Glycogen metabolism- Lecture 65

Question 1

Where is glycogen typically stored?

Answer 1

liver and muscle

Question 2

What is glycogen?

Answer 2

polymer made up of glucose units linked by direct bonds between C1 of one glucose and C4 of an adjacent glucose with branch points occurring C1-C6

Question 3

What does branching allow for?

Answer 3

increased solubility

many areas of attack to liberate for breakdown

Question 4

What is the function of glycogen in the muscles?

Answer 4

provide muscles with glucose in times of need (eg. exercise, fight, flight)

Question 5

What is the function of glycogen in the liver?

Answer 5

produces glucose which is to be released from the liver to the blood to provide glucose to other tissues

Question 6

When is glycogen broken down?

Answer 6

starvation, low carbohydrate diets, exercise, in DM

Question 7

When is glycogen synthesized?

Answer 7

high glucose availability, fed state, high energy state

Question 8

What enzymes degrade glycogen to glucose 1P?

Answer 8

phosphorylase

debranching enzyme

Question 9

What is the outcome of phosphorylase catalyzed phosphorolysis of glucose?

Answer 9

consumes one glycogen to produce one molecule of G1P and a glycogen that is shortened by one glucose unit
irreversible

Question 10

Removal of on glucose unit occurs repeatedly until about _____ residues from a branch point, at which time the _____ residue (with the 1-6 bond) is hydrolyzed by the ______ to produce ______ and ______.

Answer 10

4
fourth
1-6 glucosidase activity of debranching enzyme
glucose
now straight polymer

Question 11

How are new glucose units added to existing glycogen?

Answer 11

G1P + UTP –> UDP-G + PPi
Glycogen-Tyr-glucose + UDP-G –> Glycogen-Tyr-glucose-glucose + UDP
(irreversible)

Question 12

After building up a number of linear glucose links, a _____ transfers ______ glucose residues from the end of the chain to the C6-OH group of a glucose residue in that or another chain to form the _____.

Answer 12

branching enzyme
6-7
1-6 branching points

Question 13

What regulates glycogen synthase (GS)?

Answer 13

stimulated by high levels of G6P
directly inhibited by cAMP-PKA
activated in the dephosphorylated state
inactive in phosphorylated state (so insulin activates and epi/glucagon inactivate)

Question 14

What regulates glycogen phosphorylase (Ph)?

Answer 14

stimulated by AMP (low energy signal)
inhibited by G6P and ATP (inactivated indirectly by cAMP-PKA via phosphorylase kinase)
activated in the phosphorylated state
inactivated in the dephosphorylated state (so insulin inactivates and epi/glucagon activates)

Question 15

Describe the enzyme G6Pase?

Answer 15

present in liver but not muscle
hydrolizes G6P to G + Pi
activated by epi/glucagon

Question 16

Why didn’t G6P made from glycogen go to glycolysis in the liver like it does in the muscles?

Answer 16

F2,6BisP was the major (+) effector of PF-1K and is increased by insulin

liver: glucagon/epi decrease this activity and increase F2,6bisPhase activity (leading to decreased F2,6 levels and decreased glycolysis)
muscle: F2,6bisPhase and the F2,6 kinase are different isoforms that show the opposite properties (epi increases F2,6 kinase activity and decreases F2,6bisPase acivity which increases F2,6bisP levels, PFK activity, and glycolysis)

Question 17

How does epi effect Ca2+ in the muscle?

Answer 17

increases Ca2+ release which binds to calmodulin (subunit of phosphorylase kinase) activating phosphorylase kinase (even if it’s in low-active dephosphorylated state) –> promotion of glycogen breakdown

Question 18

In the liver in the absence of glucose, phosphorylase binds _______, preventing the phosphatase from being active and ______ and _____ which are therefore ______ and ______ respectively.

Answer 18

phosphatase-1 (PP1)
dephosphorylating phosphorylase
glycogen synthase
active
inactive