Biochemistry Gluconeogenesis II

Hello and welcome back to Educator.com, and welcome back to Biochemistry.0000

In this particular lesson, we are going to continue our discussion of gluconeogenesis.0004

Now, in the last lesson, we talked about the first bypass step, and that bypass step going from the pyruvate back to phosphoenolpyruvate happens in 2 steps.0010

We convert the pyruvate to oxaloacetate, and the oxaloacetate goes to phosphoenolpyruvate.0021

OK, now, we are just going to continue on with the process.0030

Alright, let's just jump on in.0035

OK, it is reduced to malate.0039

Now, let's continue on; now, the mitochondrial membrane does not have a transporter for the oxaloacetate that we formed.0045

You remember the first thing we did?0074

Where we left of was we actually formed oxaloacetate; that was that mechanism that involved the biotin coenzyme.0075

OK, for oxaloacetate, the oxaloacetate is actually reduced to malate, and a malate is sent back out into the cytosol.0084

It is reduced to malate then transported back to the cytosol, where it is reoxidized to oxaloacetate.0102

Yes, reoxidized to oxaloacetate, all these molecules floating around.0132

OK, this is what actually takes place.0148

We have the oxaloacetate; let me go ahead and do this in blue.0153

We have oxaloacetate plus the NADH + H⁺.0158

It goes to L-malate + NAD⁺, and then, the reverse reaction takes place once it actually is sent back out into the cytosol.0179

You have NAD⁺ plus our malate going back to oxaloacetate, and NADH + H⁺ is regenerated.0183

This part takes place; this first one takes place in the mitochondrion, and then, this one takes place outside the mitochondrion once that malate is sent back into the cytosol.0203

OK, here is the second reaction; let me go back to black here, so reaction no. 2.0215

We have our oxaloacetate molecule: C, C, C, C.0224

Excuse me.0230

I have that, and I have a carboxyl over here.0235

This is where we are going to use the guanosine triphosphate instead of the adenosine triphosphate, and what ends up happening is, one of the things that leaves is, of course, the GDP; and the other thing is now, the CO₂, this right here, is lost.0240

What you end up with is H₂, C, C, C, C, O, O, and then, of course, I will go ahead and put the phosphate down here- PO₃^2-.0260

Now, what you have is your phosphoenolpyruvate.0272

This right here is your phosphoenolpyruvate.0276

This is your oxaloacetate.0281

Let me go back to black; OK, here is the mechanism- mechanism, mechanism, mechanism.0284

OK, it is actually a pretty easy mechanism, so it is not going to be that involved.0291

Let me just go ahead and draw...yes, that is fine.0296

I will do C, C, C, C.0303

I will go ahead and draw that - woo, I swear, once you start writing all these things out, it starts to get really, really, kind of crazy - O^-.0308

I will go ahead and put my hydro...you know, that is OK.0319

I will just write H₂ here; we have 2 hydrogens attached there.0323

Now, I will go ahead and write my...let me see.0327

I have got my guanosine, ribose.0331

I have got O, P, O, P, O, P, O, here, here, and I have got that.0336

So, this is our guanosine triphosphate; here is what happens.0350

Let me go ahead and do this in red; we said carbon dioxide really likes to form, and if there is something nearby that will allow it to form, it will do so.0355

These electrons go here to reform that.0365

These electrons come back up here to actually reform the double bond.0371

These electrons come over here on a nucleophilic attack.0376

They push these electrons onto that, and what you are left with is the phosphorylated oxaloacetate that has lost its carboxyl, which is phosphoenolpyruvate, the formation of a double bond.0383

What you are left with is the following.0396

You are left with H₂, C, double bond, C, C, O^-, O, PO₃^2-.0404

This is what came from the GTP- that is it.0418

The overall reaction here...we started off with our pyruvate.0422

We added ATP; we added GTP.0433

We added bicarbonate, HCO₃2-...no, HCO₃ is -1, not -2.0440

Carbonate is -2; we formed PEP.0448

We released ADP; we released GDP.0455

We also released the PI, remember?0459

We invested the energy of ATP; we lost the ADP and the PI in the first reaction.0463

It was the energy that we used, and we also lost a CO₂.0468

Now, the ΔG for this reaction - standard ΔG - is 0.9kJ/mol.0473

We said that this is a highly exergonic reaction; that is not highly exergonic.0482

Under physiological conditions, it is highly exergonic.0487

ΔG, under physio conditions, is equal to -25kJ/mol.0492

That is huge; I mean, it is not massive, but it is big- certainly enough to make it virtually irreversible.0498

Now, this PEP - phosphoenolpyruvate - is used in many other reactions.0507

It is a molecule that the body uses for lots of other things.0519

As it is formed, it is actually, well, it is used in other reactions.0522

As it is formed, it becomes depleted because it is being used for other things.0536

As we form the PEP, we are pulling it away and using it for other things.0550

Well, Le Chatelier’s principle, as we pull away a product from a particular reaction, we are messing with the equilibrium, and what that is going to do, it is going to pull the reaction forward.0554

That is what makes this reaction so highly exergonic under physiological conditions because this product, PEP, is being used for other things.0568

It is pulling the reaction forward, under Le Chatelier’s principle.0576

As it is formed, it becomes depleted, thus pulling the overall reaction forward and making it irreversible, giving it that large -ΔG value.0581

OK, now, let's go ahead and I am going to just change pages here, and I think I am going to go back to black.0609

Recall the 2 pathways; I am going to redraw it, but I am going to redraw that thing with the mitochondrion and the 2 pathways, but without all of the information, without the enzymes.0621

Let me go ahead and draw this here; this is a mitochondrion.0630

We had our 1 pathway, which was pyruvate to pyruvate to oxaloacetate, oxaloacetate to malate, malate out to malate, malate back to oxaloacetate and oxaloacetate to phosphoenolpyruvate.0636

Now, the other pathway we had was the following; we had the pyruvate here, but it actually came from lactate, and then, of course, we had pyruvate to pyruvate, and in the mitochondrion, we had the oxaloacetate and the oxaloacetate to directly the phosphoenolpyruvate, and then, of course, that is shipped out into the cytosol to do whatever it does.0662

OK, here is the important step; the oxaloacetate to the malate - let me do this in red - NADH + H⁺ NAD⁺.0685

Oxaloacetate is reduced to malate - OK - and then, it is sent out; and then, the malate is reoxidate.0704

NAD⁺ is our oxidizing agent, and then it, itself, is reduced to NADH + H⁺.0711

Here, this step right here, this is NAD⁺ NADH + H⁺.0720

Notice what is happening; in this particular case, well, here the NAD⁺ NADH is being produced in the cytosol.0732

Here, NADH is being used up in the mitochondrion, and it is being reproduced in the cytosol.0744

Now, let's talk about what is going on here.0750

The ratio of NADH concentration to NAD⁺ concentration - OK - in the cytosol is pretty low actually, is quite small, about 8 x 10^-4.0754

That is actually quite small; there is not a lot of NADH in the cytosol.0781

OK, now, this is much lower than the NADH to NAD⁺ ratio in the mitochondrion.0788

In other words, there is a lot more NADH available - about 5 times more - in the mitochondrion than there is in the cytosol.0817

OK, let's take a look at what this means.0826

Now, one of the reactions of gluconeogenesis, in gluconeogenesis, NADH is consumed.0830

OK, in one of the reactions that takes place in the cytosol, NADH is necessary.0850

It is actually consumed in converting the 1,3-biphosphoglycerate to 3-phosphoglycerate.0855

NADH is something that is required for gluconeogenesis to actually continue.0870

This is not one of the bypass reactions; this is just one of the reverse reactions of glycolysis.0875

In glycolysis, 3-phosphoglycerate to 1,2-biphosphoglycerate, the other way around, NADH is actually being used up.0880

Well, when it is used up, the concentration is already low in the cytosol.0889

So, in order for gluconeogenesis to continue, there has to be something that actually produces enough NADH to keep it readily supplied in the cytosol.0893

What this particular pathway does is it uses the fact that inside the mitochondrion, there is NADH concentration.0903

There is plenty available, so what it does is it uses that NADH to convert the oxaloacetate to malate, and then, it sends it out into the cytosol; and then, it reuses the NAD⁺ concentration to reproduce the NADH, which goes on and constantly replenishes the NADH concentration in the cytosol.0910

It is, sort of, taking NADH that is in the mitochondrion and sending it out into the cytosol by doing this oxaloacetate to malate, malate back to oxaloacetate, to make sure that there is enough NADH in the cytosol.0931

That is what is going on; in gluconeogenesis, NADH is consumed in converting the 1,3-biphophoglycerate to 3-phosphoglycerate.0947

The NADH in the cytosol is being depleted.0959