BMSC230 Mod. 5

• Glycogen


  Can be degraded by the liver and used to maintain blood glucose levels

  
• Control/regulation of glycogen degradation and synthesis


  Glucose to Glycogen

  
• Glycogen


  • A large, branched polymer of glucose
  • Long chain of glucose residues all connected by α-1,4 glycosidic bonds and α-1,6 glycosidic bonds

  
• Branched structure of glycogen

  • Synthesis and breakdown occur at the non-reducing ends
  • If it was linear, there would only be one site where glucose could be removed or added

  
• Efficiency of glycogen storage

  • 2 ATP are needed for glycogen synthesis
  • Breakdown yields 31 ATP for every glucose fully oxidized
  • Overall efficiency of storage is close to 94%

  
• Glycogenesis (synthesis)


  Uses activated form of glucose (UDP-glucose) to add glucose units to glycogen

  
• Glycogenolysis (degradation)


  Glucose-6-P is common intermediate that connects to glycolysis, gluconeogenesis and PPP

  
• Glucose is not the substrate for glycogenesis, instead UDP-glucose is

  
• Required enzymes in glycogen synthesis

  1. Glucose enters the cell

  2. Phosphorylation of glucose to glucose-6-P by hexokinase

  3. Glucose-6-P to glucose-1-P by phosphoglucomutase

  4. Glucose-1-P to UDP-glucose by UDP-glucose pyrophosphorylase

  
• UDP-glucose pyrophosphorylase


  Catalyzes the reaction where 2 terminal phosphate groups on UDP are released as pyrophosphate (PPi)

  
• Pyrophosphate drives many biosynthetic reactions
• Glycogen synthase catalyzes transfer of glucose to growing glycogen chain


  1. Glycogen synthase adds glucose to carbon 4 of a terminal glucose moiety (non-reducing end) on glycogen, forming α-1,4 glycosidic bond

  2. Glycogen synthase can only add glucose onto a glycogen that has at least 4 glucose units

  
• Glycogenin


  Protein that can synthesize an oligosaccharide of 10-20 glucoses long that acts as a primer for glycogen synthase

  
• How branching in glycogen is formed

  1. Glycogen synthase creates α-1,4 linkages to create long strands

  2. Branching enzyme hydrolyzes an α-1,4 linkage and forms an α-1,6 link or branch point

  
• A single glycogen granule could have up to 30,000 glucose units
• Glycogen synthase


  The rate limiting and regulatory step in glycogen synthesis, sensitive to allosteric activation by glucose-6-P

  
• Regulation of glycogen synthase

  • Unphosphorylated form (glycogen synthase a) is active, phosphorylated form (glycogen synthase b) is inactive
  • Insulin and glucagon influence phosphorylation state to regulate it

  
• Glycogen breakdown: glycogenolysis


  1. Glycogen phosphorylase cleaves glucose units one by one from the non-reducing end by catalyzing a phosphorolysis reaction

  2. Phosphorolysis produces glucose-1-P which can directly enter glycolysis

  
• Debranching of glycogen


  1. Transferase moves 3 glucose units to another branch

  2. α-1,6-glucosidase hydrolyzes the α-1,6 linkage, freeing the final glucose molecule

  
• Fate of glucose-1-P from glycogenolysis

  • In liver, glucose-6-P is produced and exported as free glucose
  • In muscle, glucose-6-P is metabolized through glycolysis to produce ATP

  
• Athletes can double their muscle glycogen stores through "glycogen supercompensation"
• Regulation of glycogen phosphorylase (GP)

  1. GP is regulated by allosteric interactions and reversible phosphorylation

  2. Liver and muscle express different GP isozymes that catalyze the same reaction

  
• Regulation of GP in the liver

  • 2 layers of regulation: phosphorylation and allosteric regulation by glucose
  • Phosphorylation by phosphorylase kinase converts GP to active "a" form, dephosphorylation by phosphorylase phosphatase converts it to inactive "b" form

  
• Glucose as an allosteric regulator of GP


  Adequate glucose levels inhibit GP, as there is no need to break down more glycogen

  
• Glycogen Phosphorylase (GP)


  • Regulated by allosteric interactions and reversible phosphorylation
  • Plays different roles in liver and muscle (they express different glycogen phosphorylase isozymes/enzymes with the same function, different structures)
  • Catalyze the same reaction
  • 90% same amino acid sequence (10% allows them to be different)

  
• GP regulation in the liver

  • 2 layers of regulation: phosphorylation and allosteric regulation
  • Made of identical subunits, each subunit can be phosphorylated on a single serine residue to convert the enzyme to the active "a" form (catalyzed by phosphorylase kinase)
  • Dephosphorylation is done by phosphorylase phosphatase to convert to the inactive "b" form
  • Phosphorylation is reversible in the cell

  
• GP and Glucose


  • Glucose is an allosteric regulator as an inhibitor
  • If there are adequate glucose levels, why break more glycogen down?
  • Glucose binds to molecule "a" or "b" and starts a conformational change (blocking the active site)

  
• GP regulation in the muscle

  • Uses covalent phosphorylation & allosteric regulation
  • Muscle phosphorylase needs to be active during muscle contraction in order for glucose to act as fuel for ATP production
  • When ATP gets used up, AMP levels increase (high AMP = low energy state in cell, therefore activating muscle phosphorylase b)
  • ATP inhibits since it competes with AMP for binding site
  • Glucose-6-p also inhibits, since it represents condition where adequate amounts of glucose are present for ATP creation & muscle use

  
• Glycogen Phosphorylase "b"


  Reacts to allosteric modifiers that reflect the intracellular energy charge

  
• Glycogen Phosphorylase "a" (phosphorylated form)

  Fully active to any level of AMP, ATP and glucose-6-p

  
• Hormones that trigger glycogen breakdown

  • Epinephrine (released when quick burst of energy or sudden and vigorous muscle contraction is needed)
  • Glucagon (secreted by Pancreas when blood sugar is low)
• Regulatory cascade for glycogen phosphorylase activation

  1. Glucagon binds to its receptor on a liver cell or epinephrine attaches to a muscle cell

  2. Enzyme adenylate cyclase is activated then catalyzes the conversion of ATP to CAMP

  3. CAMP acts as a second messenger and binds to protein Kinase A which stimulates a catalytic subunit to phosphorylate target protein Phosphorylase kinase

  4. GP "b" turns to the phosphorylated form GP "a" and stimulates glycogen breakdown

  
• Glycogen breakdown in muscles

  Muscle contracts - Ca2+ is released from the sarcoplasmic reticulum - Ca2+ binds to its subunit calmodulin - Phosphorylase kinase is activated - Muscle contraction and glycogen breakdown can occur at the same time
• CAMP signaling


  • Signal transduction pathway
  • Protein modification via phosphorylation - change shape or conformation, P group is passed on to other groups
  • Protein kinases & messenger
  • Cell membrane receptor shape changer
  • Signal message from outside to inner G Protein membrane signal
  • Secondary Messengers like CAMP
  • Cell Targets

  
• Epinephrine on the Liver

  1. Epinephrine (ligand) attaches and causes conformational change in adenylyl cyclase

  2. G Protein -> ATP -> CAMP subunits attaches to new protein (2nd messenger CAMP binds to the regulatory and activates it, releases the catalytic)

  3. Activated Protein Kinase acts on enzymes like Phosphorylase

  
• When Protein Kinase A is activated by CAMP


  • It has many target proteins so phosphorylase isn't always acted on
  • It could act on glycogen synthase a, then phosphorylate it to synthase b (inactive)
  • So phosphorylase becomes active and synthase b (inactive) is not, degradation is stimulated while synthesis is inhibited

  
• Protein Phosphatase 1 (PP1)

  • Removes phosphates from phosphorylase kinase and glycogen synthase, inactivating both (inhibiting glycogenolysis)
  • Dephosphorylates glycogen synthase -> increased activity stimulates glycogenesis

  
• Insulin stimulates glycogen synthesis

  • Increases number of glucose transporters (GLUT4) in membrane which increases glucose uptake which then is converted into glucose-6-p (allosteric activator of glycogen synthase b)
  • Inactivation of glycogen synthase kinase which reduces phosphorylation state of glycogen synthase therefore activity is increased

  
• Glucose regulates liver glycogen


  • Eat meal -> blood glucose ↑ - insulin release -> store glycogen
  • Glycogen synthase activity doesn't rapidly increase post meal (glucose infusion) until it is all converted to b/inactive form. Why? Glucose binds to phosphorylase which allows for easier dephosphorylation (by PP), meaning it is in the inactive form. PP1 gets released from phosphorylase, allowing it to bind to glycogen synthase, increasing dephosphorylation and activating the enzyme

  
• PP1 binds to phosphorylase a, NOT b