Biochemistry Oxidative Phosphorylation II

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

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

In the previous lesson, we looked at complex 1, complex 2; we gave a little bit of an overview of what was going on.0008

Now, we are going to move on to the rest of the electron transport chain beginning with complex 3.0014

OK, complex 3, let's see what we have here.0020

Now, we have all of these high-energy electrons that are collected by the molecule ubiquinone, and now, it is in the form of ubiquinol, its reduced form.0033

Now, it is going to pass on its electrons.0042

It is going to pass its electrons to cytochrome c, and here is how it is going to be.0048

Let's go ahead and give a name to this.0055

This is called cytochrome bc1 complex.0059

It is also called its formal name Q:citochrome c oxidoreductase.0069

This is the name of the complex; normally, we just call it the cytochrome bc1 complex.0087

That is the common name; the formal name, when you see this :Q to cytochrome, that means something is being transferred from Q to cytochrome c, and it is an oxidoreductase.0092

Oxidoreductase, you remember from the classification, it involves the transfer of electrons.0103

OK, in the complex 3, in this particular case, electron transfer, it takes place in 2 steps.0110

It is really an unusual complex.0122

OK, the first step, what you have is a molecule of QH₂.0130

Now, it has its electrons; what it does, it releases its hydrogen ions into the inter-membrane space.0136

Again, this is part of the proton pump quality of complex 3.0145

It releases the 2 Hs into here.0150

Well, one of its electrons, it gives it over to ultimately cytochrome c, and we will talk about the actual sequence in a second; but it goes from Q to cytochrome c.0155

The other electron, now, what it does is it gives it over to another molecule of just plain old Q, and what ends up happening is this Q ends up turning into the QH radical that we talked about earlier.0166

Now, another molecule of QH₂ diffuses through the bilayer.0182

It release its 2 hydrogen ions into solution.0187

Now, we have a total of 4 hydrogen ions being passed into the inter-membrane space.0192

It, again, gives 1 electron to Q.0198

It passes it to cytochrome c; it gives its other electron over to this radical, and this radical, now, ends up becoming QH₂.0204

In the second step, 1 molecule of QH₂ is used up.0212

1 molecule of QH₂ is created; there is no net gain or loss of QH₂.0218

Ultimately, it is only this 1 molecule of QH₂ that ends up passing its electrons- 1 here to cytochrome c, the other here, another 1 to cytochrome c, the other here.0225

We have actually passed 2 electrons, and we have pumped 4 protons into the matrix.0235

That is what is happening here; complex 3, well, it is definitely not altogether that easy to follow.0242

There is a lot going on with complex 3; let's talk about what happens here.0249

Electron transfer takes place in 2 steps.0253

The first step, a molecule of QH₂ - the reduced form of ubiquinone, which is called ubiquinol - releases 2 hydrogen ions to the inter-membrane space.0257

OK, that is the P side- the positive side.0279

Now, 1 electron passes as follows.0284

This is the inner workings of the complex: QH₂ to an iron sulfur protein to cytochrome c1 and ultimately to cytochrome c.0292

This is the actual path of the electron, and 1 electron passes as follows: QH₂ to cytochrome b to Q, which, now, becomes QH radical.0307

OK, the first electron passes from QH₂; this QH₂ and this QH₂, they are the same.0347

One electron, it goes from an iron sulfur protein to cytochrome c1 to cytochrome c.0354

It is, now, here; the other electron passes to a cytochrome b and then, to Q, which, now, becomes QH- this Q radical.0360

That is what that becomes; now, for step 2, another QH₂ molecule, it does the same thing.0371

It does the exact same thing; it does the same thing.0390

It releases 2 hydrogen ions into the inter-membrane space.0400

Well, 1 of the electrons passes as QH₂, iron sulfur protein, cytochrome c1 to cytochrome c, and the second electron, it passes as QH₂ - this is the same QH₂, 1 electron goes 1 way, 1 electron goes the other way - to cytochrome b; but now, it goes to the radical, which, now, becomes our QH₂ again.0410

In step 2, 1 QH₂ is used, and 1 is created.0468

So, there is no net gain or loss.0490

OK, and like complex 1, complex 3 is a proton pump, hydrogen ion pump- that is it.0494

That is all that is going on there; OK, now, let's take a look and move on to complex no. 4.0520

OK, this is complex 4.0529

The movement is now from cytochrome c.0537

The electrons are going to finally make their way to O₂- the final destination of these electrons.0541

The name of this is cytochrome oxidase.0547

OK, let's talk a little bit about...I want to say something about the oxidase.0556

An oxidase is a general class of enzymes that catalyze oxidations, where O₂ is the electron acceptor, but O, itself, oxygen atoms, do not appear in the product.0565

Both atoms of oxygen end up being reduced to water.0622

One atom takes 2 electrons and 2 hydrogens to become 1 molecule of water.0626

The other oxygen atom takes another 2 electrons and another 2 hydrogen ions to become water.0630

Oxygen, itself, does not show up anywhere; it is pure oxidation up here in the product.0637

OK, now, let's see what actually goes on here.0645

In this particular case, this is also going to, sort of, happen in 2 steps.0654

What you see here, though, is the final result.0659

Now, you have this cytochrome c; all of the electrons have been passed to cytochrome c.0665

Cytochrome c is like a ubiquinone in a sense that it is a mobile electron carrier.0670

Again, complex 3, complex 4, we need to get the electrons from here to here.0678

Cytochrome c is what moves on the lipid bilayer, through the lipid bilayer.0682

And again, it is just another molecule that carries electrons.0690

It carries/has 2 electrons, moves over here, gives up 2 electrons, moves back, takes 2 electrons, just back and forth- that is it.0694

It does the same thing; it just an electron shuttle is what it is.0700

The net effect is the following; 4 cytochrome cs pass their electrons to, this time, a copper-copper center, passes it on to a heme group, passes the electron to another heme group, passes the electron actually to a heme copper center, and now, these electrons, here is where O₂ comes in.0705

O₂ + 4 hydrogen ions plus these 4 electrons is reduced to 2 molecules of water.0730

In the process, also 4 hydrogen ions is pumped from the matrix into the inter-membrane space.0737

This is the net effect of what complex 4 does.0744

Cytochrome c passes its electrons ultimately to O₂, reduces both atoms of oxygen in the O₂ molecule to water.0748

OK, that is very, very important- 4 protons, 4 electrons, 2 each to form waters.0758

Now, we will go ahead and do a breakdown of the individual process.0766

Again, an oxidase is a general class of enzymes that catalyze oxidations where O₂ is the electron acceptor, but O atoms do not appear in the product.0770

OK, now, this is different from the mixed-function oxidase that we saw earlier for the phenylalanine hydroxylase reaction.0779

Remember the first reaction of the catabolism for phenylalanine?0788

That enzyme, the phenylalanine hydroxylase, is also called a monooxygenase because it uses O₂ as the electron acceptor, but one of the oxygens actually ends up in the product.0794

The phenylalanine becomes tyrosine; the other oxygen atom ends up being reduced to water.0808

In this case, an oxidase, both atoms end up getting reduced to water.0814

That is the difference; even though we call this an oxidase, that other one...you remember, we called it a mixed-function oxidase?0820

That is a really unfortunate, sort of, common name for it- mixed-function oxidase.0828

You will often hear it called a mixed-function oxidase.0831

Really, it is more proper to refer to it as a monooxygenase.0837

This is strictly an oxidase; I know, enzyme names, it is enough to make you want to pull your hair out.0841

Sorry about that; OK, now, let's go ahead and talk about this.0847

Let's see; the first step, we have 2 cytochrome c molecules.0857

Two electrons are passed to this copper-copper center, and then, these electrons are passed to something called heme A.0858

These electrons are passed to something called heme a3.0867

Actually, I am going to draw it a little bit this way.0870

I am going to go to heme a3CuB.0874

It is a heme copper center, and from here, the electrons are passed to O₂.0903

However, the O₂, it actually becomes O₂^2-.0912

It is not fully reduced yet; now, 2 more cytochrome cs deliver 2 more electrons, and with 4 hydrogen ions taken up from solution, taken up from the matrix, this O₂^2- now, it becomes 2 molecules of water.0920

OK, the 2 electrons that are passed from the 2 cytochrome cs only produces O₂^2-.0968

Two other electrons from 2 other cytochrome cs pass through the same process, and now, the O₂^2- is converted to 2 molecules of water.0974

And, of course, along the way - again, complex 4 is a proton pump - 2 H⁺s are transported into the inter-membrane space.0986