Biochemistry Oxidative Phosphorylation I

Hello and welcome back to Educator.com, 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