LECTURE 6 - GLYCOGEN 1

Question 1

what is glycogen

Answer 1

branched polymer of glucose

Question 2

when do glycogen levels increase/go down?

Answer 2

increase after meals and utilized during fasting/exercise

Question 3

what are glycogen granules?

Answer 3

contain all the enzymes required for glycogen synthesis and breakdown
up to 50,000 glucose units per granule

Question 4

why store glycogen?

Answer 4

glycogen catabolism is faster
can be used under anaerobic conditions in skeletal muscle
doesn’t disturb osmotic pressure as would an equivalent amount of glucose monomers
breakdown of glycogen in muscle provides G1P faster than glucose can be taken up from the blood

Question 5

how is glycogen present in muscle?

Answer 5

uses glycogen for its own ATP
available for muscle contraction
no G6P phosphatase= cannot release glucose to blood
-2% glycogen by weight

Question 6

how is glycogen present in liver?

Answer 6

used to maintain blood glucose levels
selective G6P phosphatase expression in the liver
~10% glycogen by weight

Question 7

how is glycogen present in other tissues?

Answer 7

have small glycogen stores for their own use
energy during fasting or anaerobic glycolysis during brief periods of hypoxia/anoxia

Question 8

how is G6P made into glucose in the liver?

Answer 8

1. G6P enters the ER lumen by the G6P transporter T1
2. glucose 6 phosphatase sits outside and inside the ER membrane
   the region that chops is inside ther ER lumen
3. glucose and Pi are released
4. glucose is released to the cytosol by the glucose transporter T2
5. glucose then released out of the cell by GLUT2
6. Pi is released out of the ER by Pi transporter T3

Question 9

why are there different pools of G6P?

Answer 9

kept separate (cytosol vs ER) because used for different pathways

Question 10

what are the two ends of glycogen molecule?

Answer 10

reducing end (only 1)
non reducing end (multiple)

Question 11

to which end are glucose molecules added?

Answer 11

to the non reducing end

Question 12

what are the linkages between glucose molecules and the ones at the branches?

Answer 12

glucose residues are linked at an alpha 1-4 linkage
branches are linked at an alpha 1-6 linkage, every 8-14 glucose residues

Question 13

what do branches provide?

Answer 13

large number of non reducing ends to allow multiple sites for synthesis/degradation

Question 14

which enzyme removes a glucose from glycogen?

Answer 14

debranching enzyme
glycogen phosphorylase
rate limiting step
chops off 1 glucose + phosphorylates
turns it into G1P

Question 15

what enzyme interconverts G1P and G6P

Answer 15

phosphoglucomutase

Question 16

how is G1P converted to glycogen?

Answer 16

1. G1P is stable, doesn’t want to get rid of its Pi
2. G1P is then first converted to UDP-glucose (active intermediate) by UDP-glucose pyrophosphorylase
3. UDP glucose is then converted to glycogen by glycogen synthase (rate limiting step), this releases a UDP

Question 17

how is UDP converted back to UTP?

Answer 17

you need ATP hydrolysis for that

Question 18

what are the three required steps for glycogen synthesis?

Answer 18

1. synthesis of UDP glucose
2. elongation of a pre existing glycogen chain using UDP-glucose
3. creation of new 1,6 glucosyl brand bond

Question 19

how is UDP glucose synthesized from G1P and UTP?

Answer 19

G1P attacks UTP
picks up a phosphate + uridine this releases PPi and UMP

Question 20

how are those reactions coupled?

Answer 20

G1P + UTP is a reaction at equilibrium
coupled with H2O + PPi -> 2Pi
which is exergonic (-19.2)
this provides energy
the overall reactions is irreversible

Question 21

which enzyme synthesizes UDP glucose from G1P and UTP?

Answer 21

UDP-glucose pyrophosphorylase

Question 22

what is the reaction that restores UTP?

Answer 22

UDP + ATP <-> UTP + ADP
done by nucleoside diphosphate kinase

Question 23

which enzyme elongates a pre existing glycogen chain and what is the reaction?

Answer 23

glycogen synthase
adds UDP glucose to the non reducing
UDP-Glc+glycogen (n)-> UDP + glycogen (n+1)
-13.4, exergonic

Question 24

what is it that glycogen synthase cannot do?

Answer 24

glycogen synthase cannot link two glucose residues, it can only extend an already existing alpha 1-4 link

Question 25

which protein primes the synthesis of a new glycogen molecules?

Answer 25

glycogenin
itself is attached to the 1st glucose
only one glycogenin per glycogen molecule
uses glycosyltransferase

Question 26

what is the mechanism of glycogenin?

Answer 26

1. glycogenin attaches a glucose donated by UDPG to the OH group of its Tyr 194
2. glycogenin then extends the glucose chain by up to 7 (or 8-12) additional UDPG donated glucose residues to form a glycogen primer
3. then glycogen synthase commences glycogen synthesis by extending the primer

Question 27

which enzyme transfers chunks of glucose to make branches?

Answer 27

branching enzyme
also called amylo-(1,4->1,6)-transglycosylase

Question 28

what are the rules that the branching enzyme must respect

Answer 28

1. around 7 glucose residues are transferred to the C6-OH
   2.each transferred segment must come from a chain of 11-13 residues
2. the new branch point must be at least 4 residues away from other branch points

Question 29

what is the cost of glycogen synthesis?

Answer 29

the synthesis of glycogen from glucose costs 2 ATP per residue (1 ATP and 1 UTP)

Question 30

what are the steps of glycogen breakdown?

Answer 30

1. generation of G1P
2. debranching
3. conversion of G1P to G6P

Question 31

why is phosphorolysis better than hydrolysis of a glycosidic bond?

Answer 31

from phosphoric acid
more efficient, no need to invest ATP, the phosphate is already there
you get G1P right away

Question 32

which enzyme does the phosphorolysis of glycogen and what is the reaction?

Answer 32

glycogen phosphorylase
needs a Pi

Question 33

how does this reaction proceed even though it is slightly unfavorable?

Answer 33

in vivo, the high intracellular ratio of Pi/G1P (~100) makes the reaction exergonic

Question 34

what are the rules for glycogen phosphorylase?

Answer 34

non reducing ends only
this enzyme makes G1P from glucose located within 4 units of a branch point in the carbohydrate chain

Question 35

what are the two activities of the debranching enzyme?

Answer 35

alpha (1-4) glucosyltransferase and alpha(1-6) glucosidase

Question 36

how do the two enzymes of the debranching enzyme work?

Answer 36

1. alpha (1-4) glucosyltransferasen transfers an alpha(1-4) linked trisaccharide to the non reducing end of another branch (makes a new 1-4 link)
   the 3 units are then available for phosphorylase reaction
2. alpha (1-6) glucosidase hydrolyses the remaining glycosyl residue to yield glucose and debranched glycogen

Question 37

what % of glycogen’s glucose is converted to G1P vs plain glucose?

Answer 37

~92% converted to G1P and ~8% directly into glucose

Question 38

which enzyme converts G1P to G6P?

Answer 38

phosphoglucomutase

Question 39

how does phosphoglucomutase work?

Answer 39

the enzyme already has a phosphorylates serine
this serine phosphorylates G1P at the C6-OH
the phosphate on the C1-OH now goes to phosphorylate the serine on the enzyme
the enzyme therefore goes back to what it was in the beginning and G1P is now G6P

Question 40

what happens to G6P in the muscle vs in the liver?

Answer 40

muscle: G6P continues into glycolysis to generate ATP
liver: G6P is converted to glucose and goes to circulation

Question 41

how many ATPs are generated per glucose in glycogen breakdown?

Answer 41

glycogen breakdown generates G6P, an already phosphorylated intermediate
normal glycolysis=32 ATP
saved due to G6P=1 ATP
total=33 ATP

Question 42

what is the net balance of ATP for glycogen?

Answer 42

2 for synthesis
33 from breakdown
net=31 ATP
31/32=97%
really efficient