Glycogen metabolism

How energy (glycogen) is stored and distributed. Critical decisions - supply energy during exercise. Prevent hypoglycemia. Control glucose toxicity
- glycogen structure
- glycogenolysis (breakdown)
- glycogenesis (synthesis)
- regulation - model for other regulatory systems
Energy metabolism- more complicated than power station or car
- energy must be stored as well as distributed
- chemical energy (glucose) is supplied through the blood
- energy is used at different rates in different tissues
- energy content of the blood must be buffered (liver)
Glycogen is like a storage battery
- charged when glucose is in excess
- discharged when glucose needed (fasting or exercise)
- Glycogen present as β particles, ~30 nm diameter with up to 60,000 glucose units
Problems to overcome
- intermittent supply, demand
- toxicity of glucose
- need minimum (to avoid hypoglycemic shock)
Energy demand and supply
- constant demand (brain)
- intermittent demand (muscle)
- supply is intermittent (meals)
Liver is primary storage site (cells 10% glycogen)
- take up excess and maintain level
- muscle is secondary storage (1% glycogen)
- why secondary storage sites?
- body has 40 Kcal of glucose in fluids
- 600 Kcal in glycogen
- what happens when glycogen used up?
Structure of glycogen
- largely α 1-4 linked glucose
- cellulose is β linked
- starch is also α 1-4 linked
- amylose - pure linear chain
- amylopectin - α 1-6 branches (every 30 residues)
- glycogen has α 1-6 branches every 10 residues
- why not store as glucose solution? sucrose?
Advantages of branched structure
- glucose released from ends
- more ends - faster release (diesel vs coal)
- analogous to plates in car battery (larger - more power)
- dendrimer - polymer built from branched monomers - a way to put lots of reactive groups on the surface
- glycogen not built by polymerizing branched monomers
- polymerize simple monomers, branches by rearrangement
- optimization of structure
Comparison of plants and animals
- animals - high energy demand
- plants - lower energy demand
Enzymes of glycogenolysis
Glycogen + Pi ⇒ Glycogen(n-1) + G-1-P
-Debranching enzyme
transglucosylase (α 1-4 to α 1-4)
α 1-6 glucosidase
G-1-P ⇒ G-6-P
-Hexokinase (Glucokinase)
G + ATP ⇒ G-6-P + ADP
Glycogenolysis - breakdown of glycogen
- phophorolysis (not hydrolysis) - high energy bond to high energy bond
- yields G-1-P
- equilibrium when P/G-1-P = 3.6 (~100 under normal conditions)
- proceeds to within 4 residues of branch
- debranching enzyme transfers branch to nonreducing end of "main chain"
- debranching enzyme hydrolyzes last glucose
- phosphorylase continues
Fate of G-1-P
- conversion to G-6-P by phosphoglucomutase
- can proceed directly to glycolysis
- in liver can be hydrolyzed by glucose-6-phosphatase, glucose to blood
- energetically cheap
- hydrolyzed glucose (α 1-6 linkage) must be phosphorylated by hexokinase
- uses only ~1 ATP/10 glucose
Properties of phosphorylase - see also
- 2 subunits
- glycogen binding domain, processive phosphorolysis
- bound pyridoxal phosphate is essential
- cleavage mechanisms
- reactive intermediate (oxonium ion) - like lysozyme
- inhibited by gluconolactone (analog of oxonium)
- must be activated (phosphorylation or activating ligands)
Glycogen synthesis
- not reversal of phosphorylase (McArdle's disease - phosphorylase missing)
- UDPG is substrate
Enzymes of glycogen synthesis
-UDP-glucose pyrophosphorylase
G-1-P + UDP ⇒ UDPG + P Pi
-Glycogen synthase
UDPG + glycogen ⇒ UDP + glycogen(n+1)
-Branching enzyme
Transglucosylase (α 1-4 ⇒ α 1-6)
Glycogen synthase
- UDP donates glucose (oxonium intermediate)
- Net hydrolysis of 1 ATP in process (regenerates UTP)
- Storage uses about 3% of energy
- Activated by dephosphorylation - Inactive GS can have up to 6 phosphates
Starting chain
- Glycogenin
- Glycosylated by tyrosine glucosyl transferase
- chain extends autocatalytically (UDPG as substrate)
- glycogen synthase "primed" (like DNA polymerase)
- synthesize or degrade glycogen, not both
- phosphorylation critical
- phosphorylase runs when kinases on the ascendancy
- glycogen synthetase runs when phosphatases on the ascendancy
- control by several systems so that a single failure is not fatal
- multiple pathways complicates biochemical detection
Allosteric regulation
- b-phosphorylase activated by AMP
- inhibited by G6P, ATP (enough to keep phosphorylase "off" in resting state)
Hormonal regulation
- hormones do not enter cell
- glucagon, epinephrine stimulate glycogenolysis
- insulin stimulates glycogenesis
- Amplify signal
- Permit many stimuli
Regulation of glycogenolysis by cascade
- glucagon stimulates adenyl cyclase
- cAMP stimulates cAMP dependent protein kinase
- latter activates phosphorylase kinase
- phosphorylation switches T to R (electrostatic)
- phosphorylase binds protein phosphatase1 (PP1)
- cAMP kinase phosphorylates glycogen synthase
Phosphorylase kinase
- (α,β,γ,δ )4
- γ subunit alone active, others inhibit
- δ is calmodulin which in turn binds calcium
Allosteric regulation of glycogen synthase
- inhibited by ATP, AMP, Pi
- activated by G-6-P
Regulation of glycogenesis by cascade
- insulin stimulated protein kinase activates PP1
- PP1 dephosphorylates phosphorylase
- phosphorylase releases PP1
- PP1 dephosphorylates glycogen synthase
- glucokinase phosphorylates glucose
- G-6-P activates GS (by stimulating PP1)
Protein phosphatase 1 (PP1)
- regulated differently in muscle and liver
- bound to glycogen by G subunit (in muscle)
- insulin directed phosphorylation stimulates
- epinephrine directed phosphorylation inhibits (released from glycogen in muscle)
- in liver binds phosphorylase in R form
- released only when phosphorylase dephosphorylated, can then activate glycogen synthase
- binds to phosphoprotein phosphatase inhibitor 1 in the phosphorylated form. cAMP stimulates phosphorylation of the the latter (inactivating phosphatase)
- Low cAMP levels release the phosphatase inhibitor
Glycogen storage diseases

Stimulation of glycolysis
- fructose-2,6-bisphosphate activates phosphofructokinase1 stimulating glycolysis
- produced by phosphofructokinase 2
- synthesis, breakdown regulated by phosphorylation of FBP kinase/phosphatase
- Phosphorylation activates the phosphatase, inhibits the kinase
- review in Trends in Biochemical Sciences (26, 30-35, (2001))
- Releases glucose by hydrolyzing G-6-P
- Glucokinase phosphorylates glucose during uptake phase
- Allosteric properties of liver glucokinase give muscle "glucose preference"
- Glycogen synthase activated only when PP1 released after virtually complete dephosphorylation of phosphorylase
- free glucose highly regulated
- glycogen (highly branched) is "battery"
- main "battery" is in liver, peripheral tissues have "minor batteries"
- glycogen synthesis and breakdown highly regulated
- critical enzymes regulated allosterically (rapid control)
- hormones control circuits by phosphorylation, dephosphorylation
- regulation is complex
- Bollen, et al. Biochem J. 336, 19-31 (98) - Specific features of glycogen metabolism in the liver.