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Glycogen Synthesis (Glycogenesis) Pathway
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hey everyone in this lesson we're
talking about glycogen synthesis also
known as glyco genesis so the first
thing we're gonna get into is what
glycogen is how it is actually
synthesized what are the enzymes
involved in glycogen synthesis then
we're gonna talk about where glycogen is
stored in the body and then finally
we're gonna talk about what the role of
glycogen is in our bodies so to begin
here is a picture demonstration of what
glycogen is actually actually looks like
and as you can see it's a very complex
branching structure with a core enzyme
and we'll get into what that core enzyme
is in a moment now glycogen if you break
down the word glycogen it's actually the
prefix glyco means glucose and then the
suffix Gen means producing so it's like
glucose producing now in fact like a gin
is simply just large polymer of glucose
monomers and glucose monomers can be
arranged in a linear fashion with
linkages of alpha one four linkages and
it is also a branching structure as I
mentioned before and the branch points
are actually alpha one six linkages now
the carbons are all numbered in a
glucose molecule and again as I
mentioned before the linear linkages are
1-4 and the branching is alpha one six
another important thing to note is that
glycogen has branching points at every
eight to twelve glucose monomers which
is important because it distinguishes
glycogen from similar compounds such as
amylopectin now glycogen because of the
word glucose producing it is in fact an
app it's or s-- of glucose and that's
the key about glycogen is actually a
rapid source of glucose and it is highly
important because it actually maintains
blood glucose levels and during
prolonged fasting glycogen stores in our
body typically only last about 1224
hours or a little less than 20
for hours now I mentioned before there's
a enzyme in the core of the glycogen now
this enzyme is called glycogenin and i
like to call this enzyme the catalytic
core and it's because it actually is an
initiating enzyme it itself begins the
process of glycogen synthesis and it
does it does this by actually Auto
glycosylated itself and it'll actually
produce or attach chains of about 10 to
20 glucose monomers to itself and that's
how this whole glycogen synthesis
process actually begins so stepping back
we start with a glucose now when a
glucose enters a cell it can actually be
foster elated by hexa kinase or gluco
kinase as we've learned before another
lessons to glucose 6-phosphate now I
always like to call glucose 6-phosphate
be the hub of many different signaling
pathways it in fact can be used for
several different purposes now the first
thing it can do is actually can go back
and be D phosphorylated to glucose by
glucose-6-phosphatase if glucose is
needed however it can also go into other
pathways one of them is that glucose
6-phosphate can be processed into glucan
or 1/5 lactone and then shunted into the
pentose phosphate pathway and if you
haven't watched that lesson please do so
because it's very important now another pathway
pathway
pretty much everybody's aware of is
glucose 6-phosphate can be processed to
fructose 6-phosphate and then used for
glycolysis for energy production and the
whole topic of this video though is that
if it's not used for any of these
pathways it can be used for glycogen
synthesis by being shunted or processed
into glucose 1-phosphate for glycogen
synthesis so if the glucose 6-phosphate
is used for glycogen synthesis
does that actually happen well the first
thing that happens is that
glucose-6-phosphate is action is acted
on by the enzyme phosphoglucomutase to
form glucose 1-phosphate
and pretty much this enzyme simply just
shifts the phosphate group from carbon 6
onto carbon 1 to form glucose 1-phosphate
1-phosphate
now it's important to realize that this
step is a reversible step which means
that glucose 1-phosphate
can be reprocessed into glucose
6-phosphate if the cell requires free
glucose and this is important during
glycogenolysis which i will talk about
in another video nevertheless if glucose
1-phosphate is going to be used for
glycogen synthesis what's the next step
well the next step is actually glucose
1-phosphate being acted on by UDP
glucose pyro phosphorylates now this is
a very important enzyme it takes UTP and
it activates glucose this is what we
call the activation step UTP will
actually attach a uridine moiety to the
glucose to form UDP glucose the active
form so once we have UDP glucose what is
the next step well the next step is
glycogen synthase now this isn't very
important enzyme and what it does is it
actually will remove the UDP portion off
of UDP glucose and add the glucose to
the actually a non-reducing end of a
glycogen chain so what it does is it'll
actually just pop the UDP portion off of
the UDP glucose and it will actually pop
the glucose monomer onto a non-reducing
end of a glycogen chain of glucose
monomers and it does this again in an
alpha 1 4 linkage it forms an alpha 1 4
linkage to actually extend the glycogen
chain and again the glucose is actually
added onto a non-reducing end so all the
other all the carbons besides carbon 1
are considered non-reducing but again it
is linked at an alpha one four linkage
so once we have a very long glycogen
chain where do the branching points come
from I mentioned glycogen synthase
attaches glucose to glucose at with
alpha one four linkage but I mentioned
before branching points are alpha one
six so how does that happen well if we
can imagine a glycogen chain very long
glycogen chain with a glycogen glycogen
in core how does the branching points or
where do the branching points come from
well simply there is something called a
branching enzyme now the branching
enzyme what it does is it'll actually
remove six to eight glucose molecules
off of the terminal end of the long
chain so what it'll do is it'll actually
chop off the last six to eight glucose
molecules off a long glycogen chain and
then it'll actually attach that chain it
chopped off back onto another chain of
glycogen with an alpha one six linkage
and that's how the branching enzyme
actually produces branch points so
that's where we get the very branching
structure of glycogen and this is also
very important to point out that with
the more branch points and the more free
terminal ends that glycogen actually has
it actually speeds up the rapid release
of glucose now instead of having in our
example instead of having one chain with
one free glucose at the end where one
can be removed at a time now we have two
points where two glucoses can be removed
at a time so it the branching of
glycogen actually speeds up the process
or it allows the cell to access more
glucose quicker and faster so that's why
it is actually a rapid source of glucose
now the final thing I want to talk about
is the UDP and UTP now I mentioned
before that UTP is used to activate glucose
glucose
and UDP UDP is actually removed off of
the udp-glucose once the glucose is
actually added to a glycogen chain so
that UDP glucose has to be recycled tea
back to UTP and it's done so by the
enzyme nucleoside diphosphate kinase and
this enzyme requires one ATP to reform
UTP so I just wanted to tell you guys
that just for sake of completeness so
we've talked about the production or the
synthesis of glycogen well we never
talked about where this is actually
happening where it's stored well as I
show you here the two major sites of
storage in our body for glycogen is the
liver and the muscle now with the liver
the glycogen is actually stored in the
cytoplasm of the hepatocytes and its
weight is about 100 grams and the role
of glycogen in the liver is pretty much
to maintain blood glucose levels that's
its main role when it's actually stored
and used in the liver and again it's
used during fasting and this is this
storage of or this reservoir of glycogen
usually only lasts for about 24 hours or
a little bit less than 24 hours and
again its main rule is just to maintain
blood glucose levels now with a muscle
again if glycogen is produced in the
muscle it's actually stored in the
cytoplasm of the muscle and in totality
in muscle glycogen will actually come
out to be about 400 grams of muscle mass
is actually glycogen and its role in the
muscle is actually an energy source and
it's actually utilized during exercise
so when you are exercising when you are
having acute periods of exercise
you begin to use muscle glycogen stores
and that is used simply before rapid and
glucose a rapid source of glucose and a
rapid source of energy anyways guys that
was a lesson on glycogen synthesis I
hope you found this video helpful if you
did please like and subscribe for more
videos like this one and as always thank
you so much for watching and have a
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