Glycogen Metabolism Flashcards
where is glucose stored?
skeletal muscle and liver cells
what is glucose stored as?
glycogen
what is glycogen?
polymer of a bunch of glucose molecules connected by glycosidic bonds
it is compact!
glycogen has a main chain and branches sprouting off of it
why is glycogen branching important?
it allows glycogen to be compact
it allows for rapid addition and removal of glucose
UDP
uridine diphosphate
glycogenin
glycogen primer
fools glycogen synthase
catalyzes addition of 4 glucose molecules to itself using alpha 1,4 glycosidic linkages
hormonal response to decrease in blood glucose levels + exercise
pancreas secretes glucagon
adrenal glands secrete epinephrine to increase heart rate
glucagon
tells liver cells to break down glycogen
epinephrine
increases heart rate
tells skeletal muscle cells to break down glycogen
phosphorylated glycogen synthase is…
inactive
phosphorylated glycogen phosphorylase is…
active
what type of bonds between glucose in glycogen?
alpha 1,4 glycosidic bonds
what type of bonds at branching points in glycogen?
alpha 1, 6 bonds
After eating a starchy meal,
1) increase in insulin acts on GLUTs (glucose transporters on cell membrane) which bring more glucose into our cells
2) Hexokinase adds phosphate to C6 of glucose (makes G6P)
3) G6P broken down to pyruvate during glycolysis (increases ATP)
4) ATP inhibits some glycolysis enzymes
- -> now we use G6P to make glycogen (liver + muscle)
Glycogen synthesis step 1
ATTACH UDP TO GLUCOSE
1) phosphoglucomutase moves phosphate from C6 to C1 (creates G1P and UTP (energy))
2) UDP-glucose phosphorylase cuts 2 phosphates off UTP (gives energy)
1 phosphate on glucose + 1 on UMP = UDP-glucose
Glycogen synthesis step 2
ATTACH UDP-GLUCOSE TO GLYCOGENIN
1) glycogen synthase adds glucose to chains of at least 4 glucose
* if there are no chains this long, glycogenin catalyzes addition of 4 glucose molecules to itself using alpha 1,4 glycosidic linkages
this forms a short, linear glycogen chain
Glycogen synthesis step 3
ADD MORE GLUCOSE
glycogen synthase elongates the chain
Glycogen synthesis step 4
ADD BRANCHES
1) branching enzyme cuts off end of chain and attaches it to main chain using alpha 1,6 glycosidic bond
2) after branching enzyme shortens linear chain, glycogen synthae goes back in and elongates it
Glycogen breakdown step 1
START WITH BRANCHES
1) glycogen phosphorylase:
- cleaves alpha 1,4 bonds
- catalyzes transfer of phosphate group to free glucose
2) 1 G1P is released at a time until 4 glucose remain on branch
Glycogen breakdown step 2
DEBRANCHING ENZYME
1) cuts off branches
- has 4-alpha-glucotransferase activity: transfers 3 glucose off branch and onto main chain
- has alpha-1,6 glucosidase activity: cleaves alpha 1,6 bond (releases free glucose)
Glycogen breakdown step 3
1) after each glucose is removed via phosphorolysis, a G1P is freed.
2) phosphoglucomutase converts it to G6P
why is glycogen breakdown different in liver and muscle?
liver has glycogen-6-phosphatase, muscle DOES NOT
Glycogen breakdown in liver
HAS glucose-6-phosphatase
- removes phosphate from C6
- releases free glucose into bloodstream for other organs and tissues
Glycogen breakdown in muscle
NO glucose-6-phosphatase
-uses G6P by sending it to glycolysis to make energy
