Overview of Glycolysis
Summary
TLDRThis lecture delves into glycolysis, the metabolic pathway that converts glucose into pyruvate, ATP, and NADH. It's a three-stage process occurring in the cytoplasm, with stage one preparing glucose for cleavage, stage two breaking it into two three-carbon molecules, and stage three harvesting energy to form ATP. The lecture emphasizes the purpose of each stage, the enzymes involved, and the energy changes, concluding that glycolysis yields two ATP molecules per glucose molecule.
Takeaways
- 📚 Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, ATP, and NADH molecules, occurring in the cell's cytoplasm.
- 🔑 Glycolysis is divided into three stages for clarity and functional specificity: Stage 1 (3 steps), Stage 2 (2 steps), and Stage 3 (5 steps).
- 🔄 Stage 1 focuses on trapping glucose within the cell and preparing it for cleavage in Stage 2, involving the addition of phosphate groups to destabilize the molecule.
- 🔬 Hexokinase initiates Stage 1 by phosphorylating glucose to form glucose 6-phosphate, using ATP and creating ADP and a hydrogen ion.
- 🔄 Stage 2, the 'cleavage stage,' involves the transformation of fructose 1,6-bisphosphate into two identical three-carbon molecules: DHAP and G3P, facilitated by aldolase.
- 🔄 Stage 3 is the 'energy-harvesting stage,' where ATP is produced and pyruvate is formed, with a focus on transferring high-energy phosphate groups.
- ⚡️ The conversion of G3P to 1,3-BPG in Stage 3 is crucial as it increases the molecule's potential to transfer a high-energy phosphate group.
- 🔋 The net result of glycolysis is the production of two ATP molecules, two NADH molecules, and two pyruvate molecules per glucose molecule, considering the energy invested and harvested.
- ⚖️ The overall process is energy-efficient as it yields a net gain of two ATP molecules after accounting for the two ATP molecules used in the initial phosphorylation steps.
- 🌟 Glycolysis is a fundamental metabolic pathway essential for energy production in cells, even under anaerobic conditions.
Q & A
What is glycolysis and where does it occur in the cell?
-Glycolysis is the metabolic process that breaks down glucose into pyruvate molecules, ATP molecules, and NADH molecules. It takes place in the cytoplasm of the cell.
How many stages are there in glycolysis and what is the purpose of each stage?
-Glycolysis is typically divided into three stages. Stage 1 prepares the glucose molecule for cleavage, Stage 2 involves the cleavage of glucose into two three-carbon molecules, and Stage 3 harvests energy to form ATP molecules.
What is the role of hexokinase in glycolysis?
-Hexokinase is the enzyme that phosphorylates glucose, forming glucose 6-phosphate. This step traps the glucose molecule inside the cell and makes it more reactive for the subsequent steps.
Why is glucose 6-phosphate converted into fructose 6-phosphate in glycolysis?
-Glucose 6-phosphate is converted into fructose 6-phosphate to create a symmetrical molecule, which is necessary for the production of two identical three-carbon molecules in Stage 2.
What is the significance of the second phosphorylation step in Stage 1 of glycolysis?
-The second phosphorylation step in Stage 1, catalyzed by phosphofructokinase, further destabilizes the glucose molecule, making it more reactive and preparing it for cleavage in Stage 2.
What are the two three-carbon molecules produced from the cleavage of fructose 1,6-bisphosphate?
-The two three-carbon molecules produced from the cleavage of fructose 1,6-bisphosphate are dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P).
Why is DHAP converted into G3P in glycolysis?
-DHAP is converted into G3P because both are trioses (three-carbon sugars), and this conversion is necessary for the continuation of glycolysis, as G3P is the molecule that participates in the energy-harvesting reactions in Stage 3.
What is the role of GAPDH in the sixth step of glycolysis?
-GAPDH (glyceraldehyde 3-phosphate dehydrogenase) catalyzes the reaction that converts G3P into 1,3-BPG (1,3-bisphosphoglycerate), using NAD+ and inorganic phosphate, and producing NADH and ATP.
How does the conversion of 3-phosphoglycerate to 2-phosphoglycerate in glycolysis affect the molecule's stability?
-The conversion of 3-phosphoglycerate to 2-phosphoglycerate by phosphoglycerate mutase destabilizes the molecule, making it more reactive and preparing it for the subsequent reaction that forms phosphoenolpyruvate (PEP).
What is the final product of glycolysis and how many ATP molecules are produced per glucose molecule?
-The final products of glycolysis are pyruvate molecules and a net gain of 2 ATP molecules per glucose molecule, considering the initial investment of 2 ATP molecules in Stage 1.
Outlines
🔬 Glycolysis: An Overview
This paragraph introduces glycolysis as the metabolic pathway that breaks down glucose into pyruvate, ATP, and NADH molecules. It occurs in the cell's cytoplasm and is divided into three stages, each with a specific purpose. Stage 1 involves three steps to prepare glucose for cleavage in Stage 2, which consists of two steps. Stage 3, the most complex with five steps, harvests energy to form ATP. The paragraph emphasizes the preparatory role of Stage 1 in making glucose reactive for cleavage and trapping it within the cell.
🌟 Stage 1 of Glycolysis: Investment for Cleavage
Stage 1 of glycolysis is detailed, highlighting the steps that destabilize glucose to prepare it for cleavage in Stage 2. The first step involves hexokinase phosphorylating glucose to glucose 6-phosphate, trapping it within the cell and making it more reactive. The second step is an isomerization catalyzed by phosphoglucomutase, converting glucose 6-phosphate to fructose 6-phosphate to ensure symmetry for the subsequent cleavage. The third step uses phosphofructokinase to add a second phosphate group to fructose 6-phosphate, further destabilizing it. This stage is referred to as the 'investment stage' because it consumes two ATP molecules to prepare the molecule for the energy-yielding steps ahead.
⚡ Stage 2 of Glycolysis: Cleavage and Isomerization
In Stage 2, the cleavage of fructose 1,6-bisphosphate by aldolase into two three-carbon molecules, dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P), is described. The paragraph explains the necessity of isomerization, where triose phosphate isomerase converts DHAP into G3P, ensuring that both molecules are ready for Stage 3. This stage is termed the 'cleavage stage' as it involves the actual breakdown of the six-carbon sugar into two three-carbon molecules, setting the stage for ATP production.
🏁 Stage 3 of Glycolysis: Energy Harvest and ATP Production
The final stage of glycolysis is explored, focusing on the conversion of G3P into 1,3-bisphosphoglycerate (1,3-BPG) by GAP dehydrogenase, which also reduces NAD+ to NADH. This step increases the molecule's potential to transfer a high-energy phosphate group. Phosphoglycerate kinase then transfers this phosphate to ADP, forming ATP and 3-phosphoglycerate. The subsequent steps involve phosphoglycerate mutase and enolase to transform 3-phosphoglycerate into phosphoenolpyruvate (PEP), which is high in energy. Finally, pyruvate kinase catalyzes the transfer of the high-energy phosphate from PEP to ADP, forming pyruvate and ATP. This stage is where ATP is produced, and since the reactions occur twice (once for each three-carbon molecule from Stage 2), a net gain of two ATP molecules per glucose molecule is achieved after accounting for the ATP invested in Stage 1.
🔍 Net Result of Glycolysis
The paragraph concludes with a summary of the net result of glycolysis. For each glucose molecule, two ATP are consumed in Stage 1, and four ATP are produced in Stage 3, resulting in a net gain of two ATP molecules. Additionally, two molecules of pyruvate and two NADH are produced, which can be used in other cellular processes. The paragraph emphasizes the efficiency of glycolysis in harnessing energy from glucose despite the initial investment of energy in the early stages.
Mindmap
Keywords
💡Glycolysis
💡Hexokinase
💡Phosphorylation
💡Isomerization
💡Aldolase
💡Triose Phosphate Isomerase
💡Glyceraldehyde 3-phosphate (G3P)
💡1,3-Bisphosphoglycerate (1,3-BPG)
💡Phosphoglycerate Kinase
💡Pyruvate
Highlights
Glycolysis is the breakdown of glucose into pyruvate, ATP, and NADH molecules.
Glycolysis occurs in the cytoplasm of the cell and is divided into three stages for specific purposes.
Stage 1 of glycolysis aims to trap glucose within the cell and prepare it for cleavage in stage 2.
Hexokinase phosphorylates glucose to form glucose 6-phosphate, trapping it within the cell.
Glucose 6-phosphate is isomerized into fructose 6-phosphate to ensure symmetry for stage 2 cleavage.
Phosphofructokinase adds a second phosphate group to fructose 6-phosphate, increasing its reactivity.
Stage 2 is referred to as the cleavage stage, where fructose 1,6-bisphosphate is split into two three-carbon molecules.
Aldolase catalyzes the cleavage of fructose 1,6-bisphosphate into DHAP and G3P.
Triose phosphate isomerase converts DHAP into G3P, ensuring both three-carbon molecules are identical.
Stage 3 is the investment stage, where energy is harvested to form ATP and pyruvate.
G3P is converted into 1,3-BPG by GAPDH, generating NADH and a high-energy molecule.
Phosphoglycerate kinase transfers a phosphate group from 1,3-BPG to ADP, forming ATP.
Phosphoglycerate mutase destabilizes 3-phosphoglycerate to prepare it for further reactions.
Enolase converts 2-phosphoglycerate into phosphoenolpyruvate (PEP), a high-energy compound.
Pyruvate kinase catalyzes the transfer of a phosphate group from PEP to ADP, forming pyruvate and ATP.
The net result of glycolysis is the formation of two ATP, two pyruvate, and two NADH molecules per glucose.
Glycolysis is an essential process for energy production in cells, even under anaerobic conditions.
Transcripts
in the previous several lectures we
discuss the details of the three stages
of glycolysis so now let's actually put
all that information together into a
single lecture to actually try to make
sense of things and let's summarize our
results so glycolysis is the breakdown
of glucose into pyruvate molecules ATP
molecules and NADH molecules and all
this takes place in the cytoplasm of the
cell
now we typically break down glycolysis
into three stages we have stage 1 that
consists of 3 steps we have staged 2
that consists of 2 steps and we have the
most complex stage stage 3 that consists
of five steps now the reason we break
down glycolysis into these three stages
is because each one of these stages
actually carries out its own specific
purpose it has its own specific purpose
it carries out a specific function so
let's begin with stage one in stage one
the entire point of stage one is to take
that glucose molecule trap that glucose
molecule inside the cell so that it
can't actually leave that cell and begin
preparing that glucose molecule for
cleavage which takes place in stage two
so the entire point of stage 1 is to
prepare that molecule for stage 2 where
it basically is cleaved into two
identical 3 carbon molecules and once it
is cleaved in stage 2 it's the third
stage where we harvest some of that
energy we capture some of that energy to
form ATP molecules as we'll see in just
a moment so as we discuss each one of
these individual processes keep that in
mind because ultimately for instance in
stage one each one of these reactions
takes place and each one of these
reactions essentially wants to
accomplish that end goal so each one of
these reactions in stage 1 once the
wants the trap that molecule in the cell
and wants to destabilize the molecule
make it more reactive so that event
it is prepared for stage two to break
down into smaller molecules so let's
begin with stage one process one step
one so our glucose makes its way into
the cytoplasm of the cell what happens
is an enzyme known as hexokinase hexyl
means we have one two three four five
six carbons in our sugar kinase means
we're going to phosphorylate that
glucose so a force for group is taken
from the ATP by the hexokinase and is
added onto carbon number six and so we
form glucose 6-phosphate we break down
the ATP into ADP and also the H ion is
released as well and this reaction
releases this amount of energy so it's
an exergonic reaction that's because the
ATP is broken down into a more stable
molecule and that drives this exergonic
reaction now the point of this step is
to one destabilize the glucose to make
it more reactive and begin preparing it
for stage two and the second point is by
adding this polar component we trap that
glucose in a cell it will not be able to
exit that cell because one it can't pass
the membrane and B it cannot use any of
those transport membrane proteins
because its structure is different now
let's move on to step two in step two
the goal is to basically take that
glucose 6-phosphate and transform it
into an isomer into fructose 6-phosphate
why well because in stage two we
basically want to produce two identical
three carbon molecules and to produce
those two identical 3 carbon molecules
we have to have symmetry in our molecule
so this is not symmetric but this is
symmetric and so the glucose 6-phosphate
is transformed into fructose 6-phosphate
to make sure we get those two identical
3 carbon molecules in stage 2 so you
might ask well if I keep this molecule
the glucose 6-phosphate stage what will
happen in stage 2 well if we keep it in
the glucose that in stage 2 are going to
form one molecule that has two carbons
and one molecule that has four carbons
and that is not symmetric so that's why
we carry out step two again the entire
goal in stage one is to prepare that
glucose for cleavage which happens in
stage two and the enzyme that catalyzes
this well this is an isomerization
reaction we transform one isomer into
another and this molecule is a glucose
that contains a phosphate and so
phosphoglucomutase place spontaneously
under physiological conditions let's
look at step three so the point of step
three is to continue destabilizing that
molecule so in step one we destabilize
it increased it energy increased its
energy and made it more reactive because
we made it more polar we added a charge
and here we add a second charge and that
makes it even more reactive and more
likely to actually undergo cleavage in
stage two so we take the fructose
6-phosphate and again because we want to
add up the spoil root what type of
enzyme are we going to have well a
kinase what type of kinase well what
type of molecule isness it's a fructose
that contains a phosphate so
phosphofructokinase phosphorylates this
process and adds that was for oh group
onto this oxygen and now we have a
symmetrical molecule and once the
cleavage takes place in stage two that
will ultimately allow us to produce two
three carbon molecules and notice this
stage one because we're essentially
investing to prepare that molecule for
cleavage we actually use energy
molecules we use one two ATP molecules
in stage one that's why we call this
then
vestment stage now stage two we call the
cleavage state because this is where we
break down this molecule that we form in
stage one so we take the fructose 1 6
bisphosphate and under the guidance of
an enzyme called aldolase why aldolase
well because this is an aldol reaction
and in fact going backwards is an aldol
condensation and so that's why we call
this an aldolase so essentially what the
aldolase does is it Cleaves the bond
here and it forms these two three carbon
molecules so again the entire point of
this step was to basically create the
isomer so that once this process takes
place we produce two or three carbon
molecules and not a two carbon and a
four carbon molecule so we have two
three carbon molecules one of them is
DHAP which stands for dihydroxyacetone
phosphate and the other one is g8p which
stands for glyceraldehyde 3-phosphate
now this molecule is the one that will
go on to stage three so once we form
this gap it doesn't do anything else but
the DHAP doesn't lie directly on the
path of glycolysis and so what we have
to do is we have to take this molecule
and we have to transform it into this
molecule now just like glucose is an
isomer to fructose DHAP is an isomer to
GAAP because both of these are trioses
and a triose is a three carbon sugar so
we have one two three carbons one two
three carbons
both of these are trioses so again we
have to depend on an enzyme called
isomerase what type of isomerase will
triose phosphate isomerase trials
because these are tri OSIS and they
contain phosphate groups one each and so
triose phosphate isomerase basically
converts this GH a P the
dihydroxyacetone phosphate into the
glyceraldehyde 3-phosphate
and one stage two takes place we
essentially took so the net result of
stage two is we took the fructose 1 6
bisphosphate and we cleaned it into two
identical 3 carbon molecules these gap
molecules so let's move on to stage 3 so
essentially in this stage I've only
listed the reactions for a single gaap
molecule but you should know that all
these steps actually take place twice
because we have these two molecules that
perform in stage 2 so let's move on to
stage 3 so remember this is our
investment stage we invest energy to
prepare it for the cleavage once we
cleave it we basically go on to stage 3
and this is where this is where we're
actually going to produce those ATP
molecules and pyruvate molecules so the
entire goal here is to basically
destabilize the molecule and eventually
create a molecule that contains a high
potential to transfer force for groups
and we'll see why that's important just
the moment so let's take a look at step
6
so in step 6 what we basically want to
do is we want to transform the gap
molecule into 1 3 BPG why well because
we want to transform a molecule that has
a relatively low potential to transfer
post-oil to molecule that has a
relatively high potential to transfer
that force oil group and so we basically
take the gap we mix it with our NAD+ and
we also use a north of us an ortho
phosphate and in the presence of gap
dehydrogenase so dehydrogenase basically
means we're going to have a reaction in
which there will be a transfer of a
hydride group and so this will be
reduced into NADH we're going to release
an H ion and that phosphate the
orthophosphate will basically attack
this molecule and by onto this carbon
here and so now we have
these two phosphate groups on this
molecule so on the first position and
the third position and that's why this
molecule is essentially a molecule that
has a higher potential to actually
transfer that was Forel group and so now
in the next step we can use 1 3 BPG this
same molecule to basically transfer that
force 4 group onto an ADP molecule
thereby producing an ATP and the
molecule that catalyzes this well again
it must be a kinase why well because
this is a phosphorylation reaction and
so we use our phosphoglycerate kinase
phosphoglycerate because this is a 1 3
bisphosphoglycerate so we mix it with
the ADP because this will accept this
group here and so once the process takes
place we essentially form an ATP
molecule only for a 3 phosphoglycerate
now what happens with a 3
phosphoglycerate well in the next step
in step 8 we basically want to transform
the 3 phosphoglycerate
into a into a less stable molecule so we
want to take this and destabilize it and
that will make it more reactive so that
in step 9 it can actually react so to
destabilize this the goal is we want to
take this phosphate and bring in closer
to this negative charge so we have a
negative charge of negative 1 and a
charge of negative 2 and if we basically
decrease the distance between the
negative charge that will destabilize
this molecule and so we have an enzyme
known as phosphoglycerate mutase a
mutase is simply an enzyme that takes a
group on the molecule and changes its
position and so we have the
phosphoglycerates of phosphoglycerate
mutase will be the enzyme that will take
this group and bring it on to the second
carbon and so now we have not 3
phosphoglycerate but a 2
phosphoglycerate and notice this is an
endergonic process so under physiologic
conditions it will not be spontaneous we
have to input energy and so this
molecule will be less stable than this
molecule now so now that we have this
less stable molecule we can basically
react it in step nine and we can
transform it into a molecule that
prepares it to form that pyruvate so we
take this molecule and we use enolase to
transform it into an enol so we have
phosphoenolpyruvate or PEP we
essentially form a double bond between
these and the h and the Oh H combines
and a water is kicked off and so this is
a dehydration reaction once we form this
molecule this molecule is not very
stable and it has a very high force for
transfer potential why well because this
is essentially trapped in the enol state
and it's trapped because this oxygen
doesn't have an H it has this four spoil
group and so what must happen in the
final stages this fuzz for group must be
donated to an ADP molecule and replace
with an H and once it is replaced with
an H it can transform into the more
stable ketone state the pyruvate
molecule and that's exactly what happens
in a final stage we take the smaller
Kuehl that is high in energy so it
contains very active bonds and that
makes it a very good molecule that
actually transfers that the spore group
onto ADP and so in the presence of adp
and h plus we take this molecule and by
the action of pyruvate kinase so again
we form pyruvate in the last step and
this reaction is a phosphorylation
reaction so we're using a kinase and we
for the pyruvate in the ketone state and
that ATP molecule and because this
process takes place twice we form two
ATP molecules here we form two ATP
molecules here so a total of four ATP
molecules in stage 5 we use two ATP
molecules in stage one and the neck
result is we form two ATP molecules in
glycolysis per glucose that we actually
use up so if we sum all these individual
reactions and we basically sum up all
these individual energy values keep it
in mind that all these take place twice
and so we have to multiply these energy
values by two for these five steps we
basically get the following net result
so we have a glucose we have two adp we
have two nad plus we have two P eyes
that's our net input and then that
output is
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