Glycogen Metabolism

• Glycogen

  • Polymer of Glucose by alpha-1,4 bonds in linear and alpha-1,6 bonds in branched per 10 - 12 glucose units
  • Energy storage form in animals
  • Stored in liver and muscle
• Epinephrine

  • Other term is "Adrenaline"
  • Muscle tends to be more sensitive in adrenaline, and its release would lead to mobilization of glycogen to form glucose by a cascade of signals
• The molecule has many nonreducing ends but only one reducing end

  Reducing end of polysaccharide - anomeric carbon with a free hydroxyl group (C1)

  Nonreducing end of polysaccharide - Seen at C4
• In vivo, the glycogen reducing end is linked to a protein glycogenin

  • The number of nonreducing ends in a glycogen molecule is significant because it is the site of action of key enzymes
• Electron micrograph of a glycogen granule from rat skeletal muscle

  • Each granule (α) consists of several spherical glycogen molecules (β) and associated proteins

  
• When energy is needed, fast mobilization of glucose from glycogen is used because there is simultaneous release of glucose from its nonreducing ends, leading to a huge production of glucose
• Glycogen metabolism is the regulated release (degradation) and storage (synthesis) of glucose
• Glycogen breakdown, or glycogenolysis requires three enzymes

  1. Glycogen phosphorylase (or simply phosphorylase) catalyzes glycogen phosphorolysis (bond cleavage by the substitution of a phosphate group) to yield glucose-1-phosphate (G1P)
  2. Glycogen debranching enzyme removes glycogen’s branches, thereby making additional glucose residues accessible to glycogen phosphorylase.
  3. Phosphoglucomutase converts G1P to G6P, which has several metabolic fates.
• Glycogen degradation

  • By its specificity, after degrading glycogen, the remaining glycogen would be unsuitable for the action of the enzyme because it is not recognized.
  • It has to be remodeled by PGM for it to continue degradation
• Regulation is through

  • Allosteric control
  • Through allosteric responses, enzymes activity is adjusted to meet the needs of the cell
  • ATP, NADH, ADP are molecules that signify the energy needs of the cell
  • Muscle undergo drastic changes in energy state when allosteric control undergoes
  • Hormonal Control
  • Regulation by hormone adjusts glycogen metabolism to meet the needs of the entire organism
  • Liver is more responsive in the activity of the hormone
• Glycogen phosphorylase

  • A key enzyme in glycogen breakdown
  • Catalyzes the sequential removal of glucosyl residues from the nonreducing ends of glycogen with the addition of mono hydrogen phosphate in the reducing end forming G1P and remnants of glycogen
• Glycogen Phosphorylase

  • A dimer of two identical 97 kd subunits
  • Each subunit is compactly folded into an amino-terminal domain (480 residues) containing a glycogen binding site and a carboxyl terminal domain (360 residues)
  • The catalytic domain is hydrophobic and is located in a deep crevice formed by residues from both domains
• Phosphorylase cofactor PLP

  • pyridoxal-5'-phosphate (PLP)
  • Pyridoxamine (Vitamin B6)
• The phosphorylytic cleavage of glycogen is energetically advantageous because the released sugar already phosphorylated
• • The phosphorylytic cleavage of glycogen is energetically advantageous because the released sugar is already phosphorylated
  • The G1P, just like G6P is trapped in the cell because it is already negatively charged
• At branch of 10-14 glucosyl units, the enzyme glycogen phosphorylase would stop degrading glycogen when the branch has 4 glucosyl units remaining from the branched site
• A debranching enzyme is also needed for the breakdown of glycogen

  • Glycogen phosphorylase cannot degrade glycogen at branch points
• Two enzymes are needed for remodeling of glycogen

  • Transferase
  • shifts a block of three glucosyl residues from the tetrasaccharide from one outer branch to the other
  • The branch that would accept the trisaccharide has to be at least 4 glucosyl residues away from the branched site which also has a nonreducing end upon transformation (C4)
  • Alpha-1,6-GLYCOSIDASE
  • Debranching enzyme which hydrolyzes the alpha-1,6-glycosidic bond
  • Alpha-1,6-GLYCOSIDASE releases free glucose

  It is now suitable into a linear chain for action of glycogen phosphorylase
• These enzymes (glycogen phosphorylase and debranching enzyme) are contained inside a one big polypeptide chain
• A free glucose is released and then phosphorylated by the glycolytic enzyme hexokinase
• Phosphoglucomutase converts Glucose 1-phosphate into Glucose 6-phosphate to enter other metabolic pathways
• • The phosphoglucomutase involves a phosphorylated serine residue
  • The phosphate is transferred to C6 of glucose forming G1,6P
  • To regenerate the enzyme, the phosphate in C1 is transferred back yielding G6P

  
• The liver contains Glucose 6-phosphatase, a hydrolytic enzyme absent from muscle

  • Liver maintains glucose homeostasis of the whole organism
  • Liver releases glucose and it is transported to the bloodstream to supply glucose where it is needed
• Glucose-6-phosphatase is an enzyme present in the liver that ends at G6P during gluconeogenesis and is used to counteract Hexokinase
• Phosphorylase is regulated by allosteric interactions and reversible phosphorylation

  • Allosteric interaction - binding of small molecules that would signal the energy state of the cell
  • Reversible phosphorylation - when hormones are released, a cascade of event occurs and the glycogen phosphorylase/phosphorylation of enzyme changes its enzymatic activity
• The two tissues stored in glycogen have different regulations

  1. Muscle - uses glucose to produce energy for its own use
  2. Liver - maintains glucose homeostasis of the whole organism

  If high concentration of glucose is seen in the blood, no degradation of glucose occurs but glycogen synthesis occurs
• Glycogen phosphorylase is activated by phosphorylation

  1. Phosphorylase kinase, specifically phosphorylates Ser 14 of inactivated glycogen phosphorylase b
  2. Protein kinase A (PKA), which phosphorylates and thereby activates phosphorylase kinase
  3. Phosphoprotein phosphatase-1 (PP1), which dephosphorylates and deactivates both glycogen phosphorylase a and phosphorylase kinase
• Muscle phosphorylase is regulated by the intracellular energy charge
• Two dimeric forms

  • A usually active phosphorylase a
  • Phosphorylated serine
  • A usually inactive phosphorylase a
  • Both forms exist in equilibria between an active R state and less active T state
  • The b favors the T state
• The muscle phosphorylase b is active only in the presence of high AMP
• The muscle phosphorylase b is active only in the presence of high AMP

  • AMP - Signify low energy charge; which favors the transition to the R state
• Muscle phosphorylase b has two inhibitors leading to the T state formation

  1. ATP - competes with the nucleophilic binding site vs AMP
  2. 2 Glucose 6-Phosphate - G6P would stay in the muscle because it does not have glucose-6-phosphatase
• The transition of the phosphorylase b between the active R state and the less active T state is controlled by the energy charge of the muscle cell
• Liver phosphorylase - produces glucose for use by other tissues
• Allosteric regulation of liver phosphorylase

  • The binding of glucose to glycogen phosphorylase a shifts the equilibrium to the T state and inactivates the enzyme
  • Thus, glycogen is not proceeded when glucose is already abundant
  • Free glucose is obtained from glucose intake
• Phosphorylase kinase is activated by phosphorylation (Protein Kinase A) and calcium ions
• Phosphorylase Kinase

  • activates phosphorylase b
  • Heterotetrameric protein (αβγδ)_4 and mass is 13000 kDa
  • γ subunit has the catalytic activity
  • Under dual control : phosphorylation and [Ca2+]
  • β subunit responds to phosphorylation while δ subunit is calmodulin
• Phosphorylase Kinase

  • Ca2+ is released from the sarcoplasmic reticulum of the muscle during rigorous exercise
  • Attains maximal activity only after both phosphorylation of the β subunit and activation of the δ subunit by Ca2+ binding resulting for phosphorylase b to converted to phosphorylase a
  • Attains partial activity by either phosphorylation or activation alone
• Protein kinase A is activated when hormones are present

  • Epinephrine and glucagon signal the need for glycogen breakdown
• Epinephrine

  • Fight or flight syndrome
  • A cathecolamine derived from tyrosine
  • released from the adrenal medulla during exercise
  • stimulates glycogen breakdown in the muscle and liver to provide fuel for muscle contraction