• Storage form of glucose → in liver & skeletal muscle
  
• Readily mobilized fuel source
  

• Strenous activity produces painful muscle cramps & myoglobinuria → activity produces no increase in blood lactate levels.
  
• High muscle glycogen → patients can make glycogen but cannot mobilize it → due to decreased levels of muscle glycogen phosphorylase
  

• Morning drowsiness, fatigue, convulsions due to hypoglycemia → high levels of blood glucose and lactase after meals
  
• Liver deficient in glycogen
  
• Lack of glycogen synthetase
  

• Branching increases the solubility  of glycogen → more spots to cleave a glucose molecule → more interactions available.
  
• increase glycogen breakdown and synthesis
  

• Glycogen phosphorylase
• Glycogen debranching enzyme
• Phosphoglucomutase
  

Glycogen Phosphorylase

What functions does the enzyme have in glycogenolysis?

• Phosphorylysis → breaks glycosidic bond
  
• Rate determining step of glycogenolysis
• Removes only glucose w/ α(1→4) bonds → only within 4-5 glucose units of branch point
  
• Requires pyridoxal phosphate (PLP) covalently bound to enzyme (prosthetic group)
  

          * facilitates acid-base catalysis

          *PLP derived from Vitamin B6

Glycogen Debranching Enzyme

What functions does the enzyme have in glycogenolysis?

• Two activities
• α(1→4)  transglycolase → transfer of α(1→4)  trisaccharide to non-reducing end of glycogen
  
• α(1→6) glycosidase → removes α(1→6) bound glucose at branching point → hydrolysis reaction
  

Phosphoglucomutase

What functions does the enzyme have in glycogenolysis?

• Isomerase
  
• Catalyzes reversible conversion of glucose-1-P to glucose-6-P
  

Glucose-6-phosphatase:

Where is it found?

What distinguishes it from glucose?

What are the process in which it is involved?

• primarily found in liver
  
• glucose-6-phosphate cannot be transported out of cells, glucose can
  
• also involved in glucogenesis
  

Glycogenesis

What are the enzymes used?

• glycogen synthesis 
• UDP-glucose pyrophosphorylase
• Glycogen synthase
• Glycogen branching enzyme
  

• substrate is glucose-1-phosphate NOT glucose
• Transfer of glucose to UDP a free energy state of 0 → reaction is driven by hydrolysis of PPi
  

Glycogen Synthase

What functions does the enzyme have in glycogenesis?

• can only extend existing glycogen chain
  
• Can only make α-1,4 linkages
  

Glycogen Branching Enzyme

What functions does the enzyme have in glycogenesis?

• Makes α(1-6) bond
  
• Transfers 7 glucose unit chain to C6 hydroxyl every 8-12 glucose molecules
• Increases number of reactive ends
  

• Autoglycosylating enzyme (2 subunits)
• Contains oligosaccharide on Tyr residue
  
• Makes primers for glycogen synthase
  
• Provides scaffold
  
• also has synthase activity
  
• adds glucose molecules to itself!!
  

1. Attach glucose to glycogenin (Tyr residue)
2. Glycogenin complexes with glycogen synthase
3. Glycogen-synthase adds up to 7 more glucose residues
4. Glycogen synthase takes over once chain is > 8 residues long
5. Glycogenin dissociates from synthase but remains bound to reducing end of glycogen molecule
   

*glycogenin responsible for making the primer and not synthase

• challenge to controlling glycogen flux
  
• allosteric regulation of rate determining step of both pathways:
  

              *glycogen phosphorylase

              *glycogen synthase

Glycogen Phosphorylase

(in regards to the regulation of Glycogen Metabolism)

• Dimer of identical subunits → 2 catalytic & 2 allosteric sites
• Two conformations

*T (less active) → buried active site, low substrate affinity

         *R (more active) → accessible active site, high substrate affinity

• Allosteric Modulators

            *Activator → AMP

           *Inhibitors → ATP; glucose-6-P; glucose

• AMP binds to allosteric site in T state → Promotes T to R conformational change
  
• Inhibitors also bind allosteric site in T state → prevent T to R conformational change
  

• [ATP6] & [G6P]  high enough in resting tissue to keep phosphorylase in T form
  
• [AMP] increases in active muscle → shifts eq. from T to R; activates enzyme
  

• Does not respond to own energy state, only to [glucose]
• AMP does NOT activate liver phosphorylase → but rather the glucose binding which activates it → shifting eqilibrium from R to L
  

• Tetramer of identical subunits
• Two confromations (T and R)
  
• Allosteric modulator

          *Activate by → Glucose-6-Phosphate

          *Inhibited by → ATP, ADP, PPi

• Phosphorylase and synthase coordinately regulated
• Modulators that inhibit phosphorylase activate synthase
• Modulators that inhibit synthase activate phosphorylase
  

Hormonal Control of Glycogen Metabolism

What are the effects of phosphorylation?

• produces conformational changes that drive activity changes
• Occurs simultaneously on both enzymes → controlled by protein kinase A
• Increases glycogen phosphorylase activity
• Decreases glycogen synthase activity
  

*Flux is towards glycogenolysis

• Glucagon is released by pancrease in response to low blood glucose.
  
• Epinephrine (EPI) released by adrenal cortex in response to stress
• 2 EPI receptors on liver cells

1. Activate cAMP → 2nd messenger that activates enzymes needed for phosphorylation
   
2. Triggers Ca2+ release → partially activates phosphorylate kinase
   

• Epinephrine activates β-adrenergic receptor, causing release of cAMP
• Insulin released in response to high blood glucose → binds to insulin receptors, activating kinases that inactivate glycogen synthase kinase → drives pathway towards glycogenesis
  

Summary:

1. What type of a metabolic pathway is glycogen metabolism an example of, in terms of regulation?
2. What effect does regulation of both enzymes by the same allosteric modulators have?
3. How are the enzymes regulated?

1. Glycogen metabolism, is an example of a metabolic pathway, in which net flux is determined by regulation of two non-equilibrium enzymes.
2. Regulation of both enzymes by same allosteric modulators (in opposite directions) allows system to respond to energy state changes in cell.
   
3. Regulation of both enzymes by covalent modification (phosphorylation) allows for rapid response to blood glucose levels.