Glycogenesis and Glycogenolysis

Question 1

Glycogenesis

Answer 1

• synthesis of glycogen
  
  • begins with core protein, glycogenin
  • glucose addition to the granule beings with glucose 6-P, which is converted to glucose 1-P by phosphoglucomutase
  • glucose 1-P is activated by coupling with UDP (reacts with UTP), which permits integration into the glycogen chain by glycogen synthase (RDS)

Question 2

Action and Regulation of Glycogen Synthase

Answer 2

• Rate-limiting enzyme of glycogen synthesis; forms alpha-1,4 glycosidic bond in glucose chains of the granule
• stimulated by G6P and insulin
• inhibited by epinephrine and glucagon through a protein kinase cascade that phosphorylates and inactivates the enzyme

Question 3

Branching Enzyme

Answer 3

• Glycosyl alpha-1,4: alpha-1,6 Transferase
  
  • introduces alpha-1,6 linked branches into the granule as it grows
• Process:
  
  • hydrolyzes alpha-1,4 bond to release a block of oligoglucose
  • oligoglucose is moved and added to a different location via an alpha-1,6 linkage

Question 4

Glycogenolysis

Answer 4

• breakdown of glycogen
  
  • rate-limiting enzyme is glycogen phosphorylase (breaks bonds using P_i)
  • liberated glucose 1-P is then converted to glucose 6-P by phosphoglucomutase

Question 5

Action and Regulation of Glycogen Phosphorylase

Answer 5

• breaks alpha-1,4 linkages, releasing G1P
• cannot break alpha-1,6 bonds, and stops when it reaches branch points
• activated by glucagon in the liver to raise BGL
• activated by AMP and epinephrine in the skeletal muscle
• inhibited by ATP

Question 6

Debranching Enzyme

Answer 6

• Glucosyl alpha-1,4: alpha-1,4 Transferase and alpha-1,6 Glucosidase
  
  • ​breaks an alpha-1,4 bond adjacent to the branch point and moves oligoglucose chain to the exposed end of the other chain; forms a new alpha-1,4 bond
  • hydrolyzes the alpha-1,6 bond, released the single residue at the branch point