glycogenolysis Flashcards
where is the potential energy of glycogen found?
in the glucose residues that make up its structure
glycogenolysis overview
breakdown of glycogen into individual glucose units to form G1P
catalyzed by glycogen phosphorylase
what type of bonds does glycogen phosphorylase cleave?
alpha 1-4 glycosidic bonds
cleaves until it reaches a branch point at which point it stops
what type of bonds does debranching enzyme cleave?
alpha 1-6 bonds at the branch point
high glycogenolytic activity = formation of more…
glucose-1-phosphate
shifts phosphoglucomutase reaction towards production of G6P
where is G6P used in liver and kidneys
can enter oxidative pathway for glucose (glycolysis) or become free glucose
what enzyme turns G6P into free glucose?
glucose-6-phosphatase
only found in liver and kidneys
SO, free glucose can only form from liver or kidney glycogen
glycogenolysis catalyzing enzyme
phosphorylase
how is phosphorylase regulated?
allosteric and covalent mechanisms
different regulation in muscle vs liver
purpose of glucose release in muscle
glucose is released from glycogen to provide glucose for energy within the cell
purpose of glucose release in liver
glucose is released to provide blood glucose
phosphorylase is activated by
glycogen phosphorylation
this inhibits glycogen synthase
phosphorylase is inactivated by
glucogen dephosphorylation
glycogen synthase is activated
covalent regulation of phosphorylase is enhance by
- glucagon
- catecholamines: epinephrine and norepinephrine
how do glucagon and catecholamines covalently modify phosphorylase
they convert phosphorylase to an active form through a secondary messenger (cAMP)
cAMP’s role in covalent modification
regulates phosphorylation site of enzymes
covalent modification mechanism
- glucagon and catecholamines bind receptor on cell membrane
- adenylyl cylase is activated to produce cAMP
- cAMP causes inactive phosphorylase kinase kinase to become active by phosphorylatintg it
- active phosphorylase kinase plus ATP converts inactive (nonphosphorylated) phosphorylase b to active (phosphorylated) phosphorylase a
which type of phosphorylase is less sensitive to allosteric activation?
phosphorylated phosphorylase a
what converts phosphorylase a back into the inactive form, phosphorylase b
phosphoprotein phosphatase 1 (PP-1)
allosteric activation of phosphorylase
- energy is low, ATP is hydrolyzed to AMP –> more energy is needed
- phosphorylase a releases G1P
- AMP binds allosteric site on phosphorylase b –> increases binding of glycogen
allosteric inhibition of phosphorylase
ATP can bind allosteric site on phosphorylase b and inhibit it
G6P and caffeine also allosterically inhibit phoshphorylase b
allosteric inhibitors of phosphorylase b
- ATP
- G6P
- caffeine
muscle and liver phosphorylase are
isozymes
muscle phosphorylase mechanism
releases G1P
G1P can be converted to G6P that enters glycolysis to make energy for the cell
muscle phosphorylase is more sensitive to
intracellular ligands such as AMP for activation
muscle phosphorylase is inhibited by
- metabolites
- ATP
- G6P
- glucose
muscle phosphorylase is activated by
- stress hormones: epinephrine and norepinephrine (stimulate cAMP synthesis and PP-1 to covalently modify phosphorylase to its active form
- nervous stimulation and Ca2+ ions have same effect
liver phosphorylase sensitivity
less sensitive than muscle to intracellular ligans
insensitive to inhibition by ATP or G^P
liver phosphorylase is weakly activated by
AMP
what regulates liver phosphorylase
hormonal controls such as glucagon
