Leah here from leah4sci.com/mcat and in this video we're going to look at phase 2 of glycolysis
which is the payout phase.
In video 1, we looked at the first 5 steps of glycolysis where glucose was phosphorylated
to glucose 6 phosphate isomerized to fructose 6 phosphate phosphorylated again to fructose
1,6 bisphosphate broken into 2 pieces dihydroxyacetone phosphate and glyceraldehyde 3 phosphate and
then converted all of the dihydroxyacetone phosphate to even more glyceraldehyde 3 phosphate
so where do we stand so far?
We have 1 molecule of glucose sitting at 2 molecules of glyceraldehyde 3 phosphate with
a total investment of 2 molecules of ATP, so far we've done a lot of work and put the
energy in but now that we're on phase 2 we finally get to the payout phase where we expect
to see some energy coming out of these molecules.
As a reminder, we broke the molecule in half, we converted DHAP into glyceraldehyde 3 phosphate
so with every reaction moving forward, remember that we have 2 molecules reacting per molecule
of glucose so when we look at step 6 and we see that glyceraldehyde 3 phosphate reacts,
remember that it's 2 of them.
In step 6 we see the first release of energy when glyceraldehyde 3 phosphate is converted
to 1,3 bisphosphoglycerate or simply 13BPG, 13 bisphospho tells us we have a phosphate
group on carbon number 1 as well as carbon number 3, even though this reaction adds a
phosphate group onto the molecule we're not using a kinase in this case, we're actually
going to react G3P with any D+ along with an inorganic phosphate that happens to be
available in solution and this is the phosphate that we put on the molecule.
The enzyme that catalyzes this reaction is called glyceraldehyde 3 phosphate dehydrogenase.
Yes the name is a mouthful but this is simply G3P dehydrogenase which tells us that it's
an enzyme that acts to D hydrogen to remove hydrogen.
What hydrogen are we removing?
Notice that in the reactant we have a hydrogen atom with 2 electrons binding it with the
carbon, and in product both a hydrogen and both of its electrons are gone, that means
we're losing an entire hydride ion which is a hydrogen, 2 electrons and a negative charge.
Why is this important?
Because this right here is the fight redox reaction we see in glycolysis.
Remember that in a redox reaction something can be oxidized by losing electrons and something
else will be reduced by gaining electrons in which case here we have an NADH plus NH+.
Oh wait that doesn't seem to add up, I said we're losing a hydride, an H- and yet in the
product i'm giving you 2H's with a positive charge, what's going on here?
How does NAD+ gain a hydrogen and then give us 2 hydrogens in the product?
Very simple, remember that hydrogen is the smallest atom and it contains simply 1 proton
and 1 electron.
If I have a molecule that in addition to the molecule itself I have a proton, what I'm
really looking at is a hydrogen without its electron.
So if I take that proton and I add an electron to it I get a hydrogen so I have NAD, a proton
and an electron.
That takes care of the electron.
The proton is what we see over here, H+ is simply a proton but instead of saying plus
proton we're saying +H+ and the other electron winds up on the enzyme.
I'm sharing this not for you to memorize these details but rather to understand what happened
and understand why this is a redox reaction.
For more on its redox on the MCAT, make sure to watch my oxidation reduction video series
link below.
We said this is the first energy harvesting step, but how does getting an NADH and an
H+ help us?
Extracting the electron here, allows us to get even more energy out of it later through
processes like the electron transport chain which we'll cover in other metabolism videos
but for now let's move on to step 7 and look at this very high energy 1,3 BPG.
As a reminder, the P in the circle represents the phosphate group, but don't forget that
each phosphate has a negative charge and having 2 phosphate groups with a negative charge
so close to each other makes this molecule very unstable and when a molecule is unstable,
it is unhappy and therefore very reactive and that means it's looking to get rid of
this high energy so it can become more stable which is convenient because we have lots of
ADP floating around the solution in addition we grab an H+, dump up phosphate onto that
ADP which gives us our first ATP out of glycolysis in addition to a molecule that is now missing
a phosphate and has just an O- on carbon number 1.
What we have here is a carboxylic because it looks like the carboxylic acid but it's
missing that hydrogen atom.
1,3BPG was converted to 3 phosphoglycerate or simply 3PG.
This is a very important reaction, notice that we have ADP and without having to go
into the mitochondria for the electron transport chain, we manage to get a high energy ATP
out of this reaction.
This step is called substrate level phosphorylation because we're getting phosphorylation, we're
getting an ATP directly at the substrate level.
Not to be confused with oxidative phosphorylation which happens on the electron transport chain
in the mitochondria.
The enzyme that catalyzes this reaction is called phosphoglycerate kinase because it's
a kinase a phosphate transferring enzymes that removes the phosphate group from glycerate
but wait a minute, doesn't kinase put a phosphate group onto a molecule?
Here we took a phosphate group off a molecule well actually we can't let this go to waste.
Number 1, as we saw with the aldolase the reverse reaction is getting phosphate added
onto the molecule or think of it as yes, kinase is transferring a phosphate onto a molecule
not the molecule you're looking at or transferring the phosphate onto the ADP, now that we have
a more stable molecule with just 1 phosphate group on 3PG, we wanna convert it into a form
that is closer to pyruvate our ultimate goal which is exactly what we do in step 8, the
phosphate group ends up on carbon number 2 and carbon number 3 gets an OH forming 2 phosphoglycerate
or 2PG.
Take a look at what happened here, we have a phosphate on 3, we got a phosphate on 3.
We had an alcohol on 2, we got an alcohol on 3.
This looks like a simple straightforward isomerization reaction but the enzyme phosphoglycerate mutase
is not a straightforward isomerize.
Phosphoglycerate because it moves the phosphate group on a glycerate but mutates creating
this isomerization reaction not by moving it but rather by first mutating the molecule.
What?
The intermediate for this reaction still has the phosphate group on carbon number 3 but
it also has a phosphate group on carbon number 2, the mutase works by adding a phosphate
group on carbon 2 and then removing the phosphate group from carbon number 3 to form a very
stable intermediate called 2,3 Bisphosphoglycerate.
It's important to point out that this is a stable intermediate, in fact, 2,3 bisphosphoglycerate
of 23BPG should sound very familiar if you've already studied hemoglobin, think about it,
23BPG is a molecule that influences hemoglobin to be less effective at binding oxygen, why
would a glycolysis intermediate have anything to do with hemoglobin, how are they connected?
Actually very much so, if you think about what's going to happen next, even though not
every molecule of glucose that undergoes glycolysis will go into the kreb cycle and then the electron
transport chain, a lot of it will.
And once we start the electron transport chain in the mitochondria, we're going to need a
lot, a lot of O2 molecules because without O2, the mitochondria stop working.
So if we know we're revving up the machinery, we're sending a lot of pyruvates down the
chain, shouldn't we start telling hemoglobin to prepare, to get us all that oxygen so that
we can have energy?
And that is how hemoglobin connects to glycolysis because this stable intermediate 2,3BPG doesn't
all get converted to pyruvate, some of it escapes, gets out of the cell and tells hemoglobin
to get working.
But the rest of it are converted into 2PG which is important because this takes us one
step closer to the goal.
In step 9 we're going to remove a water from our 2PG molecule by removing an OH group from
carbon 3 and a hydrogen from carbon 2 and replace it with a pi bond between carbons
2 and carbon 3.
This gives us a phosphoenol pyruvate, this molecule is also called PEP or pep, and of
course a molecule of H2O.
An enzyme that catalyzes this reaction is called an enolase because the product is almost
a pyruvate molecule except that it has a phosphate group and it's got an enol.
The phosphate group is still sitting on carbon number 2 and the enol is hiding right over
here, remember that in order to bind into a phosphate group we have an oxygen in there,
the phosphate group is an OPO with another O and another O, and that right there is really
an alcohol bound to the phosphate group.
Next to an alkene, and if you recall, anytime you have an alkene plus an alcohol on the
same carbon atom, you have what's called an enol.
This right here is the enol explaining the enolase enzyme, an enzyme that forms an enol.
Students often asks if this is another redox reaction and the answer is no, while we are
removing a hydrogen from the molecule, we're removing an entire molecule of H2O, one of
the carbon atoms gets oxidized, the other gets reduced but the net change on the molecule
is neutral, that means there's no net oxidation or reduction, making this a non redox reaction.
We are so close to the final step that even the name of our reactant, phosphoenol pyruvate
already has our final product in there, we can sense the pyruvate coming, thing is, pyruvate
does not have a phosphate group, so let's get rid of it.
To do that we find an ADP and an H+ that are freely available in solution, we get rid of
that phosphate group and dump on to the ADP to give us a high energy ATP which you'll
recognize is our second substrate level phosphorylation reaction.
The resulting molecule has a carboxylase for carbon number 1, a carbon double bond O on
carbon number 2 because in addition to removing the phosphate group we also move the pi bond,
so just think that this pi bond jumped up over here, giving us a second bond to oxygen
and the third carbon that had 2 hydrogens now has 3 hydrogens because it lost it's pi
bond, and this right here is the long anticipated pyruvate final product.
Given that they've released up energy in the final step, this is a 1 way irreversible reaction
where the cell has to decide, am I ready to turn my phosphoenol pyruvate into pyruvate?
Or do I have to reverse this reaction to avoid having a build up of pyruvate molecules.
This reaction is catalyzed by the enzyme pyruvate kinase because it's a kinase, an enzyme that
adds a phosphate group, in this case from the molecule pyruvate not to the molecule
pyruvate, instead that phosphate group gets transferred on to the ADP.
Phew, that was a lot but if you're feeling overwhelmed, take a deep breath, take a step
back from all the crazy names and structures and actually review what happened here logically
but first, make sure you give this video a thumbs up, subscribe to my channel and hit
that bell icon so you don't miss out on any new videos and then join me in part 3 where
we review glycolysis from a much more simplified perspective where our focus is going to be
on what is happening and why rather than looking at all the details as we did in parts 1 and
2.
You can find this on our website along with the glycolysis cheat sheet by going to leah4sci.com/glycolysis
or clicking the link below.
Again, that's leah4sci.com/glycolysis .