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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?

Magnitudes

- 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

-Phosphorylase

Glycogen + Pi ⇒ Glycogen(n-1) + G-1-P

-Debranching enzyme

transglucosylase (α 1-4 to α 1-4)
α 1-6 glucosidase

-Phosphoglucomutase

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)

Control

- 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

Cascades

- 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, P_i
- 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))

Liver

- 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

Summary

- 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

Review

- Bollen, et al. Biochem J. 336, 19-31 (98) - Specific features of glycogen metabolism in the liver.