, but rather an engine that is powered
by UTP hydrolysis.
Glycogen branching enzyme:
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Glycogen Breakdown:

  • In muscle, the need for ATP results in the conversion of glycogen to glucose-6-phosphate for

entry into glycolysis.

  • Glycogen's branched structure is important: it permits glycogen's rapid degradation through the simultaneous release of the glucose units at the end of every branch.
  • Plants store glucose as starch; animals store glucose as glycogen.
  • Glycogen is considered a quick energy for the body because it is metabolized rapidly, it can be metabolized anaerobically, and glycogen can be used to alter blood glucose level.
  • Stored body fats produce higher quality energy than glycogen does.
  • Liver and muscle are two main storage areas for glycogen.

Enzymes Required for Glycogen breakdown:

Glycogen phosphorylase:

  • This is a dimer that catalyzes the controlling step in glycogen breakdown. The phosphorylase reaction results in the cleavage of the C1-O1 bond from a nonreducing terminal glucosyl unit of glycogen, yielding G1P.

Phosphoglucomutase:

  • When G1P is formed, it is converted to glucose-6-phosphate either for entry into glycolysis in

muscle or hydrolysis to glucose in liver.

Glycogen debranching enzyme:

  • This enzyme acts as an alpha (1->4) transglycosylase (glycosyl transferase) by transferring an

alpha (1->4)-linked trisaccharide unit from a limit branch of glycogen to the nonreducing end of another branch.

Glycogen Synthesis:

  • As obvious from the cause of McAdrle's disease, glycogen synthesis and breakdown occur in separate pathways.

Enzymes of Glycogen Synthesis:

UDP-glucose pyrophosphorylase:

  • This enzyme catalyzes the reaction of UTP and G1P to yield UDP-Glucose and release PPi. The

formation of PPi releases free energy that can be used nucleoside triphosphate hydrolysis to drive an otherwise endergonic reaction to completion.

Glycogen synthase:

  • This enzyme transfers the C4-OH group on one of glycogen's nonreducing ends to form alpha (1->4)-glycosidic bond. Note: For each molecule of G1P that is converted to glycogen and then regenerated, one molecule of UTP is hydrolyzed to UDP and Pi. The cyclic synthesis and breakdown of glycogen is not a perpetual motion machine, but rather an engine that is powered by UTP hydrolysis.

Glycogen branching enzyme:

  • Branching to form glycogen is accomplished by an enzyme called amylo-(1,4 -> 1,6)

transglycosylase (branching enzyme). Branches are created by transfer of terminal chain segments consisting of about 7 glucosyl residues to C6-OH groups of glucose residues on the same or another glycogen chain.

Control of Glycogen Metabolism:

  • Various types of process control glycogen synthesis; processes such as allosteric control and

enzyme-catalyzed covalent modifications of both glycogen synthase and glycogen phosphorylase.

  • There is direct allosteric control of glycogen phosphorylase and glycogen synthase. Some

effectors include ATP, G6P, and AMP.

  • When there is high demand of ATP, glycogen phosphorylase is stimulated and glycogen synthase is inhibited.
  • When ATP and G6P are high, the reverse is true and glycogen synthesis is favored.
  • In muscle, glycogen phosphorylase is activated by AMP, and inhibited by G6P and ATP.

Covalent modification of Enzymes by Cyclic Cascades:

  • Effector "Signal" Modification.
  • Glycogen phosphorylase and glycogen synthase can each be interconverted between two forms

with different kinetic and allosteric properties through reactions known as cyclic cascade.

  • This way, glycogen phosphorylase and glycogen synthase can respond to a greater number of allosteric stimuli, exhibit greater flexibility in their control patterns, and possess enormous

amplification potential in their responses to variation in effector concentrations.

  • Thus, a small change in concentration of an allosteric effector causes a large change in concentration of an active, modified target enzyme.

Glycogen Phosphorylase Bicyclic Cascade:

  • Glycogen phosphorylase exists in two forms: a and b.
  • A form is active without AMP; b form requires AMP for activation.
  • A form is Ser 14 enzymatically phosphorylated; form b is Ser 14 enzymatically dephosphorylated.
  • It should be mentioned that the activity of glycogen phosphorylase is also allosterically controlled by enzymatic interconversion through bicyclic cascade involving actions of 3 enzymes.

Glycogen Phosphorylase Activity Control:

Phosphorylase kinase:

  • It specifically phosphorylates Ser 14 of glycogen phosphorylase b.

Protein kinase A:

  • It phosphorylates/activates phosphorylase kinase.

Phosphoprotein phosphatase-1:

  • The two cascades are intimately related.
  • They are linked by protein kinase A, phosphorylase kinase, and phosphoprotein phosphatase-1.

More Glycogen Metabolism Facts:

  • Glycogen metabolism is regulated by peptide hormone insulin acting opposite to glucagon.
  • This is done together with epinephrine and norepinephrine.
  • Glucagon and norepinephrine initiate glycogen breakdown.
  • Low blood glucose causes alpha-pancreatic cells to secrete glucagon.

Glycogen is converted to G 6P and the following reaction

occurs:

  • G6P + H (^) 2O = glucose + Pi
  • If the above reaction can't occur, a type I glycogen storage disease occurs (eg., von Gierke's Disease).
  • When blood glucose level rises after a mean, insulin is released and insulin-dependent glucose transport system is activated (GLUT4).
  • The machinery of glycogen synthesis is activated.