glycogenolysis

Question 1

where is the potential energy of glycogen found?

Answer 1

in the glucose residues that make up its structure

Question 2

glycogenolysis overview

Answer 2

breakdown of glycogen into individual glucose units to form G1P

catalyzed by glycogen phosphorylase

Question 3

what type of bonds does glycogen phosphorylase cleave?

Answer 3

alpha 1-4 glycosidic bonds
cleaves until it reaches a branch point at which point it stops

Question 4

what type of bonds does debranching enzyme cleave?

Answer 4

alpha 1-6 bonds at the branch point

Question 5

high glycogenolytic activity = formation of more…

Answer 5

glucose-1-phosphate
shifts phosphoglucomutase reaction towards production of G6P

Question 6

where is G6P used in liver and kidneys

Answer 6

can enter oxidative pathway for glucose (glycolysis) or become free glucose

Question 7

what enzyme turns G6P into free glucose?

Answer 7

glucose-6-phosphatase
only found in liver and kidneys
SO, free glucose can only form from liver or kidney glycogen

Question 8

glycogenolysis catalyzing enzyme

Answer 8

phosphorylase

Question 9

how is phosphorylase regulated?

Answer 9

allosteric and covalent mechanisms
different regulation in muscle vs liver

Question 10

purpose of glucose release in muscle

Answer 10

glucose is released from glycogen to provide glucose for energy within the cell

Question 11

purpose of glucose release in liver

Answer 11

glucose is released to provide blood glucose

Question 12

phosphorylase is activated by

Answer 12

glycogen phosphorylation
this inhibits glycogen synthase

Question 13

phosphorylase is inactivated by

Answer 13

glucogen dephosphorylation
glycogen synthase is activated

Question 14

covalent regulation of phosphorylase is enhance by

Answer 14

1. glucagon
2. catecholamines: epinephrine and norepinephrine

Question 15

how do glucagon and catecholamines covalently modify phosphorylase

Answer 15

they convert phosphorylase to an active form through a secondary messenger (cAMP)

Question 16

cAMP’s role in covalent modification

Answer 16

regulates phosphorylation site of enzymes

Question 17

covalent modification mechanism

Answer 17

1. glucagon and catecholamines bind receptor on cell membrane
2. adenylyl cylase is activated to produce cAMP
3. cAMP causes inactive phosphorylase kinase kinase to become active by phosphorylatintg it
4. active phosphorylase kinase plus ATP converts inactive (nonphosphorylated) phosphorylase b to active (phosphorylated) phosphorylase a

Question 18

which type of phosphorylase is less sensitive to allosteric activation?

Answer 18

phosphorylated phosphorylase a

Question 19

what converts phosphorylase a back into the inactive form, phosphorylase b

Answer 19

phosphoprotein phosphatase 1 (PP-1)

Question 20

allosteric activation of phosphorylase

Answer 20

1. energy is low, ATP is hydrolyzed to AMP –> more energy is needed
2. phosphorylase a releases G1P
3. AMP binds allosteric site on phosphorylase b –> increases binding of glycogen

Question 21

allosteric inhibition of phosphorylase

Answer 21

ATP can bind allosteric site on phosphorylase b and inhibit it

G6P and caffeine also allosterically inhibit phoshphorylase b

Question 22

allosteric inhibitors of phosphorylase b

Answer 22

1. ATP
2. G6P
3. caffeine

Question 23

muscle and liver phosphorylase are

Answer 23

isozymes

Question 24

muscle phosphorylase mechanism

Answer 24

releases G1P
G1P can be converted to G6P that enters glycolysis to make energy for the cell

Question 25

muscle phosphorylase is more sensitive to

Answer 25

intracellular ligands such as AMP for activation

Question 26

muscle phosphorylase is inhibited by

Answer 26

1. metabolites
2. ATP
3. G6P
4. glucose

Question 27

muscle phosphorylase is activated by

Answer 27

1. stress hormones: epinephrine and norepinephrine (stimulate cAMP synthesis and PP-1 to covalently modify phosphorylase to its active form
2. nervous stimulation and Ca2+ ions have same effect

Question 28

liver phosphorylase sensitivity

Answer 28

less sensitive than muscle to intracellular ligans
insensitive to inhibition by ATP or G^P

Question 29

liver phosphorylase is weakly activated by

Answer 29

AMP

Question 30

what regulates liver phosphorylase

Answer 30

hormonal controls such as glucagon