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Oxidative Phosphorylation I

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  • Intro 0:00
  • Oxidative Phosphorylation 0:54
    • Oxidative Phosphorylation Overview
    • Mitochondrial Electron Transport Chain Diagram
    • Enzyme Complex I of the Electron Transport Chain
    • Enzyme Complex II of the Electron Transport Chain
    • Enzyme Complex III of the Electron Transport Chain
    • Enzyme Complex IV of the Electron Transport Chain
    • Complexes Diagram
  • Complex I 18:25
    • Complex I Overview
    • What is Ubiquinone or Coenzyme Q?
    • Coenzyme Q Transformation
    • Complex I Diagram
    • Fe-S Proteins
    • Transfer of H⁺
  • Complex II 31:06
    • Succinate Dehydrogenase
    • Complex II Diagram & Process
    • Other Substrates Pass Their e⁻ to Q: Glycerol 3-Phosphate
    • Other Substrates Pass Their e⁻ to Q: Fatty Acyl-CoA

Transcription: Oxidative Phosphorylation I

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

Today, we are going to start our discussion of oxidative phosphorylation.0004

We have talked about all these metabolic pathways, the catabolic pathways, the breakdown.0009

We have talked about glycolysis, the breakdown of sugars.0014

We have talked about fatty acid breakdown; we have talked about protein breakdown, amino acid degradation.0018

All of these electrons that we have collected from all of this are now going to go to this thing called the electron transport chain inside the mitochondria, and here is where we are going to use all of these high-energy electrons in order to synthesize adenosine triphosphate.0024

That is oxidative phosphorylation.0044

Let's jump on in and see what we can do; this is very, very important and quite beautiful, quite extraordinary.0048

OK, well, I will just start writing it down here.0055

Oxidative phosphorylation is the reduction of oxygen to water by all those high-energy electrons captured as NADH and FADH2 along with the concomitant of simultaneous synthesis of ATP from - AT that is P, not D - from ADP and PI, inorganic phosphate- that is it.0060

All of these electrons, they have to go somewhere; the ultimate destination of all of these electrons, the ultimate reducing agent, all of the electrons end up coming to oxygen.0140

Oxygen gets reduced to water, and we take all of the energy from those electrons.0150

We do something to them, which we will discuss in just a minute, and we use that energy to form adenosine triphosphate, which is the primary energy currency of the body.0156

In order for the body to function, it needs the ATP- that is it.0164

That is the only function of oxidative phosphorylation.0168

OK, first thing: in oxidative phosphorylation, the electrons flow through a chain of enzymes and/or molecules on the inner mitochondrial membrane.0172

All of these enzymes that are responsible for this oxidative phosphorylation, what we call the electron transport chain, they are enzymes that are embedded in the inner mitochondrial membrane.0214

They are not just floating around freely.0225

OK, 2: the free energy of this exergonic flow of electrons through the electron transport chain...exergonic, as the electrons are flowing through this electron transport chain, it is exergonic.0229

They are giving up their energy; it is a downhill flow for them.0259

All of these energy, they go downhill; where does all this energy go?0263

Well, here is where it goes; the free energy of the exergonic flow of electrons is used for the endergonic transport of hydrogen ions across the inner membrane, thus creating an electrochemical potential difference from one side of the membrane to the other; and we will show you a picture of this in just a minute to show you what is happening.0267

And finally, as these hydrogen ions, as they fall back down, this electrochemical difference, there is this gradient.0342

As the H+, as the ions, fall back down this electrochemical gradient, the free energy is used to synthesize ATP from ADP and PI- that is it.0355

That is all that is going on here; now, let's actually take a look at what is going on pictorially.0402

Electrons flow through a chain of enzymes on the inner mitochondrial membrane.0411

The free energy of the flow of electrons is used to transport hydrogen ions from the matrix outside of the membrane.0416

It, thus, creates an electrochemical potential difference from one side of the other.0423

When these protons fall back down to where they want to be, that extra energy is used to synthesize ATP.0429

Here is what is going on; this is a cut-out of the mitochondrion.0435

This is the mitochondrial matrix; this is the inner membrane.0442

This is the outer membrane, and this is the inter-membrane space, right here, in between.0446

Now, on 2 sides of it, here, you have this electron transport chain.0450

It is actually composed of 4 enzyme complexes: complex 1, complex 3, complex 4.0457

On this side, it just has the 1, 3 and 4; on this side, 1, 2, 3 and 4, the enzyme complex 2 is here.0465

It is not necessarily shown over here, but it is over here.0473

What ends up happening is the following; you remember the citric acid cycle, beta-oxidation, all of this breakdown where we are collecting all of these high-energy electrons.0478

Well, these electrons end up being given over to complex 1.0490

OK, that is the first step of the electron transport chain.0495

From complex 1, they pass on to this molecule called ubiquinone.0500

We will talk about that in just a minute; now, as far as complex 2 is concerned, complex 2 is one of the enzymes of the citric acid cycle.0505

It facilitates the conversion of succinate to fumarate, and those electrons, which are held as FADH2, they are also transferred to this molecule called ubiquinone.0518

And again, we will talk about that in just a minute; ubiquinone transfers its electrons to complex 3, and then, complex 3 transfers its electrons to something called cytochrome C.0530

Then, cytochrome C transfers its electrons to this complex 4, and complex 4 ends up taking the oxygen reducing it to water; and notice, in the process, in complex 1, in complex 3 and in complex 4 along with the transfer of electrons, what happens is hydrogen ions are actually pumped.0540

That is complex 1, complex 3 and complex 4 are hydrogen ion pumps.0562

They actually take hydrogen ions from the matrix, and they pump them out of the matrix into the inter-membrane space- right in here.0566

Now, what you have, you have a build-up of positive charge in the inner membrane space, and you have a build-up of negative charge inside the mitochondrial matrix.0574

We often call it the negative side, positive side, N-side, P-side, because you are actually moving positive charge out of here and into here.0586

Now, you are creating this electrical potential difference.0596

Now, you are creating a separation of charge; charge does not want to be separated.0602

Charge wants to be equalized, so these hydrogen ions want very, very much to get back to here.0606

That is where they want to be; we use the energy to pump them out.0612

Well, we are going to use this energy in a minute when we open up this and let the hydrogen ions flow back in naturally.0615

Well, that is the electro part of the electrochemical potential difference.0622

Well, you remember when we studied osmosis, anytime you have a heavy concentration of 1 species on 1 side of the membrane and not a lot of that species on the other side of the membrane, the species is going to want to equalize.0628

So, now, there is a chemical potential difference.0641

We call it the electrochemical gradient; that is what it is.0646

In the process of moving these electrons from 1, 2, 3, 4, we are...actually complex 1, complex 3 and complex 4 are pumping hydrogen ions out of the matrix into the inter-membrane space.0650

Now, once they have done that, once the O2 has been reduced to H2O, and you have all this hydrogen ion build-up here in the inter-membrane space - OK - and a bunch of negative charge here, now, what happens is all of these hydrogen ions will pass down their gradient.0662

Now, they naturally want to come here for electrical reasons and for chemical reasons, for concentration difference reasons.0681

As they fall back down through this complex, through this enzyme called ATP synthase, the energy that they released in going downhill, that energy is preserved and used to take ADP and inorganic phosphate to synthesize adenosine triphosphate and pump it out so that the body can use it.0689

That is what is going on here with oxidative phosphorylation.0711

This is the electron transport chain, the pumping of hydrogens into the inner mitochondrial - I am sorry - from the matrix to the inter-membrane space and then, the passage of these hydrogen ions through this enzyme called ATP synthase to produce adenosine triphosphate.0715

OK, now, let's go ahead and write some of this down.0736

We want to see it over and over and over again in visual form, in written form.0740

We definitely want to make sure to understand this as well as we can.0744

The electron transport chain, it consists of 4 membrane-embedded enzyme complexes.0748

OK, complex no. 1: it catalyzes...well, I will write it down.0773

Complex 1 catalyzes the electron flow from NADH to ubiquinone - ubiquinone is coenzyme Q, often just called Q - and the transport of hydrogen ions from the matrix to the inter-membrane space.0780

That is what is shown right here, actually here, complex 1.0825

It not only takes the electrons and passes the electrons on, but it also pumps hydrogen ions from the matrix to the inter-membrane space.0831

It is an electron pump; OK, now, complex 2: it catalyzes the electron flow from FADH2 to ubiquinone.0840

That was the succinate to the fumarate through FADH2.0870

OK, complex no. 3: it catalyzes the electron flow, now, from Q, from ubiquinone - which is actually, now, QH2, so I will just write it as ubiquinone, and we will get to the abbreviations in just a bit - to cytochrome C, as well as transporting hydrogen ions from the matrix to the inter-membrane space.0875

Complex 3 is also a proton pump, a hydrogen ion pump, transporting hydrogen ions to the IM space, and, of course, we have complex 4.0918

Complex 4, what it does is it catalyzes the electron flow from cytochrome C to O2 - the final step - as well as transporting H+ to the inter-membrane space.0933

There is lots of hydrogen ions being pumped out of the matrix and into the inter-membrane space.0967

It creates a huge electrochemical gradient.0973

OK, now, let's see here.0978

Let's see what else we can do; oh, OK, here, we have another, sort of, diagram, flowchart if you will.0984

This is a little bit of text, a little bit of pictures, so here is complex 1.0993

Electrons are transferred from NADH to flavin mono nucleotide and through the iron sulfur carriers.0997

We will talk a little bit about what all this is in just a minute when we look at details of each of the individual complexes to coenzyme Q.1003

NADH, 2 electrons passes through here goes to coenzyme Q.1010

This, right here, means that hydrogen ions are pumped to the inter-membrane space, and ultimately, they produce ATP; so that is what this little symbol means here.1017

Now, complex 2, FADH2 transfers its electrons also to coenzyme Q.1029

Now, coenzyme Q is a mobile carrier that transfers electrons to cytochrome B in complex 3.1038

We will get to that in just a little bit; complex 3, the electrons are transferred to cytochrome C.1045

OK, and again, in the process, complex 3 is also a proton pump.1052

Here, we also have the formation of ATP ultimately because of the hydrogen ions falling back down their gradient through this molecule called ATP synthase.1057

And then, complex 4, the electrons are finally used to reduce O2 to H2O.1065

2 H+ + 1/2 O2 goes to H2O, or if you prefer, 4 H+ + O2 - if you do not like the 1/2 there - goes to 2 molecules of water.1070

And again, complex 4 is also a proton pump, so it also ultimately ends up producing ATP down the road.1084

A little bit of a pictorial representation in addition to what each of the complexes do.1093

OK, now, let's take a look at the individual complexes in a little bit more detail and see what is going on.1098

Let's see; complex 1, let's see here.1107

I wonder if I should do this in...that is fine.1112

I will just stick with black; it is not a problem, so complex 1.1116

What we are doing is actually taking electrons from...electrons are flowing from NADH to ubiquinone.1125

The name of this is called NADH dehydrogenase.1139

I have not got to learn to write a little bit better.1148

The name of this complex is the NADH dehydrogenase complex.1152

It is also called...its formal name is actually NADH:ubiquinone oxidoreductase.1163

The actual systematic name of this enzymes is NADH:ubiquinone oxidoreductase.1181

That is what this, sort of, symbol means.1185

You remember, when we talked about enzymes, there is, sort of, a systematic name for enzymes.1190

This is the systematic name; we just call it NADH dehydrogenase.1194

OK, now, before we do that, we want to talk about what ubiquinone is.1198

First, what is ubiquinone or if you like, ubiquinone.1203

It is up to you; it does not really matter.1217

It is called coenzyme Q or just Q- that is it.1220

Most of the time, we will just be calling it Q; OK, Q is a fat-soluble molecule, which carries electrons- that is all it is.1228

In other words, it accepts electrons from 1 molecule, and it gives its electrons to another molecule- that is all it is.1247

It is a mobile, so it is fat-soluble; because these enzymes are embedded in the membrane, if the enzyme is here and the enzyme is here, sometimes the electrons cannot make the jump.1255

It is a little bit too far physically.1265

It gives its electrons to ubiquinone, which dissolves in the lipid bi-layer in the fat, travels over to the next enzyme and gives these electrons that way.1270

Because again, this is a physical thing that is going on; electrons are being transported.1279

Electrons can be transported, but they cannot jump huge distances.1282

This molecule Q is what actually moves through the fat and takes it from 1 complex to the other.1288

That is all that is happening here; it is a fat-soluble molecule, which carries electrons.1293

OK, it can accept 1 or 2 electrons.1300

It can accept 1 electrons, or it can accept 2 electrons; and by the same token, it can give 1 or give 2.1315

That is what is nice about this; it can accept 1 or 2 electrons diffused into the lipid bi-layer of the inner membrane and pass its electrons down the electron transport chain- that is all it does.1323

Let's take a look at this.1358

This is what it looks like; this is coenzyme Q.1362

This is ubiquinone, do a little red here.1365

When it accepts 1 electron, the symbol, now, becomes QH.1373

What is happening is actually H, and let's just say 1 electron.1380

Let's do it that way, H+ and 1 electron; it turns into this.1385

The H attaches here; 1 electron from this carbonyl jumps up to balance the charge.1390

The other electron jumps down and becomes here; this is actually a radical.1395

OK, they call this a semiquinone form; the name, itself, does not really matter.1400

All that matters is that it has gone from Q; it has accepted 1 electron plus it has taken a hydrogen ion from solution to become QH.1403

This is 1 electron that it is carrying; if it takes another H and another electron, if it gets the second electron, now, it becomes QH2.1411

This is the fully-oxidized form; it is Q.1424

This is the half-reduced form; this is the fully-reduced form, QH2.1427

This is called ubiquinol because now, it is an alcohol.1431

It starts off as Q; in complex 1, when it passes its electrons to Q, Q turns into QH2.1443

It is the QH2 that travels through the lipid bi-layer until it gets to the next complex, and it gives its electrons over to whatever complex it is going to give it to, complex 3; and then, it goes back to Q, travels back, collects more electrons as Q turns into QH2, goes and passes its electrons- back and forth.1450

That is all that ubiquinone does; I just wanted you to see what actually happens atomically, molecularly.1470

Notice, that is there; there is the radical.1477

OK, nothing strange happening here.1481

OK, now, let's go ahead and move on to - OK - what we were discussing- complex 1.1485

2 electrons pass from NADH to flavin mononucleotide.1497

NADH gives up its 2 electrons as a hydride ion to FMN, then, passes these electrons one at a time through a series of iron-sulfur proteins - boom, boom, boom, boom, boom, boom, boom, - as they make their way over to Q, so that Q can become QH2.1512

2 electrons pass from NADH - this is the beginning of the electron transport chain - that was collected from all of those catabolic steps like from glycolysis, from beta-oxidation, things like that.1575

They pass to FMN as a hydride ion; the FMN passes these electrons one at a time through a series of iron-sulfur proteins - boom, boom, boom, boom, boom, boom, boom - as they make their way to Q, so that Q can become QH2.1587

OK, now, what are iron sulfur proteins; what are these things?1600

Well, the FeS proteins or iron-sulfur centers are proteins that contain various arrangements of iron sulfur bonded together- that is it.1608

That is all it is; you can have 1 particular arrangement like this.1643

You can have an iron ion that is bound, that is coordinated, to 4 sulfurs that happen to be attached to the cysteine amino acids in the protein, or you might have this kind of arrangement.1648

You might have Fe; you might have S, S, Fe - oops - here, S, cysteine, S, cysteine, S, cysteine, S.1671

These cysteine residues, these are the amino acids in the particular iron-sulfur protein.1689

They happen to coordinate an iron ion.1695

Here, as part of the iron-sulfur protein, you have the cysteine residues that are coordinating an iron, but now, it is also coordinated by sulfur that is not attached to cysteine, just plain old inorganic sulfur.1699

These things are the iron-sulfur centers of the iron-sulfur protein that is part of this bigger enzyme complex, and all they do is they are just individual steps in the transfer of electrons.1712

What is interesting about these iron-sulfur proteins is they only transfer 1 electron at a time.1728

Fe2+ becomes Fe3+ when it is oxidized.1732

Fe3+, when it gains an electron, becomes Fe2+.1736

When it passes it to the next one, it becomes Fe3+; the next one becomes Fe2+, then it passes its electron.1740

It passes its electron; it passes its electron on down the chain- that is it.1746

That is all these things do; it is their individual steps for single electrons to pass.1750

That is what is important; these iron-sulfur proteins, they pass electrons one at a time until the electrons get to ubiquinone.1754

And, as we said, ubiquinone will accept 1 electron, then, it will take another electron to become ubiquinol; and now, when it is ubiquinol, it will go ahead and move on through the lipid bi-layer to pass on its electrons to the next complex in the electron transport chain.1761

OK, now, let's see.1777

Let me go back to black here.1785

Along with the transfer of electrons, 4 hydrogen ions are also transported by complex 1 from the matrix to the inter-membrane space.1789

Along with the transfer of electrons, 4 hydrogen ions are transported from the matrix to the inter-membrane space.1807

Again, we have the N-side going to the P-side.1831

Remember, negative side, that is what N stands for, going to the positive side because there is a build-up of positive charge.1838

Negative side is the matrix; positive side is the inter-membrane space.1857

That is where all of these protons are collecting.1861

OK, now, let's take a look at complex no. 2.1866

Let's do this in blue, so complex no. 2.1871

The name of this complex is succinate dehydrogenase.1879

A succinate dehydrogenase, it is the only enzyme in the citric acid cycle where the enzyme is not actually floating around in the matrix.1887

It is actually embedded in the inner mitochondrial membrane.1897

During the citric acid cycle, when succinate becomes fumarate, it passes through here.1902

It passes its electrons to flavin adenine dinucleotide, so the FAD ends up becoming FADH2; and that FADH2 is what transfers its electrons to ubiquinone.1906

What is happening is that electrons are passing from succinate to Q via FAD.1921

OK, this is the only membrane-bound enzyme of the citric acid cycle.1936

OK, let's take a look and see what this looks like here, image of complex 2.1963

Here we go; OK, here we have an image of complex 2.1971

Basically, the succinate comes in, and it is oxidized to fumarate.1978

It gives its electrons to FAD; FAD becomes FADH2.1987

FADH2, now, gives its electrons 1 at a time to the iron-sulfur centers, again, step-by-step passing electrons down the electron transport chain through complex 2; and eventually, the electrons end up again at ubiquinone.1992

Ubiquinone takes 1 electron; it takes another electron to become ubiquinol.2007

Now, that same pool of ubiquinol goes on to pass on its electrons.2012

Now, notice this thing right here- heme.2017

This heme is not directly involved in the electron transfer process.2022

Electrons do not actually flow through this in the electron transport chain.2028

Let's just go ahead and write out what is going on here.2032

Electrons pass from succinate to FAD, and, of course, the FADH2, now, passes the electrons through the iron sulfur.2036

OK, now, the heme, it is actually called heme B.2053

OK, it appears to prevent the formation of hydrogen peroxide and this highly-reactive superoxide radical species.2064

Basically, what happens, it is there to prevent the leakage of electrons.2094

In other words, ultimately, these electrons are going to end up passing through complex 3 and complex 4 and going to oxygen so that we can reduce the oxygen to water.2100

Well, it is possible that some electrons here might leak out and actually go to oxygen before they are actually supposed to, before they have passed through complex 3 and complex 4.2111

It appears that this particular heme group in complex 2 is there to trap these leaking electrons, these, sort of, wildcard electrons before they actually reach O2.2123

If they actually get to O2 before they have passed through complex 4, in complex 4, the O2 is reduced to water.2140

That is what it is supposed to be, but if the electrons end up reaching O2 before they get through complex 4, what ends up happening is that the oxygen is actually reduced to hydrogen peroxide; and it is also reduce to this thing- the O2- radical.2148

These are called reactive oxygen species, and they are highly, highly, highly reactive species; and they basically go around doing damage to everything they come in contact with.2163

The idea, it appears that this particular heme group is there to prevent that from happening if any electrons happen to leak and try to escape before they actually run through the process they are supposed to run through.2180

We do not want oxygen to reduce to peroxide and superoxide radical.2193

We want them to reduce to water; water is safe.2198

Water will not do any damage; these will do damage.2201

That is what the heme is; it is not in complex 2.2204

It is not directly involved in the electron transport process.2206

The heme B appears to prevent the formation of H2O2 and O2 reactive oxygen species by electrons, which might bypass the chain and go directly to O2.2213

That is complex 2, reasonably straightforward; nothing strange going on.2245

OK, before I get to complex 3, I want to talk about a couple of other substrates that actually end up also passing their electrons to ubiquinone.2250

Other substrates also pass their electrons to Q.2262

One of those is glycerol 3-phosphate.2279

You remember, when we were talking about beta-oxidation, remember, we were not talking about fatty acid breakdown?2285

Well, the fatty acid breakdown, one of the by-products is glycerol 3-phosphate.2292

Well, glycerol 3-phosphate, it is converted into dihydroxyacetone phosphate via the action of FAD, flavin adenine dinucleotide.2300

The FAD oxidizes the glycerol 3-phosphate to dihydroxyacetone phosphate.2314

In the process, what you end up with is the FADH2.2320

FADH2 also passes its electrons to Q to turn it into QH2.2324

Now, we have electrons passing through complex 1, electrons passing through complex 2.2330

Now, we have electrons passing through here also going to Q, and you remember when fatty acids - I will just write it as fatty acyl-CoA - are broken down - OK - they also produce the FADH2.2338

Now, the FADH2 passes its electrons to something called ETF.2365

It passes its electrons to something called ETF-Q oxidoreductase, and then, this passes its electrons onto Q.2373

Basically, this is just...we are accounting for all of the electrons that come from all of the catabolic processes.2387

The NADH, the succinate, the glycerol 3-phosphate and the fatty acyl-CoA, all of these electrons all end up being collected by ubiquinone.2394

Now, ubiquinone can go ahead and pass its electrons onto the next carrier in the electron transport chain.2406

That is all that is happening here.2413

OK, now, we can close this off by saying so.2417

Now, we have a pool of ubiquinol, the ubiquinone being reduced to ubiquinol, which can diffuse into the bilayer and pass its electrons to complex 3; and complex 3 and 4 is going to be our discussion for the next lesson.2421

Thank you for joining us here at; we will see you next time for a continuation of oxidative phosphorylation.2464

Take care, bye-bye.2470