Biochemistry Fatty Acid Catabolism I

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

We just finished discussing the citric acid cycle and the breakdown of carbohydrates.0004

Now, we are going to start discussing the breakdown of the fats, of the lipids.0010

We are going to be discussing fatty acid catabolism, so let's just jump right on in.0016

OK, the oxidation of fatty acids to acetyl-CoA, you notice that fatty acids are also catabolized to acetyl-CoA just like the pyruvate was that entered the citric acid cycle.0022

It is broken down into acetyl-CoA, and these acetyl-CoAs actually enter the citric acid cycle.0050

The oxidation of fatty acids to acetyl-CoA is a major source of energy for organisms.0055

In fact, it is probably the major source.0070

OK, complete oxidation of fatty acids - I will just go ahead and abbreviate it as FAs - releases about 40kJ/g.0075

For a gram of fat, it releases about 40kJ of heat - that is a lot of energy - more than twice that released by an equal mass of either protein or carbohydrate.0101

OK, now, vertebrate cells obtain the fatty acids that they need - obtain FAs - for catabolism from 3 sources.0137

In other words, the fatty acids that the cell needs in order to burn to get energy, they come from 3 different sources.0168

There are 3 ways that the fatty acids get to the cells, in other words.0176

One of them is diet.0180

What you eat, it finds its way directly to the cells.0184

Two: is the fats that are stored in cells already.0189

Sometimes, it will just mobilize those fats that are already there and use them for breakdown, and the third way is the fats that are synthesized by one organ then, transported to the tissues that need them.0199

OK, let's talk about diet first.0237

No. 1: diet, dietary fats are absorbed by the small intestine.0242

Now, we will just go ahead and list how it actually does this.0268

In your biochemistry books, there is probably some illustration of some sort that talks about how it goes from the intestine, how it gets here.0272

You are more than welcome to look at it; in fact, I encourage you to take a look at it.0279

What I am going to do here, I am not really going to be using any diagrams.0283

I am just going to be listing the process; it is important to know what happens, but we want to concentrate on the second one, the fats that are stored in cells, how those fats are actually mobilized, and that is when we are going to be looking at diagrams and pictures and following reactions very carefully.0286

Dietary fats are absorbed by the small intestines, and it happens as follows.0301

Bile salts emulsify dietary triacylglycerols.0309

Triacylglycerol, just fat, the fat that you ingest, you are ingesting it as triacylglycerols.0325

The bile salts that the body secretes actually emulsify them.0332

OK, now, 2: lipase enzymes, they break down these triacylglycerols - which I will abbreviate as TAGs - to monoacylglycerols, diacylglycerols, free fatty acids - free fatty acid, those are just those long chain carboxylic acids, we just call them free fatty acids - and glycerol.0336

OK, now, no. 3: these breakdown products, they are what is actually absorbed by the intestine.0380

These breakdown products are absorbed by the intestine.0395

OK, now, no. 4: these breakdown products - well, I will write that again - are reconverted back to the triacylglycerols, then, these TAGs are combined with cholesterol, cholesteryl esters and - excuse me - specific proteins into particles called chylomicrons; and certainly, there is a picture of a chylomicron in your book.0408

These triacylglycerols that are reformed, they are combined with cholesterol and the esters of cholesterol.0492

Cholesteryl ester is actually an ester of cholesterol, and these specific proteins, they are, sort of, combined into this particle.0502

The proteins are on, sort of, the outside of it, and these particles are called chylomicrons.0510

OK, I want to say a word about these specific proteins.0517

Now, these specific proteins are called apolipoproteins, and they are responsible for actually transporting the lipids through the bloodstream because fats, as you know, are not water soluble.0521

So, they cannot just float around in the blood or the lymph.0551

They have to be solubilized - if you will - by these proteins.0557

They attach to proteins, and the proteins carry them to where they need to be.0561

These specific proteins are called apolipoproteins, and they are responsible for transporting insoluble lipids through the bloodstream and lymphatic system.0565

OK, one of these proteins, APOC2 is an example.0604

That is the name of it- APOC2.0613

OK, excuse me; let's see what happens next.0617

Alright, no. 5: these chylomicrons are transported via blood and lymph to the various tissues.0623

OK, now, no. 6: in the capillaries, when they reach the end of their respective journeys, the enzyme lipoprotein lipase - or lipase if you want, I say lipase - recognizes the APOC2 that I mentioned earlier - one of those lipoproteins, it recognizes the APOC2 - and starts breaking down, hydrolyzing - yes, that is fine - the triacylglycerols, the TAGs to free fatty acids and glycerol- that is it.0653

And then, the fatty acids, at this point, that is when they enter the cells.0725

Fatty acids enter cells of the tissues, at this point.0732

OK, now, no. 8: OK, the fatty acids, once they are in the cell, the FAs are either oxidized to release the energy, or they are re-esterified to store as triacylglycerols.0745

They are reesterified - in other words, the free fatty acids and the glycerol are put back together in order to store them for future use - to the triacylglycerols, which they were originally, for storage, and that is it.0786

OK, now, let's go ahead and talk about...this is how the fats that we ingest in our diet, how they get to the particular cells.0807

Now, the other way that free fatty acids, that triacylglycerols, the fats, are actually mobilized in order to be used are there are the fat that is actually stored in the cells, how do we use that fat?0818

Well, now, we will talk about that.0832

No, 2: let's go ahead and do this one in blue.0836

No. 2: fats that are already stored in adipocytes - adipocytes, however you want to pronounce it - fat cells.0840

They are cells where fat is stored as just as fat, just straight triacylglycerols.0854

OK, now, excuse me.0863

Fats stored in adipocytes are mobilized by the hormones glucagon and epinephrine, which are secreted by the body in response to low blood glucose levels.0868

If your blood glucose rises too high, at that point, that is when the body pumps in insulin.0925

That is the hormone that starts to break that down to regulate high blood glucose.0929

If your blood sugar drops too low, glucagon and epinephrine are secreted, in order to, in this particular case, mobilize free fatty acids.0935

That is all that is going on here; OK, now, let's run through the process.0944

Now, in this particular case, what I am going to do is I am going to list the process 1, 2, 3, 4, 5, and then, we will actually take a look at a diagram of a cell, so that we can see it visually as well.0948

I would like you to have it written out, and I would like you to also have it visually, so we will go through this twice.0958

If there is anything in the written part that you do not necessarily understand, we will go through it again, and we will see it.0965

When we see it pictorially, it should make sense what is happening in the cell.0969

OK, no. 1: now, the hormone, it stimulates the enzyme - well, you do not have to - it stimulates adenylyl cyclase via a G protein to produce cyclic adenosine monophosphate.0974

There is this enzyme that is in the outer membrane, and it stimulates this adenylyl cyclase via something called a G protein to produce this thing called cyclic adenosine monophosphate.1023

OK, now, 2: cyclic adenosine monophosphate - cAMP - it is a secondary messenger inside the cell, which induces a protein called cAMP-dependent protein kinase to phosphorylate 2 things, right?1037

A kinase does 1 thing, so this cyclic adenosine monophosphate is a secondary messenger.1085

It stimulates this cAMP-dependent protein kinase to phosphorylate 2 things.1090

OK, the first thing that it phosphorylates is something called a hormone-sensitive lipase, and we know what lipases do.1108

Lipases break down lipids, triacylglycerols.1118

OK, it also phosphorylates something called a perilipin.1122

It also phosphorylates something called the perilipins, which are proteins on the outer surface of the lipid droplets that is in the cell.1128

The fat that is in the cell that is, sort of, aggregated on the surface of that fat collection are these things called perilipins on the outer surface of the lipid droplets in the adipocytes.1157

OK, no. 3: OK, hormone-sensitive lipase, once it phosphorylates this thing called hormone-sensitive lipase, the hormone-sensitive lipase, which is floating around in the cytosol, it actually moves around in the droplet, moves toward the lipid droplet, and it starts hyrdolyzing the triacylglycerols.1182

It starts breaking them down - the triacylglycerols - into free fatty acids and glycerol.1230

It basically takes those triacylglycerols and just cuts them off to release the free fatty acids and to release the glycerol.1245

OK, now, let's go to no. 4.1253

Now, the free fatty acids, they pass from the adipocyte into the blood stream.1259

The free fatty acids pass from the fat cell - from the adipocyte - to the bloodstream where they combine with, where they bind to a protein called serum albumin - bind to serum albumin protein - which then, carries them to the various tissues - cells - where they are needed.1270

We have these cells, adipocytes, it is where the fat is stored.1337

If that fat is needed for some purpose, let's say you have not eaten all day, but the body needs some energy, and it wants to actually burn some fat, it is going to burn some of the fat that you have stored.1342

It is going to mobilize this fat, and once it breaks it down into the free fatty acids, it sends those free fatty acids outside of the fat cell into the blood stream, so that the blood stream, the serum albumin, can take those bound-free fatty acids to the cells, to the muscle tissue, in order to mobilize it and extract the energy.1355

That is all that is happening there; now, that takes care of the free fatty acids.1375

What about the glycerol?1379

OK, the glycerol left behind, and then, we have to account for everything.1380

We are not just going to leave molecules just hanging around and say nothing about them.1389

The glycerol that is left behind is phosphorylated by - you guessed it - glycerol kinase because that is what kinases do - they phosphorylate - to glycerol 3-phosphate, not glyceraldehyde-3-phosphate- glycerol 3-phosphate.1393

It is this; glycerol goes to glycerol 3-phosphate, which is then, converted into dihydroxyacetone phosphate, which is then, converted to glyceraldehyde-3-phoaphate; and you can see where we are going.1428

Glyceraldehyde-3-phosphate continues on with glycolysis.1452

It has ways of handling what is left over, and that is all that happens.1459

OK, now, what we want to do, now, that we have actually written this out, let's go ahead and take a look at a picture of this, and see what it looks like; so let's see here.1463

I think I have got it here; yes, here we go.1473

OK, do not worry; there is a lot going on here, but it just seems that way.1477

Once we actually start talking about what is going on, you are going to realize that everything is really, really, very, very clear.1482

Let's identify some things here; this is the fat cell.1488

OK, and this thing right here, this is the lipid droplet.1493

It is just a collection, an aggregate of fat- triglycerols.1498

This is the cell; now, on the cell surface, there is a receptor for the particular hormone.1504

In this particular case, whether it is glucagon or epinephrine, it does not really matter.1507

The hormone attaches to the receptor; it has been secreted by the body.1512

It attaches to the receptor; this receptor, now, via something called the G protein - do not worry about it if you do not know what the G protein is.1516

For our purposes, it is not necessarily...for those of you that have taken cell biology and have studied biosignaling, you know what a G protein is.1523

For those of you that do not, you will, so it is not a problem.1530

Just understand that through G protein, it stimulates this enzyme called adenylyl cyclase to use ATP to create something called cyclic adenosine monophosphate.1534

That is the secondary messenger; that is created inside the cell.1545

Well, this cyclic adenosine monophosphate travels to this thing, the cAMP dependent kinase, which we call PKA.1550

It stimulates the PKA to phosphorylate 2 things.1560

This enzyme ends up phosphorylating this thing called hormone-sensitive lipase, and it also ends up phosphorylating the perilipins.1565

Remember, we said the perilipins are these proteins that are on the surface of the droplet.1572

That is where the perilipins are; what they do is they actually restrict access to this lipid droplet, but once they are phosphorylated, they actually open up.1577

They open up, and they allow things to have access to the lipid droplet.1587

In this particular case, the thing that is going to have access to the lipid droplet is the hormone-sensitive lipase.1591

Once that is phosphorylated, it starts to move towards the lipid droplet; it makes its way in between the perilipin proteins, and then it just starts to hydrolyze it, starts to break them up.1597

Now it is in contact with the lipid droplet, here is just the triacylglycerol - right, a fat - it hydrolyzes them, and it releases into the cytosol the free fatty acids; and here is the glycerol left behind.1606

The free fatty acids, they enter the bloodstream and they bind, and they do whatever it is that they do.1623

They bind non-covalently to the serum albumin protein, and the serum albumin protein carries these free fatty acids to the cells that actually need the energy; and we already said what happens to glycerol.1629

Glycerol is phosphorylated, and it ultimately enters the glycolytic cycle.1641

That is all that is happening here, stimulation via hormone production of cyclic adenosine monophosphate.1646

Cyclic adenosine monophosphate stimulates PKA to phosphorylate hormone-sensitive lipase and the perilipins.1654

Hormone-sensitive lipase ends up hyrdolyzing the triacylglycerols to free fatty acids and glycerol.1661

Free fatty acids are released into the bloodstream to go where they needed.1666

Glycerol is rephosphorylated into - well, not re...it is phosphorylated - and it enters the glycolytic cycle- that is it.1670

The glycolytic pathway- sorry.1679

Pathway cycle it is the citric acid that is actually a cycle; glycolysis is a serial pathway- that is it.1681

OK, now, let's actually talk about fatty acid oxidation, itself.1689

Alright, now, let's talk about fatty acid oxidation.1698

I think I am going to go back to...I don't know.1703

Should I do red or should I do...no, I think I will go back to black.1705

We have got fatty acid oxidation.1710

OK, yes, OK, now, the fatty acids from the bloodstream, now, the serum albumin is carrying these free fatty acids to the cells that are going to need them - need the free fatty acids - in order to break them down and recover their energy.1719

The fatty acids from the bloodstream, they enter the cells where needed.1750

OK, now, fatty acid oxidation, it takes place in the mitochondrion.1767

Oxidation takes place - it seems like everything takes place in the mitochondrion, doesn't it - in the mitochondrion.1777

OK, now, fatty acids of 12 carbons or less, they enter the mitochondrion without the help of a transporter.1791

In other words, they just pass through the outer membrane and the inner membrane in going to the matrix.1825

They get into the mitochondrial matrix - we will see a picture in just a minute - very, very easily- the ones that have 12 carbons or less.1830

Fatty acids of 12 carbons or less, they enter the mitochondrion without the help of a transporter.1837

Now, those fatty acids with greater than or equal to 14 carbons - once this chain starts to get a little long, which is the majority, by the way - they must undergo 3 reactions of something called the carnitine shuttle in order to enter the matrix from outside- reactions of the carnitine shuttle in order to enter the mitochondrial matrix.1843

And again, we will see a picture in just a minute.1896

In other words, they do need some help getting in; they cannot just go across the membrane- the 14 carbons or more.1902

OK, let me go ahead and list the reactions, and then, we will go ahead and take a look at a picture of this.1910

The first reaction that they have to undergo is the following; let me go ahead and do this in blue.1915

Our first reaction is, we have a free fatty acid.1921

This is going to be R, C, O^-.1927

OK, actually, you know what, I think I am going to write this a little bit lower because I need a little bit more room.1936

Let's put it down here; we have R.1947

We have C; we have some free fatty acid, and let's see.1951

We are going to have CoA.1957

Coenzyme A comes in. ATP is used.1963

AMP + PPI is released, then, we end up with R, C, O, O, S-CoA; and the enzyme that does this is called - oops, let's do the enzyme in red - fatty acyl-CoA synthetase.1975

The first reaction that has to happen is these free fatty acids are converted into the coenzyme ester- the thioesters, so R, C, O, O to R, C, O, S-CoA.2007

That is the first reaction that has to take place; now, the second reaction that has to take place is the following.2020

We have this, now, the R, C, O, S-CoA, and go ahead and do that.2027

Carnitine comes in, and CoA - I will just write it as CoA-SH - leaves, and what you end up with is now, this transesterification.2043

The coenzyme A is taken off, and carnitine is attached as an ester.2055

R, C, O, O, well, I will just go ahead and write carnitine because it is the oxygen on the carnitine that is this.2060

And then, our third reaction is going to be the following: R, C, O, carnitine, and it is reconverted once it is inside the matrix back to what it was supposed to be.2075

CoA-SH comes in, and carnitine leaves; and it is reconverted back into what it is supposed to be, which is S-CoA.2094

Let me go ahead and write the enzymes; let's do this.2111

This is called carnitine acyltransferase no. 1, and this is CAT no. 2- carnitine acyltransferase no. 2.2115

The reactions that have to take place are the following for any carbon, for any free fatty acid that has 14 carbons or more.2133

The free fatty acid is converted into the acyl-CoA.2141

The CoA is converted into the carnitine ester, and then, it is transported into the matrix at this point; and then, the carnitine ester is reconverted back to the acyl-CoA.2146

It is a way of taking this that is on the outside of the mitochondrion, getting it inside the mitochondrion and getting it here.2155

That is what we want; we are trying to get to this point because it is at this point that now, the mitochondrion can actually oxidize it, start turning it into the acetyl-CoA that it is going to turn it into.2165

OK, let's go ahead and take a look at this, what is actually going on in terms of...do a little diagram of what is going on here.2175

Let's see; aha, alright, here is a picture of the mitochondrion.2185

Basically, these free fatty acids are on the outside; they are out here.2189

OK, this is the cell; the mitochondrion is an organelle inside the cell.2193

This is the cytosol; they need to get in here.2198

What we have, no. 2, this is the outer membrane of the mitochondrion.2202

This, right here, that is the inner membrane of the mitochondrion.2206

4: the stuff that is in orange, that is the matrix.2213

That is where it needs to be; that is where it all happens.2217

This white space is actually the intermembrane space; it is in between the outer membrane and the inner membrane.2221

Now, what I am going to do is I am going to take a little section of it like this.2227

I am going to do this in black, I think; I am going to take a little section of it, and I am going to blow it up, so that you see it.2233

Actually, let me do it on this side because I am actually going to have the cytosol on the left.2240

Let's go ahead and take this portion right here; OK, there is the cytosol, the outer membrane, the intermembrane space, the inner membrane and the matrix.2246

We are going to have 1, 2, 3, 4, 5 things.2254

I am just going to magnify this, and we are going to see what is going on.2258

Let me go ahead and draw this out.2262

Actually, I do not need to make the space that big.2266

This is going to be the...that is the outer membrane.2270

Let's go ahead and do the inner membrane here.2277

OK, here, we have the cytosol; here, we have the intermembrane space.2283

That is the white; this is the outer membrane, and here, we have the actual matrix.2295

This is the orange part; OK, these free fatty acids that are out here, we need to get them from the cytosol into the matrix.2300

The carnitine shuttle does this; here is how it does it.2314

OK, I am going to draw something here, and I am going to draw a little something here.2317

Yes, that is fine; I guess I can draw it right there.2323

I am going to go ahead and draw this in blue.2327

This is our carnitine acyltransferase I, and I will go ahead and draw a little something here.2332

Hold on; let me do that and that.2340

This is this little transporter that we have.2350

OK, it basically splits it in half; it is basically something that the molecule can pass through on the inner membrane.2357

Let me go ahead and just list this; this is the outer membrane.2365

This is the inner membrane.2374

OK, I have just taken this, and I have magnified it- cytosol intermembrane space matrix.2379

Here is what happens; the first reaction goes ahead and takes place out here.2384

It is converted into its acyl-CoA, and then, we have the following.2389

Let me go ahead and do this in...I wonder if I should do this in...now, what color should I do it in?2394

That is fine; I will go ahead and leave it as blue.2401

OK, we have R, C, O, S-CoA, and we have carnitine.2406

This is the second reaction that is going to take place.2415

This is going to go that way; this is going to go that way.2420

The acyl-CoA and the carnitine are going to react.2423

It is going to release the coenzyme A, and what you are going to end up with is this R, C, O, O, carnitine.2429

Now, we are not sure whether this actually takes place outside in the cytosol and then goes into the intermembrane space or if it actually happens in the intermembrane space, but the CATI, they acyl tranferase is right there on the outer membrane.2438

At this point, this is transported across the intermembrane space and into here.2455

Now, we have R, C, O, carnitine.2464

Now, at this point, it is going to react with CoA, again.2473

This is the third reaction, and it is going to release carnitine; and it is going to be reconverted back to its R, C, O, S-CoA.2484

This is what we want; it is on the outside.2497

We want it to be on the inside; now, a carnitine comes back out through here to start the cycle again.2502

Alright, now, let's go ahead and just write a couple of things here.2510

This is the carnitine shuttle.2515

This is the carnitine shuttle, and let's just list a couple of things here.2520

Oh, let me put my other enzyme; my other enzyme is this one right here.2530

OK, this is my CATII.2541

Let's go ahead and do this in red- the 3 reactions of the carnitine shuttle.2548

The first one is taking a free fatty acid - this thing - turning it into acyl-CoA.2554

Once we have turned it into acyl-CoA, it actually reacts with carnitine.2563

The coenzyme A leaves; it turns it into a carnitine ester.2568

This carnitine ester passes through the intermembrane space, goes through this particular transporter - this transporter - and it brings it into the matrix as the carnitine ester.2573

Now, we can convert it back to the acyl-CoA that we need.2586

It reacts with the CoA that is inside the matrix; we have produced this.2590

We have released the free carnitine; now, the free carnitine passes back through the transporter, goes back over here, so it can start the cycle all over again.2595

That is the shuttle; it is taking an acyl-CoA on the outside of the - out in the cytosol - mitochondrion, and it is bringing it inside of the mitochondrial matrix.2602

That is what the carnitine shuttle does; that is all that is going on here.2614

Reaction 1 takes place here, so I will go ahead and put reaction 1 there.2617

This, right here, is reaction 2 of the carnitine shuttle, and this, right here, is reaction 3 of the carnitine shuttle; and this transporter is what actually affects it- that is it.2624

OK, now, let's go ahead and say a couple of words here.2636

The carnitine acyltransferase, the CATI, the one that actually converts the acyl-CoA to the carnitine ester, CATI is inhibited by malonyl-CoA, an intermediate in fatty acid biosynthesis.2645

This is to ensure that a synthesis - in other words, anabolism - and breakdown - catabolism - do not happen at the same time, do not happen simultaneously.2694

When the body is actually synthesizing fats - a fatty acid - one of the intermediates is this thing called malonyl-CoA.2730

Its presence actually stops, it actually inhibits this enzyme of the carnitine shuttle.2740

It stops the actually breakdown of the fat because that is the thing.2748

There are some times when the body needs fat, so it will synthesize it.2751

There are times when the body needs to break down the fat; if they happen simultaneously, nothing is really changing.2755

When it is producing, when it is biosynthesizing the fatty acid, it actually shuts down the breakdown of the fatty acid, so that you actually have a build-up of the fatty acid because the body needs it, otherwise, it is just going to make it-use it, make it-use it- nothing changes.2762

This is a point of regulation - a very important point of regulation - for the fatty acid catabolism.2778

OK, now, the final word here on the carnitine shuttle.2785

The carnitine shuttle is the rate-limiting step in fatty acid oxidation.2798

This carnitine shuttle, the rate at which it actually can enter the mitochondrion, that is ultimately the rate at which fatty acids can be oxidized.2813

This is what controls the rate; everything else is a very fast reaction.2822

This is what controls the rate; the carnitine shuttle is the rate-limiting step in fatty acid oxidation, and as such, is a point of regulation.2827

Now, remember what we said, the rate-limiting steps, the highly exergonic steps in pathways, these are the points that the body uses to actually regulate the flow through these pathways- major theme.2845

This is what you want to concentrate on,2861

Carnitine shuttle is the rate limiting step in fatty acid oxidation, and as such, is a point of regulation for this process, how is that?2870

OK, we are going to go ahead and stop the lesson here.2880

In the next lesson, we are actually going to talk about the actual oxidation of the fatty acid.2884

Thank you so much for joining us here at Educator.com, bye-bye.2889