Lecture 17: Glycogen Metabolism Flashcards
what does glycogen yield?
glucose 1-P… isomerized to glucose 6P
glycogen phosphorylase
remove 1 glucose at a time
glycogen synthase adds….
adds one glucose at a time
inorganic phosphate used…
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
what joins the glycogen?
alpha 1-4 and alpha 1,6 glycosidic bonds
branched points
have non reducing ends
nonreducing ends of glycogen
the carbon that is opposite the reducing end in ring structure
reducing end
linear glucose molec that can be oxidized by Cu2+
glucogen core complex
glycogenin protein
about 50,000 glucose moelcs
alpha 1,6 branches every 10 residues
with branch points every 10 residues, and 50,000 residues, about how many (nonreducing)ends?
1 branch every 20 residues
so 2500 ends
what does glycogen degredation accomplish for animals?
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
what do muscle cells lack?
glucose-6-phosphatase
so it can’t release glucose into blood
key enzymes in glycogen metabolism
glycogen phosphorylase glycogen synthase branching and debrancing enzymes brancing--> synth debranching--> breakdown
glycogen degredation starts…
with phosphorolysis cleaveage rxn using Pi
initiated by glycogen phosphorylase
What accounts for the favorable actual free nrg change of the rxn (standard is 3, real is -6)
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
glycogen phosphorylase structure
PLP (vitamin B6) covalently attached to it
involved in activating the Pi used for rxn
glycogen phosphorylase
stays bound to glycogen and successively removes glucose one at a time till it gets close to branched point
cant cleave at branched point
which enzyme is present in liver cells BUT ABSENT from muscle cells, that determines what will happen to the glucose-6P
glucose-6Phosphotase
what converts G-1P to G-6P
phosphoglucomutase
also frees a serine
is there a difference in glycolytic yield between glucose1-1P and glucose? Explain
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
branch points
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
SO how does cell deal with this (when glycogen phosphorylase, can’t work any more to cleave)
debrancing enzyme! It transfers to rest of the glycogen chain to the nonreducing end near by
debranching enzyme:
opens up branch position so we can cleave to get a molec of glucose that can be a substrate
complete degredation: releases….
90% glucose-1P
10% glucose
this is good since G1P has a 1 higher ATP yield
glycogen synthase
add glucose to nonreducing endsof glycogen by this enzyme
needs activated form of glucose: UDP-glucose
UDP-glucose
activated form of glucose
why does rapid conversion of PPi–>2Pi result in a more favorable reaction?
energetically much more favorable b/c
pyrophosphate doesnt stay put, has to get degraded
its thermodynamically less favorable
ASK ASK ASK!!!!
glycogen synthase
what???? no se
use UDP, recycle it regenerate ATP and UTP
Glycogen branching enzyme
chain of 7 glucose residues moved to new branch point
no closer than 5 residues from nearest branch
what would happen to the efficiency of glycogen metabolism if the branching enzymes were defective?
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
how is the first glucose chain synthed when a glycogen particle is “built”
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
glucose chain extension
use UDP as substrate
