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Lecture Comments (8)

1 answer

Last reply by: Professor Hovasapian
Thu Apr 7, 2016 1:22 AM

Post by Jinhai Zhang on April 6, 2016

Since in general biology, we learned that some bacteria which dislikes O2 can use S2- as electron acceptor. And they are still called anaerobic?  

1 answer

Last reply by: Professor Hovasapian
Tue Aug 27, 2013 11:26 PM

Post by Eduardo Cesar Melo Barbosa on August 27, 2013

Professor Hovasapian, on your first slide for this lecture, you wrote that DHAP is converted into Glyceraldehyde-3- Phosphate and the enzyme used is triose phosphate kinase, but isn't triose phosphate isomerase? Or are both names used?

3 answers

Last reply by: Professor Hovasapian
Mon Jun 3, 2013 2:36 AM

Post by Gaston Dominguez on March 12, 2013

Professor Hovasapian, These are fantastic lectures. I have no doubt i'm going to get an A in my biochem exam because you've helped clear the concepts so well. Thank you!

Glycolysis IV

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
  • Feeder Pathways 0:42
    • Feeder Pathways Overview
    • Starch, Glycogen
    • Lactose
    • Galactose
    • Manose
    • Trehalose
    • Sucrose
    • Fructose
  • Fates of Pyruvate: Aerobic & Anaerobic Conditions 7:39
    • Aerobic Conditions & Pyruvate
    • Anaerobic Fates of Pyruvate
  • Fates of Pyruvate: Lactate Acid Fermentation 14:10
    • Lactate Acid Fermentation
  • Fates of Pyruvate: Ethanol Fermentation 19:01
    • Ethanol Fermentation Reaction
    • TPP: Thiamine Pyrophosphate (Functions and Structure)
    • Ethanol Fermentation Mechanism, Part 1
    • Ethanol Fermentation Mechanism, Part 2
    • Ethanol Fermentation Mechanism, Part 3
    • Ethanol Fermentation Mechanism, Part 4
    • Ethanol Fermentation Mechanism, Part 5
    • Ethanol Fermentation Mechanism, Part 6

Transcription: Glycolysis IV

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

In the last lesson, we finished off our discussion of the glycolytic pathway.0004

I am going to continue, and talk a little bit more about glycolysis; but what I am going to talk about in this lesson is the feeder pathways to glycolysis- basically all of the carbohydrates in the body, how do they enter the glycolytic pathway.0009

And then, after that, I am going to talk a little bit about the fates of pyruvate.0024

You remember pyruvate was the final molecule; the 2 molecules of pyruvate that glucose was converted into, I am going to talk a little bit about what it is that happens to them- lactic acid fermentation and ethanol fermentation.0027

Let's go ahead and get started.0040

OK, as you can see, this first page here, there is a lot going on.0044

What I have here are the feeder pathways, the pathways that other carbohydrates in the body take in order to get to glycolysis.0051

For example, when you eat a regular table sugar, sucrose, what does the body do to that?0060

How does it take that sucrose, and how does it metabolize it?0067

Does it go through the glycolytic pathway?0070

Does it take another pathway?0072

What is it that happens?0074

Your starch that you eat, glycogen, what happens to these things?0077

And that is what is going on here; I am just going to run through each individual pathway.0081

We are not going to go and get into a lot of detail, but I do want you to see where each thing goes, where it comes from.0085

I think it is really, really important; what I have here are - in boxes - the major sugars that we are concerned with.0092

I have the starch and the glycogen, fructose, sucrose.0099

Here is glucose, of course, the central position; well, it is not central here, but it is glycolysis, glucose, trehalose, lactose, galactose and mannose.0104

These are the primary carbohydrates for the body, and we want to see what happens to them.0113

Now, in red, you are going to see the enzymes that actually catalyze these particular transformations.0119

And, of course this, in blue, 1, 2, 3, I will talk about them in a little bit.0125

It is going to talk about the dietary pathway for a particular carbohydrate or the cellular, the tissue pathway for a particular carbohydrate; and, of course, there is the liver pathway.0130

That is going to be 1, 2, 3, and I will get to those when I actually talk about those individually.0141

I guess the best place to start is right in the middle; I am going to start with starch, and I am going to start with glycogen.0146

Look over here; we have the starch.0154

Let’s say you happen to eat some starchy food.0156

Well, the dietary pathway, what happens is the enzyme alpha-amylase starts to break down the starches.0160

You have alpha-amylase in your saliva that actually does the first phase of breakdown.0173

And then, of course, once it goes into your stomach, that enzyme is actually neutralized; and another alpha-amylase takes over in your intestine.0178

Eventually, what happens is it is all converted into glucose, and once it is glucose, then, it can start the glycolytic pathway.0189

Hexokinase + ATP goes to glucose 6-phosphate, and, of course, it will just continue on and get to the end of the glycolytic pathway.0197

Starch passes through glucose and into the regular glycolytic pathway.0207

Now, as far as glycogen is concerned, you remember when we talked about the carbohydrate glycogen?0213

Your body stores glucose units as this polymer called glycogen, and if for any reason, if your blood sugar drops too low, the body mobilizes this glycogen.0218

Remember, it is highly, highly branched; it goes and it breaks off all those glucose units, and it sends them into the glycolytic pathway.0229

What happens, as far as this cellular tissue pathway for glucose to enter the glycolytic pathway, a glycogen, an inorganic phosphate comes in; and this enzyme called phosphorylase actually breaks off individual monomers from the ends of the glycogen molecule.0237

It is converted to glucose 1-phosphate; well, glucose 1-phosphate is then converted to glucose 6-phosphate by the enzyme phosphoglucomutase, and now that it is glucose 6-phosphate, it can just continue on into the glycolytic cycle.0257

You see how that happens, dietary and then cellular tissue pathway.0271

OK, let’s talk about galactose; actually, let’s talk about lactose.0277

Lactose is that disaccharide, which is glucose and galactose.0283

What happens is lactase ends up breaking up that disaccharide, and the molecule of glucose goes on normally into the glycolytic pathway.0289

The galactose monomer, it goes through a couple of conversions.0299

It is turned into uridine diphosphate galactose, uridine diphosphate glucose, and then that is converted to glucose 1-phosphate; and then the glucose 1-phosphate, as we see here in the center, is converted to glucose 6-phosphate, and then it can go on and continue in the glycolytic pathway.0303

OK, now, let me see; we have taken care of lactose.0322

We have the glucose; we have starch and glycogen.0325

In the case of mannose, mannose is converted into mannose 6-phosphate, and then it is converted to fructose 6-phosphate.0328

It is a little further down the glycolytic pathway, but that is how it enters glycolysis, which is right here, this central pathway right here.0337

Trehalose, the enzyme trehalase breaks it up into glucose monomers, and glucose enters the glycolytic pathway, as usual.0346

The sucrose, the table sugar that you eat, it is a disaccharide of glucose and fructose.0357

Well, the sucrase breaks it up; the glucose enters the cycle normally.0362

Fructose, it has 2 possible fates; one possible fate is when fructose, by the action of the hexokinase + adenosine triphosphate, is converted to glucose 6-phosphate.0368

It can enter the glycolytic pathway that way at fructose 6-phosphate or in the liver.0380

What the liver does is this fructokinase converts fructose to fructose 1-phosphate, and then, fructose 1-phosphate aldolase takes the fructose and breaks it up into 2 molecules- glyceraldehyde and dihydroxyacetone phosphate.0387

Now, the glyceraldehyde is converted by triose kinase to glyceraldehyde-3-phospahate, and now, it can enter the second phase of glycolysis.0401

It comes in down halfway through the glycolytic pathway, and the dihydroxyacetone phosphate, as we know, is converted by the triosephosphate kinase, again, into glyceraldehyde-3-phospahte; and it enters the glycolytic pathway that way- that is it.0410

I am not sure about the extent to which your particular teacher is necessarily going to have you memorize this entire thing.0428

Perhaps he or she will want you to know, at the very least, maybe the dietary pathway for starch and the cellular tissue pathway for glycogen, but I did want you to see it.0434

All of the carbohydrates that you eat, somehow or other, they all end up in the glycolytic pathway- that is it, that is what is going on here.0445

OK, great, now, let’s talk about the fates of the pyruvate that is formed during glycolysis.0457

We take this glucose molecule; we break it up.0465

We subject it to 10 steps, and we end up with these 2 molecules of pyruvate.0469

Well, what happens to pyruvate?0474

Well, we know what happens to pyruvate aerobically; well, I am going to tell you right now.0477

Let me see; let me go do this in blue, I think.0484

When there is plenty of oxygen to go around, under aerobic conditions, the pyruvate that is formed, pyruvate continues on and is oxidized to a molecule called acetyl coenzyme A; and it enters the citric acid cycle - the Krebs cycle - eventually becoming CO2 and H2O once it enters the electron transport chain.0496

Now, the NADH from glycolysis, remember we actually formed 2 molecules of NADH- those 4 electrons?0549

It passes its electrons to the electron transport chain becoming NAD+ again; this is important.0565

Under aerobic conditions, when there is enough oxygen available in the body, when the cells have enough oxygen to work with, the NADH that is formed in glycolysis can actually pass its electrons to that oxygen because that is what the electron transport chain is.0581

It takes those high energy electrons from these coenzymes, and it passes them along a chain and ultimately its oxygen that ends up taking those electrons.0595

It ends up being reduced; that is what oxygen does.0604

Oxygen oxidizes things; it, itself, gets reduced.0606

When there is plenty of oxygen, the NADH can actually turn back into NAD+ when it gives up its hydride, when it gives up its electrons.0610

That is very important because we need to regenerate the NAD+ so that glycolysis can continue with step 5.0618

We need that NAD+ to make the…I am sorry; it is step 6.0626

We need that NAD+ because we need to oxidize that glyceraldehyde-3-phosphate.0631

If we ran out of NAD, if NADH cannot give up its electrons, glycolysis comes to a halt because there is not going to be enough NAD+ to keep glycolysis going.0637

That is what is going on; under aerobic conditions, when there is plenty of oxygen, everything is fine.0648

NADH goes to NAD+ and glycolysis can continue; now, what happens under anaerobic conditions?0653

Well, under anaerobic conditions or hypoxic conditions - low oxygen or no oxygen - what is the NADH going to do?0660

It has to give up its electrons somehow because it has to turn back into NAD+, so that it can go back and actually continue doing its work on glycolysis.0667

Well, the body has figured out a way to do that, now, the anaerobic fates of pyruvate.0675

Alright, under hypoxic conditions - well, I am going to say hypoxic anaerobic, so either low oxygen or no oxygen, anaerobic conditions - NADH cannot pass its electrons to oxygen because there is no oxygen.0692

It cannot pass its electrons to O2 to become NAD+ again, and a depletion of NAD+ stops glycolysis.0728

We do not want that to happen; when glycolysis stops, very, very bad things happen.0755

Now, NAD+ must be regenerated somehow.0762

OK, now, cells do this by transferring their electrons - the NADH - to pyruvate, directly to pyruvate, to form lactate and or ethanol.0779

In other words, we are reducing pyruvate; we are passing electrons to pyruvate in the form of hydride, in the form of hydrogen, so transferring electrons to pyruvate.0819

Alright, we are reducing pyruvate.0831

OK, the first of these methods is lactic acid fermentation.0838

Let me go ahead and actually start this on the next page; I think I am just going to do this in red for a change of pace.0845

The first fate of pyruvate is lactic acid fermentation.0852

This is what your body does- lactic acid fermentation.0858

When you are heavily exercising, heavily exercising, heavily exercising, your body starts to crave oxygen.0862

You start to run out of oxygen; when there is not enough oxygen for the body for regular aerobic cellular respiration to continue, the cell has to be able to regenerate that NAD+ to keep glycolysis going.0868

In order to do that, it takes the pyruvate that is formed in glycolysis; and it converts it to lactate.0882

That is the sore that you feel when your muscles start to give up, when your muscles start to fail.0889

That soreness that you feel, that is the lactic acid, the lactate building up in your muscles.0895

OK, lactic acid fermentation, here is the chemistry.0900

This is CH3; we have our pyruvate molecule, and it is going to go like this.0908

NADH + H+ in and NAD+ out.0921

NADH gives its electrons to pyruvate and converts it to lactate.0927

C, C, C, this stays; H3, this becomes OH, and H, O-.0933

This is pyruvate; this is lactate.0945

This is actually L-lactate; I am going to draw it this way.0953

I am going to put the OH over here; it is L-lactate.0959

OK, we have taken this carbonyl, this double bond, and we have added 2 hydrogens across the double bond.0963

OK, the carbonyl is here; we have added a hydrogen to the oxygen, a hydrogen to the carbon.0972

We have reduced the pyruvate; in the process of reducing it, we have actually recovered NAD+.0977

Now, glycolysis can continue; now, the body can do something else with the lactate.0983

That is what is going on; now, the ΔG for this reaction - I will go ahead and put that there - it is pretty high actually.0988

ΔG is -25.1kJ/mol, or I should say low, depending on the number is high, but it is negative.0997

So, this is a highly exergonic reaction; OK, that is it.1005

Glucose is converted into pyruvate via glycolysis.1016

Now, pyruvate is converted into - let me make this arrow a little bit longer – lactate, and we have NAD+ in NADH.1022

That is the cycle; glucose - step 6, NAD+ in, NADH out - goes to pyruvate.1045

This is glycolysis, what we just did.1053

Under anaerobic conditions, NADH gives its electrons, gives its hydrogens to pyruvate, reducing it to lactate; and then, the process recovering NAD+, so that the glycolytic cycle can continue, otherwise, the glycolytic cycle will stop and bad things happen.1057

OK, now, I probably should have done that…that is OK.1076

All right, some cells like erythrocytes, they do not have mitochondria.1082

They have no mitochondria, so they cannot metabolize under aerobic conditions because that is where oxidative phosphorylation takes place- in the mitochondria.1096

They have no mitochondria, so they do this lactate acid fermentation even under aerobic conditions.1109

That is how they regenerate the NAD+.1121

They do this even under aerobic conditions all the time.1125

OK, that is lactic acid fermentation; now, we will talk about the other fate of pyruvate- ethanol fermentation.1138

I think I will go back to black for this one just for a change of pace.1145

Ethanol fermentation, OK, yeast ferments glucose to ethanol.1150

The pyruvate, that glycolysis, so yeast takes glucose, goes through glycolysis, converts it to pyruvate; but now, instead of the lactic acid, instead of lactate, it actually produces ethanol.1171

That is how we get our ethanol; that is how we get our drinking alcohol.1184

OK, what we have here is C, C and C.1188

We have that; we have that, and we have this.1194

This is a 2 step process actually.1199

I will go ahead and do this.1202

CO2 is released, and the enzyme is pyruvate decarboxylase; and it also requires a coenzyme called TPP.1207

TPP is a thiamine pyrophosphate.1224

It is the coenzyme that is derived from vitamin B1, which is thiamine, and it also requires magnesium ion.1228

This is the enzyme; it requires this, and it requires that.1237

Let me go ahead and write what TPP is, and I will talk about it in a minute.1241

I will do this mechanism; it is very, very important.1246

TPP, profoundly important coenzyme, as you know, vitamin deficiency, bad things start to happen.1248

TPP is thiamine pyrophosphate, and do not worry.1256

We will be drawing all of this out in just a minute; the first thing that it does is it converts it into C, CH3.1265

It converts it into acetaldehyde; it takes the pyruvate, and it actually knocks off this carboxyl group.1275

It goes away a CO2; this COO-, this carboxyl group goes away a CO2 leaving just the acetaldehyde, and then, the second step.1282

This is where the NADH comes in.1298

It gives its electrons over; it regenerates NAD+, and it produces our famous ethanol.1302

There is H there; there is H there, and there is OH here.1313

Here we go, and this is alcohol dehydrogenase.1319

OK, there you go.1329

Alright, now, let's see what else do we want to say.1335

The ethanol fermentation pyruvate first step is catalyzed by pyruvate decarboxylase to form acetaldehyde, and acetaldehyde is converted into ethanol.1340

Let me go ahead and write those out; this is ethanol.1350

This is acetaldehyde, and this is pyruvate.1355

OK, let me go, now, thiamine pyrophosphate.1367

OK, let's go ahead and talk about the thiamine pyrophosphate, and let's talk about this first reaction- the pyruvate decarboxylase reaction.1373

OK, let me do this in blue.1383

I wonder if I can draw the structure; well, that is OK.1388

TPP, as we said, it is a coenzyme that is derived from thiamine, which is vitamin B1.1392

You ingest the thiamine - the vitamin B1 - and the body converts it into its active form, which is a thiamine pyrophosphate.1411

It adds a pyrophosphate onto this structure that I am going to draw in a second, and then, it goes ahead and does what it does with this enzyme pyruvate decarboxylase.1418

OK, let's go ahead and work on some structures here.1428

Let me go ahead and do this in blue; I think I will do this structure in black actually.1432

I have got a little 6-membered ring here.1438

I have got a nitrogen up here; I have got a nitrogen up here.1443

This is aromatic; I have NH2.1447

I have got CH3; I have got a CH2 group here.1452

I have got a C; I have got a...aha, here is where we get really, really interesting.1459

There, this is carbon; this is sulfur, and I will go ahead and put that there.1468

I will go ahead and do that; Do not worry.1475

I do not think you are going to have to memorize the structure for thiamine pyrophosphate.1479

It is the mechanism that is important.1484

This is going to be a plus charge on there because we have 4 things attached.1487

We have, of course, a hydrogen there, the sulfur; and let me see what else do we have.1492

Aha, we have CH2; we have CH2.1496

Now, we have our pyrophosphate; we have O, P, O, P, O- there we go.1501

Let me go ahead and make sure all the oxygen and charges are there.1509

So, this is TPP; this is thiamine pyrophosphate.1512

What is important is this thing right here, this ring.1515

OK, this is called a thiazolium ring.1521

This is this thiazolium ring; all of the chemistry takes place at that carbon.1526

OK, and you are accustomed to this already; just like when we did NAD+, all of the chemistry takes place at one place.1532

The rest of the molecule is just used for...basically, the enzyme uses it to hold it, to bind it, to make sure is a handle.1539

All of the chemistry usually takes place in one location over and over again; it is not multiple locations.1550

OK, now, the H, this H right here, the H on the...let me number these by the way.1556

This is no. 1, no. 2, no. 3, no. 4 and no. 5 on this thiazolium ring.1566

The H on the no. 2 carbon of the ring - well, there are 2 rings, so the thiazolium ring - of the thiazolium ring is acidic, and when it is lost, when it is deprotonated, it creates a nucleophilic carbanion.1574

It creates a nucleophile, a nucleophilic carbanion, a carbon that is actually carrying a little bit of a…well, a negative charge.1610

As you will see in a minute, that negative charge is stabilized, but it is carrying a negative charge, so it is highly nucleophilic, OK, carbanion that adds to carbonyls, OK, that attacks that carbonyl carbon.1622

OK, now, let's go ahead and see if we can make sense of what is going on.1640

Let's go ahead and do this mechanism; I am going to try and do the mechanism on one page.1645

Hopefully, I will be able to do it; if not, well, that is OK.1650

We will go to 2 pages; let me see if I should go here first, and then here.1655

No, that is OK; I want to have it all on 1 page.1664

Let me go ahead and do this in black, and then, I will probably do the electron moving in red.1668

Alright, we are going to concentrate just on that ring.1673

Let me go ahead and draw this first; I have got N.1679

I have got C, S here.1686

I have got R1, R2, and I have got a double bond, double bond there.1690

This is a positive charge; I have this H, and I have CH3.1696

OK, everything starts when…we said this proton here, on this no. 2 carbon of the thiazolium ring, is acidic.1701

I will do it this way; when it is lost, what you end up with is this carbanion.1714

We have got R1; I have got N.1719

I am going to draw this a little bit better: N, S here.1725

So, there is now, an electron there, and it is negatively charged; so this is plus.1734

This is CH3; this is R2.1741

Have I forgotten anything else?1743

No, I think everything is good, and now, we have our pyruvate.1745

Let me go ahead and draw my pyruvate molecule.1750

I will go ahead and draw the pyruvate in blue; this is C, C and C.1755

I have got O-; there is that carbonyl, right?1760

Pyruvate, and this is CH3; what it is going to do is it is going to attack the carbonyl like we said.1766

So, I will do this in blue; it attacks the carbonyl, and this goes and actually grabs a hydrogen ion, and it becomes the following.1771

Let me go just here because I am going to need some room.1783

Let me see; I have got...well, you know what, it is OK.1790

Let me go back to black; I have got R1, boom.1796

This is N; this is C, S there, there, there.1800

Now, I have got myself a C; I have got a COO-.1808

Here, I, sorry; I decided to do this in blue.1816

I think it is probably best if I do this in blue, so let me go ahead and just make sure that everything else is on here.1820

I have go a double bond, a positive charge; let me go ahead and write my R2 here, my CH3 here.1827

Now, let me go to blue, and let me do...and I am also going to move my arrow.1834

I am going to put my arrow over here, so I have got some room.1844

Now, I am in blue, and now, I have got C; I have got C.1848

I have got C; this is H3.1853

This is now OH, and this is O and O-.1855

OK, so far so good; this carbanion, right?1863

Nucleophile, it attacks the carbonyl; the carbonyl, this goes ahead and grabs an H from the surrounding medium to become an alcohol group right here.1867

Now, what happens is the following.1876

Oops, let me do this electron movement in red.1879

That bounces down there; this bond breaks.1885

This bond breaks; this goes off as CO2.1891

This bond breaks and forms a double bond here, and it forces these electrons onto the nitrogen, which is carrying a positive charge.1894

So, it actually clenches the positive charge on that.1901

What we end up with is the following; let me go back to black.1907

CO2 is gone, and now, we have...let me see, boom, boom.1915

I will do N; I will do S, that.1927

No, that is not there anymore because these electrons moved.1932

OK, this is R, and now, we have this double bond, which I will do in blue in just a minute.1936

This is there; let me see.1945

This is R2; this is CH3.1948

Now, I will go back to blue; now, we have a double bond.1950

We have a C, and we have an OH, right?1954

And we have a CH3, so that is what is there.1959

Now, this actually undergoes a little bit of a resonance.1964

These electrons were pushed onto nitrogen.1970

Now, what happens is these electrons are going to go like that, and they are going to go back up to the carbon here, so, we have this resonance structure.1975

Let me draw this in black; we have got...let me see: nitrogen, sulfur, that, that, that.1987

Now, we have a single bond; we have a C.1998

No, I said I was going to do that in blue; I keep forgetting I do not want to do that in black.2003

This is R2; this is R1.2007

This is CH3; now, let me go to blue.2010

Now, I have this carbon with 2 electrons on it and that.2014

It has a hydroxy group; it has a CH3 group, and this, right here, is your resonance stabilization, right here.2018

This double arrow, single double arrow- resonance.2026

Once we form this, these electrons can bounce back up here and actually stabilize.2030

This negative charge on the carbon is actually stabilized because it can redistribute itself and actually come, and the nitrogen is actually sharing that negative charge.2036

That is why this works; OK, now, what we have is the following.2045

Once we have this; let me see.2053

Where are we?2057

Let me make sure hydroxy, FOTEP.2059

I wonder if I should go ahead and do it that way; OK, that is fine.2070

I will go ahead and go here; I know there is a lot going on.2074

Do not worry; we will make sense of everything here.2082

This is N, and this is S.2084

We have that; OK, and then we have - let's see - our R1.2095

We have R2; we have that.2106

We have our CH3; OK, and now, I have got the C, and I have got the O, and I will write the OH that way.2108

I have got the CH3; OK, and let's see.2120

I am going to draw a little bit of an H+ here.2125

Because this carbanion, basically, what I am going to do, it takes a hydrogen ion from the medium to become that.2129

OK, it forms this; there is resonance stabilization here.2138

A hydrogen ion attaches to here; now, you have this thing attached.2145

OK, now, here is what happens.2150

These electrons move here.2154

These electrons go and grab a hydrogen ion.2159

OK, and what you are left with is where I definitely should have gone the other page.2165

Now, what happens is, what ends up leaving is our acetaldehyde.2173

I will do this in blue; once this grabs this H, this bond over here to the thiazolium ring, actually breaks; and what you end up with is the following molecule.2183

You end up with C, C, O, H, CH3.2196

That is this thing; this COO, that is this.2203

This H is this H, and this CH3 is that CH3.2207

This is our acetaldehyde; that is what the enzyme ends up actually releasing, and then once this happens, you are back to the form that you were before.2211

OK, this goes and grabs this H+ that becomes this H.2224

This comes here; this H is this H.2232

This O is this O; this carbon is this carbon, and this CH3 is this CH3.2235

OK, this H that this bond grabs, it recovers the thiazolium that is protonated, and then, it can begin the cycle again.2240

Let's run through this one more time; we have the thiazolium ring on the TPP.2249

We lose a proton, and we form this nucleophilic carbanion.2254

This nucleophilic carbanion attacks the pyruvate, the alpha-keto on the pyruvate - OK - and when it does that, it is now, attached to it covalently.2257

Now, of course, it is primed because now, there is a place for electrons to actually go.2269

This is called an electron sync; this positive charge on the nitrogen pulls electrons toward it, and when it does that, it makes it very, very easy for this negative charge on the oxygen to actually form a second double bond with a carbon to form CO2.2276

CO2 is a very stable molecule; this happens a lot in biochemistry.2291

Decarboxylation forming CO2 happens very, very easily.2295

It forms the CO2; this CO2, this bond, breaks, so CO2 is lost in this reaction, and what you end up with is this structure right here.2301

Well, these electrons push back up; they form the carbanion again.2311

This is a resonance structure; this negative charge is actually shared between the carbon and the nitrogen.2316

If you want, you can think of this actually grabbing another hydrogen from here to become this thing.2324

OK, a hydrogen comes in and becomes this, and now, these electrons go over here to form a double bond.2330

These electrons are pushed, and they grab a hydrogen ion to recover the thiazolium, which is protonated, and, of course, the acetaldehyde goes away.2342

Yes, this is the pyruvate decarboxylase reaction; this is the first part of the ethanol fermentation process.2357

We will not go ahead and talk about the second part that is just a dehydrogenase reaction.2363

We may talk about it later, we may not, but I definitely wanted you to see this.2368

This is our first introduction to a major coenzyme- the thiamine pyrophosphate.2372

OK, we are going to go ahead and close off our discussion of this phase of glycolysis for today.2377

Thank you so much for joining us at; we will see you next time, bye-bye.2383