Glycogen

Glycogen

1. Characteristics

   • -Polymer of alpha(1-->4) linked D-glucose with alpha (1-->6) linked branches every 8-10 residues
   • Most prominent in muscle & liver cells.
   • -Granules contain: enzymes for synthesis & breakdown & regulatory proteins -Glucose removed from nonreducing ends (w/out C1-OH)

2. Glycogen Breakdown (Glycogenolysis)

   • 3 Enzymatic Steps
     • 1. Glycogen Phosphorylase
       • 1. Glycogen + P (n residues)--> glycogen + G1P (n-1 residues) 2. Must be at least 5 glucose units from branch.
       • 1. Dimer of identical 842 residue subunits. 2. Rate controlling step: Allosteric interactions, covalent modification-phosphorylation, dephosphorylation
       • 1. Glycogen binding site allows several glucose units to be cleaved without unbinding/rebinding between catalytic cycles
       • Control Mechanisms
         • 1. Inhibitors & Activators
           • 1. Inhibitors & activators interact differently with phosphorylase a & b 2. Extermely sensitive regulation
           • Allosteric Inhibitor
             • ATP, G6P, glucose
           • Allosteric Activator
             • AMP
         • 2. Phosphorylation/Dephosphorylation
           • 1. Alters enzymatic control similar to allosteric 2. Presence of Ser 14-phosphoryl group causes similar change as AMP binding shifting T --> R
           • Accounts for observation that phosphorylase b requires AMP for activity while phosphorylase a does not
           • P causes conformation change
       • 2 Dimers
         • 1. Phosphorylase A
           • Phosphorylated form. Phosphoryl group esterified to Ser 14
             • Does not require AMP
         • 2. Phosphorylase B
           • Dephosphorylated form
             • Requires AMP
         • Both have large N terminal & smaller C-term domain
           • 1. N terminal: phosphorylation site, allosteric effector site, glycogen binding site. 2. Catalytic site located at the center of subunit
           • 1. Able to accom
       • Binds pyridoxal-5-phosphate (PLP) as cofactor
         • 1. Phosphate group which acts as acid-base catalyst
       • Properties
         • Catalyzes bond cleavage by phosphate sub
         • Conformational Change
           • 1. T (inactive) interconverts with R (active) 2. T state has buried active site, low sub aff 3. R state has accessible active site, high sub affinity
           • 1. AMP binding: Concerted T to R transition. 2. ATP binds T state inhibiting T to R shift
           • AMP allosteric activator. Binds to R, promotes switch
             • 1. AMP binds & T to R transition. Cover T state active site, prevent T substrate binding, increase binding affinity for P to R state 2. ATP binds T state inhibiting T to R transition
     • 2. Glycogen Debranching Enzyme
       • 1. Remove & transfer a trisaccharide from branch to nonreducing end of another branch 2. Acts as Alpha (1-4) transglycosylae by transferring an Alpha (1-4) linked trisaccharide and forms new alpha (1-4) linkage for glycogen phosphorylase
       • Has separate active sites at Steps 1 & 2
       • Breaks branch bonds
         • 1. Forms new alpha (1-4) linkage for glycogen phosphorylase
           • Transferase reaction
         • 2. Lase alpha (1-6) bond is hydrolyzed
           • 1. NOT PHOSPHORYLATED 2. Products: Glucose & debranched enzyme 3. Glucosidase reaction
     • 3. Phosphoglucomutase
       • 1. Similar to Phosphoglycerate mutase. 2. Phosphoryl group transferred to G1P making G1,6P which dephosph to G6P. 3. Near Eq, also functions in reverse
       • Converts G1P to G6P
         • 1. G6P converted to glucose for transport to other tissues
         • 1. Phosphoryl group transferred to G1P making Gl,6P which dephosphorylated to G6P 2. Near EQ, also functions in reverse
         • Can enter glycolysis or Hexose monophosphate pathway
         • liver
           • 1. G6P can be converted to glucose for transport to other tissues 2. G6P translocase & Glucose-6-phosphatase (G6Pase) 2. Glucose transported out by GLUT2
         • G6Pase
           • 1. Muscle & other tissues lack G6Pase.
         • Type I Glycogen Storage Disease
           • Intracellular conc. of G6P increases, Liver & kidney conc. of glycogen increases, and inability to increase conc. of glucose in blood

3. Glycogen Synthesis

   • 1. Glycogen synthesis does not require glycogen phosphorylase
   • 3 Enzymes Required
     • 1. UDP-glucose pyrophosphorlase
       • Creation of "activated" UDP-glucose
         • 1. Combines G1P with uridine triphosphate (UTP). Creates "activated" UDP-glucose 2. Delta Go is approx 0 as phosphoanhydride exchange 3. Coupled to inorganic pyrophosphate, deltaG=-19.2 KJ/mol
         • 1. Common biosynthetic strategy of cleaving nucleotide triphosphoate forming pyrophosphate 2. Total elimination of PPi prevents reverse rxn
     • 2. Glycogen Synthase
       • Transfer of glucose from UDP glucose to glycoen
         • 1. Combine 2 enzyme reactions 2. 1 molecule of UTP cleaved to UDP for each glucose incorporated into glycogen
         • 2. UTP replenished through nucleotide diphosphate kinase
       • Generates only alpha (1-4) linkgaes
         • 1. Glucosyl unit of UDPG transferred to glycogen at nonreducing C4-OH end forming alpha (1-4) glycosidic bond
       • Inhibited by 1,5-gluconolactone
       • 2 forms
         • 1. Form a: dephosphorylated & active 2. Form b: phosphorylated
         • Allosteric Control
           • 1. Strongly modulcated by physiological conc. of ATP, ADP, P 2. Phosphorylated form b almost totally inactive, dephosphorylated form a activated by G6P
       • Function
         • 1. Glycogen synthase activity varies with conc. of G6P & fraction of dephosphoryated a 2. many phosphorylation sites 3. glycogen "Primes" glycogen synthase. Cannot join 2 glucose residues
       • Initiated by 1 glycogenin
         • 1. Attaches glucose moeity to OH group on Tyr 194 residue 2. Extends glucose chain by 7 additional UDPG donated glucose residues 3. Glycogen primer. Glycogen synthase can start glycogen synthesis
         • Branching performed by amylo-(1,4 --> 1,6) transglycosylase
           • 1. Transfer a 7 residue segment from end of 11 residue chain to C6-OH of glucose 2. Branch at least 4 residues away from branch point
     • 3. Glycogen Branching Enzyme
       • Creates branches alpha (1-6) linkages

4. McArdle's Disease

   •  1. Obvious glycogen synthesis does not require glycogen phosphorylase 2. Lack muscle glycogen phosp & cannot break down glycogen

5. Control of Glycogen Metabolism

   • Synthesis/Breakdown
   • Control
     • 1. Allosteric Control
       • Muscle/Phosphorylase/Synthase
         • 1. Allosteric control by ATP, G6P, AMP
         • 1. Activate Phosphorylase by AMP 2. Inhibit Phosphorylase by ATP & G6P
         • 1. Activate synthase by G6P 2. Inhibit??
     • 2. Hormonal Control
       • Binding to specific hormone receptors triggers 2nd messengers
         • 1. Common 2nd messengers: cAMP and Ca+2 released into cytosol
         • cAMP
           • 1. When [cAMP] increases, PKA activity increases, phosphorylation rates increase & dephosphorylation decreases 2. Small change in cAMP leads to large change in phosphorylated enzyme
           • Enzymes Phosphorylated
             • 1. Metabolic flux toward glycogen breakdown. Phophorylase active and synthase inactive 2. [cAMP] decreases,
       • Liver
         • 1. Glucagon binding generates intracellular cAMP. Results in glucose mobilization. 2. Glucagon critical for supplying glucose to tissues depending primarily on glycogen for energy needs (not muscle)
     • AC & HC are superimposed
       • 1. Interconversion of a & b forms of phosphorylase and synthase accomplished through hormone regulated enzyme catalyzed phosphorylation & dephosph
       • Phosphorylase a
         • 1. Activated even without AMP stimulation 2. Synthase inactive unless dephophorylated and G6P present
       • Relationship between Phosphorylase a/b of glycogen synthase/phosphorylase
         • 1. Protein Kinase A phosphorylates phosphorylase kinase and then this phosphorylates glycogen phosphorylase b to a