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Lecture Comments (5)

1 answer

Last reply by: Professor Hovasapian
Fri Oct 24, 2014 10:02 PM

Post by Tim Zhang on October 22, 2014

Your lectures helped me a lot. However I faced a really difficult question on this topic, could you help me solve this?  
The question ask that average human requires about 2,000 kcal of energy per day, which is equivalent to about 3 mol of glucose per day. It is a lot calories! but why don't humans spontaneously combust?

2 answers

Last reply by: tiffany yang
Wed Nov 13, 2013 10:07 PM

Post by tiffany yang on November 13, 2013

Dear professor,
I have an exam this friday, but I'm confused on the transaldolase reaction....isn't ketose being transferred as well? just like all the other nonoxidative reactions? so why is that one called transaldolase reaction, instead of transketolase rxn? Thanks! Your so amazing!

ALso, I read from my teacher's study material that there are four modes for PPP, depending on the needs , in mode one, where cell only needs ribose, then there WILL be nonoxidative reaction, so that fructose 6 phosphate and glyceral 3-phosphate will use the non oxidative PPP pathway to go back to ribose.(because we need ribose) THis I understand;

however, I don't understand why, when both ribose and NAPDG are needed, then there won't be non oxidative reaction happening. (the reason one the study guide was that because the product ribose is needed, therefore we don't want the non oxidative part to recycle those ribose, because we need those ribose.)

seems like these two are contradicting. Thanks.

The Pentose Phosphate Pathway

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • The Pentose Phosphate Pathway Overview 0:17
    • The Major Fate of Glucose-6-Phosphate
    • The Pentose Phosphate Pathway (PPP) Overview
  • Oxidative Phase of The Pentose Phosphate Pathway 4:33
    • Oxidative Phase of The Pentose Phosphate Pathway: Reaction Overview
    • Ribose-5-Phosphate: Glutathione & Reductive Biosynthesis
    • Glucose-6-Phosphate to 6-Phosphogluconate
    • 6-Phosphogluconate to Ribulose-5-Phosphate
    • Ribulose-5-Phosphate to Ribose-5-Phosphate
  • Non-Oxidative Phase of The Pentose Phosphate Pathway 19:55
    • Non-Oxidative Phase of The Pentose Phosphate Pathway: Overview
    • General Transketolase Reaction
    • Transaldolase Reaction
    • Final Transketolase Reaction

Transcription: The Pentose Phosphate Pathway

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

We have been discussing glycolysis and gluconeogenesis, and we are going to close out our discussion of this unit by discussing the pentose phosphate pathway, so let's go ahead and jump right on in.0004

OK, we know that the major fate of glucose 6-phosphate, actually, is glycolysis.0018

The major fate of our glucose 6-phosphate is glycolysis, and we spent a large amount of time discussing that.0029

Quick recap: glycolysis- we formed pyruvate; pyruvate becomes acetyl-CoA.0043

Acetyl-CoA enters the citric acid cycle, and it goes on to the electron transport chain and oxidative phosphorylation- all of which, we will be discussing, not a problem.0049

However, another important fate of glucose 6-phosphate is the pentose phosphate pathway.0061

Another pathway for the glucose 6-phosphate is the - I will just call it PPP - the pentose phosphate pathway.0070

OK, this is the formation of pentose phosphates - 5 carbons sugars - that are ultimately used by the cell to form things like RNAs and DNA and ATP, NADH, FADH2 - things like that, I will also put - and coenzyme A.0082

That is another fate for the glucose 6-phosphate; instead of glycolysis, it can actually take another pathway.0129

Now, in this pathway - in PPP - NADP+ is the electron acceptor.0135

We saw in glycolysis that it was the NAD+ that ended up oxidizing and accepting the electrons.0145

Here, it is NADP+; it is still doing the same thing.0152

It is just oxidizing; it is taking a couple of electrons away, taking a couple of hydrogens away - that is it - and then, ultimately giving them over to the electron transport chain.0156

It is the electron acceptor, the oxidizer, and the NADPH that is formed upon oxidization of these biomolecules is needed, used in anabolic pathways - in other words, biosynthesis - and anti-oxygen radical chemistry.0166

It basically acts participates in anti-oxidant chemistry.0216

OK, now, the pentose phosphate pathway has an oxidative phase and a non-oxidative phase.0223

OK, let's go ahead and diagram this out, so that we will have a look at it; and then, we will go ahead and take a look at the individual reactions of the pathway.0247

It is a rather short pathway; it is not a problem.0256

There is only 1 part of it - the non-oxidative - that looks like it is a little involved, at least it will on the page, but again, it really only consists of 3 reactions.0258

Let me see; do I want to do it here?0268

No, you know what, I think I will start on the next page.0270

Let me go ahead and put that there, and I think I am going to go ahead and do this in...I think I will do this in blue, why not?0274

Just for a change of pace, OK, let's go ahead and write oxidative up here.0281

Yes, I will write oxidative phase up here, and then, I will write the non-oxidative over here; and hopefully, we can work this out.0288

We have got our glucose 6-phosphate.0300

I will go ahead and write everything out, and in a couple of steps, we are going to end up forming our 6-phosphoglucanate.0306

Again, it is going to be up to your teacher whether he or she wants you to actually memorize this or just know that it exists.0324

Whether you actually have to know what is formed, be able to reproduce it or be able to, at least, passively identify it, label it, know the enzymes, it is going to be up to them, but we will have discussed it.0330

6-phosphoglucanate to ribulose, I will go ahead and write everything out, and then, I will go back and fill in all of the enzymes and all of the cofactors, ribulose 5-phosphate, and finally, we have ribose 5-phosphate.0340

OK, in this phase right here, we have NADP.0367

I always forget the P; I am so used to NAD all the time.0376

So, if it happens like that, please forgive me, and again, I am hoping that you will confirm all of this with the illustrations that are in your book to make sure that I have not forgotten anything.0380

There is so much information in just a simple pathway like this that it is very, very easy to forget things.0388

NADP+ comes in; we have NADPH comes out, and then, I will go ahead and do that.0393

I will go ahead and do this; I will explain what the GSH and the GSSG is in just a second and we will be talking about later, and let me see.0403

6-phosphoglucanate to ribulose 5-phosphate, CO2 actually ends up coming out there.0414

And then, again, another oxidation takes place because it is the oxidative phase.0420

NADP+ and NADPH comes out, and here, we will go ahead and do this; and we will just call this reductive biosynthesis anabolic pathways- that is it.0425

You know what, let me rewrite this oxidative just a fact, I am going to go ahead and do this in red.0451

This is the oxidative phase, and the non-oxidative - here, we will just diagram it out - phase.0468

Now, let me go back to blue.0475

OK, we are going to have the ribulose come here like that.0479

This is going to recycle our glucose 6-phosphate, the non-oxidative phase.0485

It is not going to go on to from the ribose 5-phosphate, which gets siphoned off and goes on for use in RNA, DNA and things like that; but it recycles this, so that it can keep producing this NADPH, so that it can keep doing whatever it is supposed to be doing - reducing the glutathione - so that it can be involved in antioxidant chemistry or keep producing NADPH, so that biosynthesis can actually keep taking place.0490

OK, that is a general scheme; glucose 6-phosphate is converted to 6-phosphoglucanate in 2 steps, and then, 6-phosphoglucanate, it loses its CO2 molecule, becomes ribulose 5-phosphate, and ribulose 5- phosphate is isomerized to ribose 5-phosphate.0512

In the non-oxidative phase, if the cell does not happen to need any ribose 5-phosphate, it will come to here, and the ribulose and the ribose 5-phosphate will recycle to glucose 6-phosphate and keep going like that.0529

OK, now, let me go ahead and go to red here.0543

In the oxidative phase, the products are ribose 5-phosphate, NADPH and CO2.0549

We have our NADPH; we have our NADPH.0574

We have our CO2, and we have our ribose 5-phosphate.0577

Those are our major products of this pathway.0580

And again, the NADPH is used to reduce glutathione, which protects the cells against damage by hydrogen peroxide and the hydroxyl radical- very powerfully reactive species, very damaging.0585

A radical, if you remember, is a species with an odd number of electron.0634

There is one electron there that is very, very reactive.0638

It also is used - in other words, the NADPH - to support reductive biosynthesis.0643

OK, the ribose 5-phosphate is used...well, we know that already.0665

We have mentioned that; the ribose 5-phosphate is used as a precursor for use in the formation of other molecules, the coenzyme A, RNA, DNA- things like that.0676

OK, now, for cells that do not require the ribose 5-phosphate - the non-oxidative phase, the non-ox phase - recycles the pentose back to the glucose 6-phosphate.0688

That is this pathway right here, and it does it for the continued production of the NADPH - back and forth, back and forth, back and forth, that is it.0725

This is the pentose phosphate pathway; it will either go and form the ribose 5-phosphate, and then, the process goes and produces some NADPH to be used, and if the cell does not need it, it just continues the cycle like this, so that it produces NADPH for whatever it is that the cell needs.0752

OK, now, let's go ahead and take a look at the individual reactions of the oxidative phase.0767

I think I am going to go back to black for this one.0776

Let's look at glucose 6-phosphate here, 1, 2, here, here, here, here.0780

Let's go ahead and do this, bottom, top, bottom, C, O, and I have got PO32-.0787

The first step, I will go ahead and do that.0798

Let me get a little bit bigger; what the heck.0803

I have got that; I said we have got NADP+ coming in.0808

We have got NADPH coming out.0815

The enzyme for this is glucose 6-phosphate dehydrogenase, and it also requires magnesium 2+, most dehydrogenases do.0821

OK, now, this particular reaction, what it does is the following.0837

Yes, it is fine; OK, it actually ends up forming...that is that.0844

That is that; yes, I remembered it there.0854

OK, C, O, PO32-, we end up forming this thing called...actually, I will put the names in a minute.0857

And then, from here, let's go ahead and come down here.0869

OK, from here, an enzyme called - well, I will put the enzymes...lay out...let me put it here - lactonase.0874

This is a lactone; It is an ester, which is involved in a ring.0883

OK, 6-phosphoglucono-delta-lactone, I will write the names in just a minute.0888

It is just an ester, C, double bond O, C; it just happens to be in a ring.0892

That is what a lactone means; OK, now, this lactonase, actually, opens up the ring, and what you end up with is the following.0897

You end up with the...yes, let me put it...yes, it is fine, 1, 2, 3, 4, 5.0905

Let me make sure I have enough room here; I will go ahead and put the carbonyl up here.0913

It is that, and then, we have OH; and we have H.0920

We have H over here; we have OH over here, and then, we have our OH, our OH, and then, of course, we have our PO32-.0925

So, we actually open this up; now, the other reaction is going to be this one.0937

We are going to have NADP+ come in, NADPH come out, and this is also where CO2 comes out.0944

You know what, I am actually going to...sorry about that; I am going to put the CO2 on top, so that I can write the enzyme name underneath.0959

CO2 also leaves, and the enzyme here is 6-phosphoglucanate dehydrogenase.0965

OK, and what we end up forming is the following; the CO2, that is this CO2 right here that goes away.0984

The dehydrogenase part, what it, actually, ends up doing, it, actually, takes this hydrogen away, and it takes this hydrogen away to form a double bond at this carbon.0991

What you end up with is the following 5-carbon fragment.1002

I will go ahead and stay here, 1, 2, 3, 4, 5.1007

What you end up with is OH, H; what you end up with is a carbonyl.1012

You end up with OH; you end up with OH, and you end up with our PO32-.1018

This is the ribulose 5-phosphate; now, what happens here is this enzyme, pentose phosphate isomerase or isomerase, and what it does is it flips this and this.1023

The carbonyl, it turns this from a ketose into an aldose.1044

We have C, C, C, C, C, and now, the carbonyl ends up up here with an H; and the alcohol ends up here.1049

This is the ribose 5-phosphate, and I will go ahead and put a circle around the P for PO32-.1064

Now, let me go ahead and write the names in; I will go ahead and write this in blue.1070

This is our glucose 6-phosphate.1074

OK, this is 6-phosphogluco-delta-lactone.1078

Well, delta gluco, glucono, I do not know.1089

Glucono-delta-lactone, it does not really matter- gluco, glucono.1096

OK, this right here, this is 6-phosphoglucanate.1101

This is 6-phosphoglucanate.1108

That is that molecule right there; OK, this is ribulose 5-phosphate, and this is the ribose 5-phosphate.1116

You remember the names; the ketose has the U-L: ribulose-ribose, xylulose-xylose- things like that.1128

This is the ketose, ketone; this is the aldehyde version.1138

This is the oxidative phase; from here to the lactone to the phosphogluconate, it opens it up, and then, it decarboxylates.1142

This takes the CO2 off; it oxidizes, right?1151

It takes away couple of hydrogens, converts it to a carbonyl, and then, it flips the carbonyl and the alcohol, so it looks like that.1156

This is the oxidative phase; alright, let me see.1164

Is there anything else that I need to write here?1168

I have got the enzyme names; I have got the NADP, the NADPH.1172

I have the CO2, so again, CO2 is lost; that is this CO2.1175

OK, actually I should probably put this as +H+ because this is the H1180

This is the H; that is those 2 Hs right there.1184

In any oxidation with the NAD or NADP, you end up with NADH + H+, NADPH + H+.1188

OK, OK, now, let's go ahead and take a look...this is the oxidative phase.1196

Now, we will go ahead and take a look at the non-oxidative phase.1203

OK, it is going to look like there is a lot on this page, but it is not a problem.1207

We will just draw it all out, and we will see what we have got.1212

I think I am going to go back to black for this one; I hope you do not mind.1217

Hopefully, there is enough room; I think I have got enough room to actually do this.1221

I am going to start up here, so ribose 5-phosphate, and it is going to go there, sedohep 7-phosphate; and then, we have fructose 6-phosphate.1225

OK, we have xylulose 5-phosphate, and we have glyceraldehyde-3-phosphate; and that will form our erythrose 4-phosophate.1251

Let me see; that goes that way, and this goes this way.1270

And again, I hope to God that I have enough room here.1277

Fructose 6-phosphate and I have glyceraldehyde-3-phosphate.1282

I have xylulose 5-phosphate.1291

Now, let's go ahead and see if we cannot...we are going to have a mirror image of this.1295

I think I am going to have to make this a little bit smaller.1300

I have got xylulose 5-phosphate, and a glyceraldehyde-3-phosphate.1304

Geez, I hope I have enough room to do this, fructose 6-phosphate.1315

I have erythrose 4-phosphate that comes from this.1320

Yes, I should have enough room; it should not be a problem.1330

I have glyceraldahyde-3-phosphate, and I have got sedohep 7-phosphate; then I have something coming this way and this way, and I have got, again, a xylulose 5-phophate, and I have got ribose 5-phosphate.1334

There is a mirror right here - OK - right down the middle, and, oh, let me...a little bit more here.1355

Let's go to fructose 6-phosphate, sedohep, sedohep, and here is another fructose 6-phosphate, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 6.1364

OK, we are good; now, the enzymes that take care of this... actually, let me do the enzymes in blue.1380

I will tell you what is going on; let me just go ahead and write everything in.1388

Transketolase, transaldolase- these enzymes are absolutely extraordinary.1395

They do the extraordinary.1400

It is just really...when you see what they do, it is is amazing, and another transketolase reaction right here.1404

OK, now, let's go ahead and write in red glucose 6-phosphate.1414

That ends up going there; this ends up going here.1423

This ends up going here; this ends up going here.1426

This ends up going here; OK, here is what is happening.1429

Let me go ahead and do this in black.1432

The transketolase enzyme, it transfers a 2-carbon fragment; it is what it does, and we will talk about the reaction in just a minute, but I wanted you to see what actually happens with this whole non-oxidative phase.1438

This is the non-oxidative phase that we are going to be discussing here; let me just write that.1452

Sorry about that; I did not write it on top, so this is the non-oxidative phase.1456

This is where the pentose phosphates that is formed - the ribose 5-phosphate and the ribulose 5-phosphate - are actually recycled back to glucose 6-phosphate to keep going in that non-oxidative cycle in order to keep producing the NADPH.1461

OK, here is what happens; in the first transketolase reaction - let me do this in...that is fine, I will do it in red - ribose 5-phosphate and xylulose 5-phospahte react and 2 things get produced.1479

The 2 things that come out of this reaction are glyceraldehyde-3-phosphate and sedoheptulose 5-phosphate.1495

This is a 5 carbon; this is a 5 carbon.1500

There is 10 carbons altogether; what they produce is a 3-carbon and a 7-carbon sugar.1503

That is what is amazing; it is the carbons that again, they switched around.1509

It is 3 and 7; it is still 10.1511

Those come together, they form erythrose 4-phosphate, which is a 4 carbon and fructose 6-phosphate, which is a 6 carbon.1514

So, we went 5-5 which is 10, 7-3, which is 10 to 6-4, which is 10.1522

Now, this fructose 6-phosphate is converted to glucose 6-phosphate by the enzymes in the glycolytic pathway, remember?1527

And then, we are trying to get to glucose 6-phosphate because we are recycling it back to the oxidative phase.1533

OK, now, this erythrose 4-phosphate and the xylulose 5-phosphate - this is a 4 carbon - another xylulose 5-phosphate, they react - this is 9 - to produce a fructose 6-phospahte, which is a 6-carbon and glyceraldehyde-3-phosphate, which is a 3-carbon.1541

Again, the carbons are conserved, 9 carbons.1560

That is 1/2 of the mirror image; well, the same thing happens on the other end.1564

Ribose 5-phosphate and xylulose 5-phosphate - a 5 and 5 carbon - produce sedoheptulose 7-phosphate, glyceraldehyde 3-phosphate, which is a 7-3 carbon- 7-carbon sugar, 3-carbon sugar.1570

They react in the transaldolase reaction to form a 6-carbon sugar and a 4-carbon sugar, fructose 6-phosphate and erythrose 4-phosphate.1580

The erythrose 4-phosphate - just like this other side, this is happening in tandem - reacts with xylulose 5-phosphate.1590

So, we have 4-carbon and a 5-carbon, which is 9 carbons.1597

They produce a 6-carbon fructose 6-phosphate, a 3-carbon glyceraldehyde-3-phosphate.1600

Now, the glyceraldehyde-3-phosphates that are produced - this is 3-carbon, this is 3-carbon - they form a 6-carbon, a fructose 6-phosphate, which is converted to glucose 6-phosphate, and now, we can start the oxidative phase again.1605

What happens is this; I will do this in red, 1, 2, 3, 4, 5 and 6.1618

6 pentose phosphates, which is 30 carbons, are converted through this pathway to 1, 2, 3, 4, 5, 6-carbon sugars.1633

Once again, 1, 2, 3, 4, 5, 6, 5-carbon sugars, which is 30 carbons, are converted to 5 6-carbon sugars, which is again 30 carbons.1657

The carbons are conserved, but we changed them to fructose 6-phospahate, so that fructose 6-phosphate can be converted to glucose 6-phosphate and start this oxidative phase of the cycle again, in order to produce the NADPH - this is absolutely amazing - in application of the transketolase reaction once, the transaldolase reaction once and the transketolase reaction a second time to actually do this.1667

This is the non-oxidative phase of the pentose phosphate pathway.1691

It is absolutely beautiful; you will see something like this in your book.1695

Either use this one or the illustration in your book to make sure you understand.1699

What is happening is this and this are coming together to form this and this; this and this are coming together to form this and this.1703

This and this coming together to form this and this, and so on.1708

5 and 5 is 10; 3 and 7 is 10.1712

4 and 6 is 10; 5 and 4 is 9.1715

3 and 6 is 9; OK, the carbons are conserved.1718

They are just being shifted back and forth, transferred from one substrate to another.1722

Now, let's take a look at the individual reactions, the transketolase and the transaldolase.1726

Let's see.1732

OK, there we go; let's go back, and now, let's go back to black.1738

This is going to be the general transketolase reaction.1743

What you have is 1, 2, 3, and, of course, I will just put R1 here.1756

We have an OH; we have an H.1765

We have that, and we have another OH and an H2.1769

This is going to be added to some...let's see.1774

We have got...that is fine; we will just go ahead and do C.1783

We will do R2, aldehyde and that.1787

Notice, we have this ketose and TPP.1791

Yes, I do not think I am going to do the mechanism here.1800

OK, TPP, this is the transketolase, and what we end up with is COO, R1, H+, C, C, C, R2, OH, H.1803

This is there, and we get another OH and an H2.1828

This is the ketose donor.1832

This is the aldose acceptor; I will tell you what that means in just a minute.1839

OK, P, notice, TPP is required coenzyme, thiamine pyrophosphate, we saw it a little bit earlier.1846

OK, the enzyme transketolase, what it does is it catalyzes the transfer of a 2-carbon fragment from a ketose to an aldose, in other words, from a ketone sugar to an aldehyde sugar.1854

Here is our 2-carbon fragment.1892

This is our ketose; this is our aldose.1896

We are moving this over to here; notice, this moves.1899

This molecule becomes that molecule, and this aldose accepts, that is why aldose acceptor.1905

It accepts the 2-carbon fragment; the ketose donates that 2-carbon fragment.1910

Nothing changes; here is that 2-carbon fragment.1914

This molecule has turned into this molecule; this has given this 2-carbon fragment over to that.1917

That is what the transketolase enzyme does.1925

OK, now, let's go ahead and take a look at the first transketolase reaction.1929

Let me see; should I...yes, that is fine.1936

I guess I can do it on here; yes, I will go ahead and do it here.1940

Let's go ahead and go to blue, so 1, 2, 3, 4, 5; I have got 1, 2, 3, 4, 5.1945

OK, I have got OH; I have got O.1953

I am going to skip all of the hydrogens, OH, and I have got O; and I have got - excuse me - the phosphate.1958

This is our ketose; this is the general reaction.1967

Now, we are going to look at our specific reaction; this is our ketose.1971

Now, we have our aldose, which is going to be 1, 2, 3, 4, 5.1976

We have the xylulose 5-phosphate reacting with the ribose 5- phosphate.1981

That was the first reaction in the thing that we just drew in the top right or the top left.1986

This is an aldose; this is an aldehyde sugar.1991

We have OH, OH, OH, O and phosphate - those 2 - and it is going to go to...that is what is going to transfer over.1995

Let me go back to blue here, make sure I am blue.2012

OK, I have got C, C, C, form glyceraldehyde-3-phosphate, O phosphate plus our sedoheptulose 7-phosphate.2017

Let's go 1, 2, 3, 4, 5, 6, 7.2035

This is the fragment that moved over; that is the OH.2041

That is the carbonyl; this one, actually, is over here, and then, we have OH.2046

We have OH; we have OH, and we have our phosphate.2053

Here is what was moved; this went from here.2059

It was transferred over to this one; 5 carbons + 2 carbons gives us a 7-carbon.2064

Where do I write this?2073

This is our xylulose 5-phosphate; this is our ribose 5-phosphate.2077

This is our glyceraldehyde-3-phosphate, and this is our sedoheptulose 7-phosphate.2083

That is the first transketolase reaction; that is the first step of what we did when we saw it go like this.2091

These two switched; this gives it over to this to become this.2097

OK, now, let's go ahead and take a look at the transaldolase reaction.2102

Let me go back to blue here; OK, now, we have then, for the transaldolase reaction.2109

Alright, we have got our 7-carbon fragment, 1, 2, 3, 4, 5, 6, 7.2122

Let me go ahead and put know what, I need to make more room here, 1, 2, 3, 4, 5, 6, 72134

This is going to be OH; this is going to be a carbonyl.2153

This is going to be OH, and then, we have 1, 2, 3.2158

Yes, and then, we have our O phosphate.2162

This is our sedohep 7-phosphate; this is going to react, now, with the glyceraldehyde-3-phospahte, which is a 3-carbon, C, C, C, OH and O, P, phosphate.2170

What it is going to form is the following.2188

1, 2, 3 and then, we are going to be 3 and 6.2193

Yes, what is going to move...well, let me just draw it out first, and then, I will tell you what moves.2197

We have got 1, 2, 3, 4.2200

That is that, OH, OH, O, P, and then, plus our fructose 6-phosphate, which is going to be 6-carbon, 1, 2, 3, 4, 5 and 6.2206

I hope I have enough room here, OH, carbonyl, OH, OH, OH and O and P.2224

Here is what ended up moving over.2235

These 3 were moved over onto this to make this.2239

That is this right here; this is what the transaldolase reaction does.2248

It takes a 3-carbon fragment, pushes it onto the glycerldehyde-3-phosphate to create a 6-carbon fragment.2253

I have started off with a 7-carbon sugar, now, I am left with a 4-carbon sugar.2259

This became that; this became that upon transfer of this to that.2264

OK, now, let me go ahead and write the transaldolase reaction.2272

The transaldolase reaction condenses - right, when we are putting 2 molecules together, it is a condensation reaction - a 3-carbon fragment with the glyceraldehyde-3-phosphate to from our fructose 6-phospahte.2280

Here, we have fructose 6-phosphate; here, we have our erythrose 4-phosphate.2312

Here, we have glyceraldehyde-3-phosphate, and here, we have our sedoheptulose 7-phosphate- there we go.2320

Fructose 6-phospahte and a tetrose is a 4 carbon sugar- that is it.2332

7-3, 10, 4-6, 10, we wanted to produce a 6-carbon sugar, so that it can become glucose 6-phosphate.2343

OK, that is the transaldolase reaction; now, let's go ahead and look at the final transketolase reaction.2350

I think I will go ahead and do this one in black.2359

Now, we are going to react a xylulose 5-phosphate with the erythrose to actually form our next fructose.2363

Let's go ahead and do 1, 2, 3, 4, 5...we have got 1, 2, 3, 4, 5, and this is our ketose.2376

Remember, we are transferring a 2-carbon fragment.2385

This is OH, and this is O; and this is phosphate, and we are going to react this with our 4-carbon, 1, 2, 3, 4.2390

This is an aldose, OH, OH and our O, P.2398

OK, you know what, let me write these out.2407

This is xylulose 5-phosphate, and this is our erythrose 4-phospahte.2411

Let me go back to black, and what we are going to end up transferring, of course, is this onto this because that is what transketolase does.2419

It transfers a 2-carbon fragment from a ketose to an aldose- transketolase.2427

It transfers a 2-carbon fragment from a ketose to an aldose.2432

Here is our ketose; here is our aldose.2438

So, what we are going to end up forming, we are going to be left with 3-carbon fragment, which is going to be our glycerldehyde-3-phosphate.2442

OK, we produce an aldose, and we produce a ketose, OH.2449

This is O, and this is our phosphate.2455

This is glyceraldehyde-3-phosphate - now, let me go back - + fructose 6- phosphate, 1, 2, 3, 4, 5 and 6.2459

What we have is OH; we have that.2471

We have that; we have this, and we have this, and we have this.2475

Here we go; we are down to fructose 6-phosphate again.2483

Those 3 sequential reactions produce the fructose 6-phosphate.2488

The 3 sequential reactions on the mirror image produce that again.2494

Here, we had another 2-carbon fragment.2498

That ended up over here.2507

Another 2-carbon fragment is transferred from a ketose to an aldose.2512

If you know nothing else, recognize that.2531

A transketolase reaction takes a 2-carbon fragment from a ketose to an aldose.2534

It just moves it over- that is it.2538

And the final word on the pentose phosphate pathway, the enzymes, all these enzymes of the pentose phosphate pathway, they are cytosolic.2542

There you go; this closes out our discussion of glycolysis and gluconeogenesis and the pentose phosphate pathway.2559

Thank you so much for joining us here at; we will see you next time for a discussion of the citric acid cycle, bye-bye.2568