Glycogen metabolism
Summary
TLDRThe script delves into glucose's role as a primary energy source, detailing how it's converted into ATP. It explains the storage of excess glucose as glycogen in liver and muscle cells, highlighting the process of glycogen synthesis involving enzymes like glycogen synthase and branching enzyme. The script further explores glycogen breakdown, triggered by hormones like glucagon and epinephrine, and the regulatory impact of insulin and glucagon on glycogen metabolism, emphasizing the body's energy management post-meal and during fasting.
Takeaways
- đŹ Glucose is a six-carbon molecule essential for energy production in the form of ATP.
- đ The body stores excess glucose as glycogen in skeletal muscle and liver cells for energy reserves.
- đł Glycogen is a large, branched molecule composed of glucose units linked by glycosidic bonds, allowing compact storage and rapid glucose release.
- đź After consuming a meal, like tacos, glucose absorption from the intestine raises blood sugar levels, prompting the pancreas to secrete insulin.
- đ Insulin activates glucose transporters (GLUTs) to increase glucose uptake by cells, facilitating energy production and glycogen synthesis.
- đ Glycogen synthesis involves four steps: UDP glucose formation, attachment to glycogenin, elongation of the glycogen chain, and branching for efficient storage.
- đ The enzyme glycogen synthase is crucial for glycogen chain elongation, requiring at least four glucose molecules for activation.
- đ Glycogenin acts as a primer for glycogen synthesis, tricking glycogen synthase into building a new glycogen chain.
- đââïž During physical activity or fasting, hormones like glucagon and epinephrine stimulate glycogen breakdown to release glucose for energy.
- ⥠Glycogen phosphorylase initiates glycogen breakdown by cleaving alpha-1,4 bonds and transferring phosphate groups to release glucose-1-phosphate.
- đ The debranching enzyme complex helps in glycogen breakdown by transferring glucose molecules and releasing free glucose for immediate use.
- đĄ Glycogen metabolism is regulated by insulin and glucagon, with insulin promoting synthesis and glucagon promoting breakdown.
Q & A
What is the primary role of glucose in the body?
-Glucose is a six-carbon molecule used to make energy in the form of adenosine triphosphate (ATP).
Where does the body store excess glucose?
-The body stores excess glucose in skeletal muscle cells and liver cells in the form of glycogen.
What is glycogen and how is it structured?
-Glycogen is a large molecule or polymer made up of glucose molecules linked together by glycosidic bonds, with a main chain and multiple branches, allowing it to be compact and capable of rapid addition and removal of glucoses.
How does the body respond to an increase in blood sugar after a meal?
-The pancreas responds by secreting insulin, which activates glucose transporters (GLUTs) on the cell membrane, allowing more glucose to be brought into cells.
What is the role of the enzyme hexokinase in glucose metabolism?
-Hexokinase adds a phosphate group to the sixth carbon of glucose, creating glucose 6-phosphate, which is then broken down during glycolysis to make ATP.
What are the four main steps in glycogen synthesis?
-The steps are: 1) attaching a uridine diphosphate (UDP) molecule to glucose, 2) attaching the glucose part of the UDP-glucose molecule to a glycogen primer called glycogenin, 3) adding more glucose molecules to the primer, and 4) adding branches to the glycogen molecule.
What is the purpose of the branching enzyme in glycogen synthesis?
-The branching enzyme cuts off a chain of about six to eight glucose residues and attaches it to the side of the linear glycogen strand by creating an alpha 1,6 glycosidic bond, forming branches.
How does the body break down glycogen during fasting or exercise?
-The body breaks down glycogen by using the hormone glucagon and epinephrine, which signal liver and skeletal muscle cells to convert glycogen back into individual glucose molecules.
What is the difference between glycogen breakdown in the liver and skeletal muscle cells?
-In liver cells, glucose 6-phosphatase removes the phosphate from glucose 6-phosphate, releasing free glucose into the bloodstream. Skeletal muscle cells lack this enzyme and use glucose 6-phosphate for energy production through glycolysis.
How do insulin and glucagon regulate glycogen metabolism?
-Insulin promotes glycogen synthesis by activating glycogen synthase and inactivating glycogen phosphorylase. Glucagon does the opposite, promoting glycogen breakdown by activating glycogen phosphorylase and inactivating glycogen synthase.
What is the significance of the alpha-1,4 and alpha-1,6 glycosidic bonds in glycogen structure?
-Alpha-1,4 glycosidic bonds link glucose molecules in a linear fashion, while alpha-1,6 bonds create branches in the glycogen structure, allowing for efficient storage and quick release of glucose.
Outlines
đŹ Glucose Metabolism and Glycogen Synthesis
This paragraph explains the role of glucose as a primary energy source in the form of ATP. It details how excess glucose is stored as glycogen in liver and muscle cells, highlighting the structure of glycogen as a highly branched polymer. The paragraph also describes the process of glycogen synthesis involving enzymes like hexokinase, glycogen synthase, and glycogenin, and the steps involved in creating the glycogen molecule, including the formation of UDP-glucose and the addition of glucose molecules to the glycogen chain.
đ Glycogen Breakdown and Energy Release
The second paragraph delves into the process of glycogen breakdown when blood glucose levels drop, such as during fasting or exercise. It describes the hormonal response involving glucagon and epinephrine, which trigger the breakdown of glycogen into glucose-1-phosphate by enzymes like glycogen phosphorylase and the debranching enzyme. The paragraph explains the difference in glycogen metabolism between liver and muscle cells, with liver cells releasing free glucose into the bloodstream and muscle cells using glucose-6-phosphate for energy production through glycolysis. It concludes with an overview of the regulation of glycogen metabolism by insulin and glucagon, and the activation states of glycogen synthase and phosphorylase.
Mindmap
Keywords
đĄGlucose
đĄAdenosine Triphosphate (ATP)
đĄGlycogen
đĄGlycosidic Bonds
đĄInsulin
đĄHexokinase
đĄGlycolysis
đĄGlycogen Synthase
đĄGlycogenin
đĄBranching Enzyme
đĄGlucagon
Highlights
Glucose is a six-carbon molecule essential for energy production in the form of ATP.
The human body stores excess glucose as glycogen in skeletal muscle and liver cells.
Glycogen is a large polymer made of glucose molecules linked by glycosidic bonds, allowing for compact storage and rapid glucose addition or removal.
Insulin secretion by the pancreas in response to high blood sugar levels facilitates glucose uptake by cells.
Glucose 6-phosphate is a key intermediate in the metabolic pathway leading to ATP production.
Glycogen synthesis occurs in four main steps, starting with the formation of UDP-glucose.
Glycogenin plays a crucial role in initiating glycogen synthesis by catalyzing the attachment of four glucose molecules.
Glycogen branching enzyme and glycogen synthase work together to create a branched glycogen structure.
Glucagon and epinephrine stimulate glycogen breakdown in the liver and skeletal muscle cells during fasting or physical activity.
Glycogen phosphorylase is responsible for cleaving alpha 1,4 bonds and releasing glucose 1-phosphate during glycogen breakdown.
Debranching enzyme is involved in transferring glucose molecules and extending the glycogen chain during breakdown.
Glucose 6-phosphatase in liver cells releases free glucose into the bloodstream, while muscles use glucose 6-phosphate for energy production.
Glycogen metabolism is regulated by insulin and glucagon, with insulin promoting synthesis and glucagon promoting breakdown.
Glycogen synthase activity is increased by insulin through dephosphorylation, while glycogen phosphorylase is activated by glucagon through phosphorylation.
Glycogen serves as the major form of glucose storage in the body, stored primarily in the liver and skeletal muscle cells.
The glycogen structure consists of alpha-1-4 glycosidic bonds for linear chains and alpha-1-6 bonds at branching points.
High insulin levels after a meal promote glycogen synthesis, while high glucagon and epinephrine levels during fasting promote glycogen breakdown.
Transcripts
glucose is a six
carbon molecule that's used to make
energy in the form of adenosine
triphosphate
or atp glucose is such an important
energy source that our body stores
excess glucose in skeletal muscle cells
and liver cells in the form of glycogen
glycogen is basically an enormous
molecule or polymer
that's made up of glucose molecules
linked together by glycosidic bonds
you can think of glycogen having a main
chain and there being multiple branches
sprouting off of it
these branches allow glycogen to be
compact and capable of rapid addition
and removal of glucoses
it's kind of like growing a plum tree in
a tiny house with a short ceiling
the short ceiling limits the tree's
vertical growth but the tree is able to
branch off
so that it can still grow and produce
many plums into tight space
now let's say that you just wrapped up a
delicious lunch you had tacos
glucose is absorbed from the intestine
and that causes our blood sugar to go up
the pancreas responds to high blood
sugar by secreting insulin
insulin axon glucose transporters on the
cell membrane
which are called glutes and makes them
bring more glucose into all cells in our
body
inside the cell an enzyme called
hexokinase adds a phosphate group to its
sixth carbon
creating glucose 6-phosphate then
glucose-6-phosphate is broken down
during glycolysis
making atp as a by-product over time atp
levels start to rise and that inhibits
certain enzymes in glycolysis
when that happens the extra glucose
6-phosphate can be used to make glycogen
that usually takes place in the liver
and muscle cells
there are four main steps in glycogen
synthesis
first is attaching a uridine diphosphate
or udp molecule to glucose
second is attaching the glucose part of
the udp glucose molecule to a glycogen
primer called glycogenin
forming a short linear glycogen chain
which serves as a primer
third is adding more glucose molecules
to the primer
a bit like forming a conga line
[Music]
and fourth is adding branches to the
glycogen molecule
so starting with step one to make udp
glucose
an enzyme called phosphoglucomutase
moves the phosphate from the sixth
carbon of glucose six phosphate to the
first carbon
creating glucose one which uniquely
comes in the form of uridine
triphosphate
or utp in the presence of glucose
1-phosphate and utp
an enzyme called udp-glucose
pyrophosphorylase cuts
phosphate molecules off of utp which
give the energy necessary to complete
this reaction
so only one phosphate remains attached
to uridine
and then glucose one phosphate is added
to it
that makes two phosphates so the
resulting molecule is called udp
glucose
once many glucose molecules are
converted into udp glucose molecules
we're ready to create glycogen an enzyme
called glycogen synthase
catalyzes the attachment of the glucose
part of udp glucose to another glucose
residue at the end of the glycogen
branch
forming an alpha 1 4 glycosidic bond
it's almost as if the glucose molecules
are holding hands
and in addition to prolonging the
glycogen chain another byproduct of this
reaction is udp
but it turns out that glycogen synthase
can only elongate an already existing
glycogen chain that's at least four
glucose molecules long
so if there aren't at least four glucose
molecules linked up together already
then glycogen synthesis requires a
protein called glycogenin
glycogenin plays the role of fooling
glycogen synthase by catalyzing the
attachment of four glucoses to itself
creating a short chain connected with
alpha one four glycosidic bonds
by doing that it's able to tell glycogen
synthase hey we have a chain here that
looks kind of like an old glycogen
molecule
and glycogen synthase falls for it
and elongates this short chain on
glycogenin by attaching lots of glucose
molecules to it through alpha-1-4
glycosidic bonds
this elongates the chain and creates a
new linear glycogen molecule
next an enzyme called the branching
enzyme goes to the end of the chain and
cuts off a chain of about six to eight
glucose residues in length
the branching enzyme then attaches that
chain to the side of the linear glycogen
strand by creating an alpha 1
6 glycosidic bond so there's now a bond
between the first carbon of the glucose
on the small cleaved segment
and the sixth carbon of a glucose that's
part of the linear chain
and as soon as you've shortened the
linear chain glycogen synthase will
elongate it once again
this happens over and over again
resulting in a branched glycogen tree to
serve as stored energy
now let's say it's been a couple of
hours since those tacos
and you decide to go for a run because
you're fasting your blood glucose levels
take a dip
in response the pancreas secretes the
hormone glucagon
and the adrenal glands secrete
epinephrine to increase your heart rate
it turns out that glucagon tells the
liver cells to break glycogen down into
individual glucose molecules
and epinephrine tells skeletal muscle
cells to do the same thing
in both the liver and skeletal muscle
cells glycogen breakdown starts with the
branches
first an enzyme called glycogen
phosphorylase cleaves the alpha 1
4 bonds between individual glucose
residues and catalyzes the transfer of a
phosphate group to the free
glucose the result is that the enzyme
releases one glucose one phosphate
molecule at a time
it keeps on doing this until exactly
four glucose molecules were made on the
branch
next a de-branching enzyme literally
cuts off glycogen branches
it has a component called four alpha
glucanotransferase
which transfers three out of the four
glucose molecules off of the branch and
reattaches them to the linear glycogen
chain instead
extending it as a result the same
debranching enzyme has another component
known as alpha 1-6
glucosidase which cleaves off the
alpha-1-6 glycosidic bond and releases a
free glucose
so for each glucose that's removed via
phosphorylysis
there's a glucose 1-phosphate that gets
liberated and it's converted to
glucose-6-phosphate by
phospho-glucomutase
the difference between glycogen
breakdown in the liver and what goes on
in the muscles
results from different enzymes in those
two tissues
in liver cells glucose 6-phosphatase
removes the phosphate off of the sixth
carbon
releasing free glucose into the
bloodstream for other organs and tissues
to use
skeletal muscle doesn't have this enzyme
so it simply uses the glucose
6-phosphate by sending it into the
glycolysis pathway to make energy
that can help you with that run
glycogen metabolism is primarily
regulated by two pancreatic hormones
insulin and glucagon now a general rule
of thumb
is that glycogen synthase is active when
it doesn't have a phosphate
whereas glycogen phosphorylase is active
when it does have a phosphate attached
to it
so in liver and skeletal muscle cells
insulin binds to a tyrosine kinase
receptor on the cell surface
and that ultimately activates a protein
phosphatase which goes around removing
phosphates from glycogen synthase
making it active as well as from
glycogen phosphorylase
making it inactive this promotes
glycogen synthesis
and decreases its breakdown
on the other hand glucagon in the liver
cells bind to a g-protein-coupled
receptor on the cell surface
which activates adenolyl cyclase which
converts atp to cyclic amp
or camp camp then activates protein
kinase a
which adds a phosphate to glycogen
phosphorylase kinase
which activates it glycogen
phosphorylase kinase adds a phosphate to
glycogen phosphorylase
increasing its activity and promoting
glycogen breakdown
it also adds a phosphate glycogen
synthase decreasing its activity and
therefore decreasing glycogen synthesis
all right as a quick recap glycogen is a
multi-branched compact structure that's
made of alpha-1-4 glycosidic bonds
between the glucose molecules
and alpha-1-6 bonds at the branching
points
glycogen is considered the major form of
glucose storage in the body
and it's primarily stored in the liver
cells and skeletal muscle cells
after a meal high insulin levels promote
glycogen synthesis
whereas during fasting high glucagon and
epinephrine levels promote glycogen
breakdown
you
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