Glycogen Metabolism Flashcards
How is glycogen stored?
Large cytosolic granules
10% of liver mass and 1-2% of muscle mass
What is the structure of glycogen?
a1->4 linked glucose with a1->6 branches
Where does the addition and removal of glucose in glycogen occur?
The non-reducing ends
What is the function of glycogen phosphorylase and what cofactor does it require?
Removes glucose from non-reducing ends until it reaches four glucose molecules from a branch point
Requires pyridoxal phosphate
What is the function of glycogen debranching enzyme?
Has two activities:
Transferase activity moves 3 glucose moelcules from branch to non-reducing end
Glucosidase activity removes remaining glucose molecule
What is the function of phosphoglucomutase?
Converts glucose-1-phosphate converted to glucose-6-phosphate
What is the function of glucose 6-phosphatase?
Converts G6P to glucose.
Phosphatase is expressed in the liver and kidney, but not muscle and adipose tissue
Where does glycogen synthesis take place?
Liver and Skeletal muscle
What is the first step in glycogen synthesis?
G6P to G1P via phosphoglucomutase
Reversal of reaction in glycogen breakdown
What is the function of UDP-glucose pyrophosphorylase?
Converts G1P to UDP-glucose
What is the function of glycogen synthase?
Adds glucose residue of UDP-glucose to the non-reducing end of glycogen
Removes UDP
What is the function of glycogen branching enzyme?
Adds a1–>6 branches
Removes 7 residues, 3 residues down to form a branch
What two things does the initiation of a new glycogen chain require?
Primer and glycogenin
Primer is usually a preformed polyglucose chain
Glycogenin can act as both a primer and enzyme
What is the function of glycogenin?
Binds to UDP-glucose and removes UDP. This continues six more times through the addition of glucose and removal of UDP.
Describe the structure of glycogen phosphorylase
Made up of two subunits, each exists in an a and b form
a = catalyticall active b = less active
How is glycogen phosphorylase regulated?
By phosphorylation.
Phosphorylated = a form, active
Not Phosphorylated = b form, less active
What is the function of phosphorylase b kinase?
To phosphorylate glycogen phosphorylase b and activated it.
How is phosphorylase b kinase activated in the muscle?
epinephrine, increased Ca, increased AMP
How is phosphorylase b kinase activated in the liver?
Glucagon
What inhibits phosphorylase b kinase?
ATP, G6P, and insulin
Stimulate dephosphorylation via phosphatase
What is the function of Phosphoprotein phosphatase 1 (PP1)?
Dephosphorylates glycogen phosphorylase a
What signaling pathway initiates glycogen breakdown?
cAMP, PKA
How does glucose concentration affect phosphorylase activity?
Phosphorylase can act as a glucose sensor. Glucose can allosterically inactivate phosphorylase a and promote dephosphorylation
How is glycogen synthase regulated?
Phosphorylation
a form, active - not phosphorylated
b form, inactive, - phosphorylated
How is the dephosphorylation of glycogen synthase stimulated?
Insulin, G6P, glucose
What stimulates the phosphorylation of glycogen synthase?
Glucagon and epinephrine
What are three regulations of carbohydrate metabolsim specific to muscle?
Lack enzymatic machinery for gluconeogenesis
Lack receptors for glucagon
Isozyme of pyruvate kinase not phosphorylated by PKA
-so glycolysis is not turned off when cAMP is high
How do glycogen storage diseases present?
Hypoglycemia
Liver damage and enlargement
Myopathy
