Glycogenesis/Glycogenolysis Flashcards
Where is most glycogen stored?
Skeletal muscle and liver
serves as a store for muscle to use for ATP production
serves a store for liver to maintain blood glucose levels (eg during a fast)
What is the benefit of branching glycogen?
More soluble than a single chain (amylose) and provides more non-reducing ends for more glucose residues to be added, accelerating the rate of glycogen synthesis
Branches are located, on average, how many residues apart?
8
When does Phosphorylase stop cleaving glycogen?
4 residues from the branch point
What is the active form of glycogen phosphorylase, and is it phosphorylated?
Active form is phosphorylase a, and it is phosphorylated
What is the active form of glycogen synthase, and is it phosphorylated?
Active form is glycogen synthase a, and it is NOT phosphorylated
What activates glycogen phosphorylase?
Phosphorylase b kinase, which is activated by PKA and Ca2+
What inactivates glycogen synthase?
PKA, phosphorylates it to glycogen synthase b
Allosteric vs covalent activation of glycogen phosphorylase and glycogen synthase
Glycogen phosphorylase b and glycogen synthase b are the inactive forms, yet can be allosterically activated by AMP and G6P respectively to perform their functions. Phosphorylase b kinase can be activated by ca2+
Effect of PKA on glycogen synthase and glycogen phosphorylase
PKA phosphorylates phosphorylase kinase, activating it to phosphorylate phosphorylase b to phosphorylase a
PKA also phosphorylates synthase to inactivate it, producing decreased glycogenesis, and increased glycogenolysis
- synthase has multiple phosphorylation sites, more sites phosphorylated=more inhibition
Mechanism of Methyl Xanthine (coffee, tea, cocoa) inhibition
Inhibit the function of cyclic nucleotide phosphodiesterase
This inhibits the conversion cAMP to AMP, further allowing glycogenolysis to continue producing glucose
Effect of Ca2+ on Glycogen metabolism
Ca2+ release from the SR complexes with Calmodulin
This then binds with phosphorylase kinase b and allosterically triggers it to activate phosphorylase b to phosphorylase a and activate glycogenolysis
Effect of G6P on glycogen synthase
Can activate glycogen synthase b to perform its activity without needing to be converted to glycogen synthase a
Effect of epinephrine on glycogen metabolism
Released from adrenal medulla
Muscle: Activates cAMP cascade to trigger glycogenolysis
Liver: if epi is low, trigger rise in Ca2+, if high, triggers cAMP cascade
Effect of Glucagon on glycogen metabolism
Secreted from pancreatic ⍺-cells in response to low blood glucose
Muscle: no effect bc no receptors
Liver: production of glucose from glycogenolysis and gluconeogenesis via cAMP cascade
