Lecture 17: Glycogen Metabolism

Question 1

what does glycogen yield?

Answer 1

glucose 1-P… isomerized to glucose 6P

Question 2

glycogen phosphorylase

Answer 2

remove 1 glucose at a time

Question 3

glycogen synthase adds….

Answer 3

adds one glucose at a time

Question 4

inorganic phosphate used…

Answer 4

to gen glucose 1-P when glycogen is degrated
free b/c doesnt require ATP investment to get glucose 6P for glycolysis
glycogen synth requires 1 ATP net investment per glucose

Question 5

what joins the glycogen?

Answer 5

alpha 1-4 and alpha 1,6 glycosidic bonds

Question 6

branched points

Answer 6

have non reducing ends

Question 7

nonreducing ends of glycogen

Answer 7

the carbon that is opposite the reducing end in ring structure

Question 8

reducing end

Answer 8

linear glucose molec that can be oxidized by Cu2+

Question 9

glucogen core complex

Answer 9

glycogenin protein
about 50,000 glucose moelcs
alpha 1,6 branches every 10 residues

Question 10

with branch points every 10 residues, and 50,000 residues, about how many (nonreducing)ends?

Answer 10

1 branch every 20 residues

so 2500 ends

Question 11

what does glycogen degredation accomplish for animals?

Answer 11

liver glycogen=short term nrg source
provides a means to store and release glucose in response to blood glucose levles

muscle glycogen=readily available source of glucose during exercise
supports anaerobic and aerobic nrg conversion pathways w/in muscle cells
CANT RELEASE GLUCOSE INTO BLOOD

Question 12

what do muscle cells lack?

Answer 12

glucose-6-phosphatase

so it can’t release glucose into blood

Question 13

key enzymes in glycogen metabolism

Answer 13

glycogen phosphorylase
glycogen synthase
branching and debrancing enzymes
brancing--> synth
debranching--> breakdown

Question 14

glycogen degredation starts…

Answer 14

with phosphorolysis cleaveage rxn using Pi

initiated by glycogen phosphorylase

Question 15

What accounts for the favorable actual free nrg change of the rxn (standard is 3, real is -6)

Answer 15

glucose 1 won’t stay put, so there is not build up of it
it gets used by downstream rxns,
this drives the reaction to the right, makes it favorable

Question 16

glycogen phosphorylase structure

Answer 16

PLP (vitamin B6) covalently attached to it

involved in activating the Pi used for rxn

Question 17

glycogen phosphorylase

Answer 17

stays bound to glycogen and successively removes glucose one at a time till it gets close to branched point
cant cleave at branched point

Question 18

which enzyme is present in liver cells BUT ABSENT from muscle cells, that determines what will happen to the glucose-6P

Answer 18

glucose-6Phosphotase

Question 19

what converts G-1P to G-6P

Answer 19

phosphoglucomutase

also frees a serine

Question 20

is there a difference in glycolytic yield between glucose1-1P and glucose? Explain

Answer 20

YES: yield of G1P is 1 ATP higher than glucose
glucose-1P can just move the phosphate to get glucose-6P, which can be used in glycolysis
glucose requires an ATP to get phosphorylated to glucose-6P

Question 21

branch points

Answer 21

once about every 10 glucose residues
about 4 residues from branching, glycogen phosphorylase, can’t work any more to cleave

so we have to transfer the chain
then take away the glucose

Question 22

SO how does cell deal with this (when glycogen phosphorylase, can’t work any more to cleave)

Answer 22

debrancing enzyme! It transfers to rest of the glycogen chain to the nonreducing end near by

Question 23

debranching enzyme:

Answer 23

opens up branch position so we can cleave to get a molec of glucose that can be a substrate

Question 24

complete degredation: releases….

Answer 24

90% glucose-1P
10% glucose

this is good since G1P has a 1 higher ATP yield

Question 25

glycogen synthase

Answer 25

add glucose to nonreducing endsof glycogen by this enzyme

needs activated form of glucose: UDP-glucose

Question 26

UDP-glucose

Answer 26

activated form of glucose

Question 27

why does rapid conversion of PPi–>2Pi result in a more favorable reaction?

Answer 27

energetically much more favorable b/c
pyrophosphate doesnt stay put, has to get degraded
its thermodynamically less favorable

ASK ASK ASK!!!!

Question 28

glycogen synthase

Answer 28

what???? no se

use UDP, recycle it regenerate ATP and UTP

Question 29

Glycogen branching enzyme

Answer 29

chain of 7 glucose residues moved to new branch point

no closer than 5 residues from nearest branch

Question 30

what would happen to the efficiency of glycogen metabolism if the branching enzymes were defective?

Answer 30

we would have a very long chain and we could only release one glucose at a time

with branches, more than one glucose can be released from a glycogen at a time

it would be a ton slower if branching didnt happen

Question 31

how is the first glucose chain synthed when a glycogen particle is “built”

Answer 31

glycogenin protein causes glucosyltransferase
first chain comes from UDP glucose– it donates a glucose at the reducing end.
its added on to the tyrosine
it is covalently attached to the glycogenin molec (just the first molec)

all other glucose molecs get added by the UDP molec

Question 32

glucose chain extension

Answer 32

use UDP as substrate