Biochemistry Glycolysis IV

Hello and welcome back to Educator.com, 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