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Lecture Comments (7)

1 answer

Last reply by: Professor Hovasapian
Mon Feb 8, 2016 12:42 AM

Post by Widad Hassan on February 7, 2016

hello sir,
i have a question concerning the oxidation ofthe heme of myoglobin:
i read somewhere that the oxidation of fe(II) is based on acid catalysis, but how does this work, since it's only possible during desoxyMb?

thank you :)

1 answer

Last reply by: Professor Hovasapian
Fri Jan 24, 2014 3:49 AM

Post by Udoka Ofoedu on January 23, 2014

Hey sir ,
  Please where are the lectures for post-translational modifications in proteins . Thank you !

2 answers

Last reply by: Shannen Brown
Tue Jun 25, 2013 6:37 PM

Post by Shannen Brown on June 25, 2013

Hi there!

I was wondering if any of the lectures had protein folding?
I had a quick look but just thought I'd double check in case I missed it.

Thanks :)

Protein Function I: Ligand Binding & Myoglobin

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Protein Function I: Ligand Binding & Myoglobin 0:30
    • Ligand
    • Binding Site
    • Proteins are Not Static or Fixed
    • Multi-Subunit Proteins
    • O₂ as a Ligand
    • Myoglobin, Protoporphyrin IX, Fe ²⁺, and O₂
    • Protoporphyrin Illustration
    • Myoglobin With a Heme Group Illustration
    • Fe²⁺ has 6 Coordination Sites & Binds O₂
    • Heme
    • Myoglobin Overview
    • Myoglobin and O₂ Interaction
    • Keq or Ka & The Measure of Protein's Affinity for Its Ligand
    • Defining α: Fraction of Binding Sites Occupied
    • Graph: α vs. [L]
    • For The Special Case of α = 0.5
    • Association Constant & Dissociation Constant
    • α & Kd
    • Myoglobin's Binding of O₂

Transcription: Protein Function I: Ligand Binding & Myoglobin

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

Today, we are going to start our discussion of protein function.0004

We have already taken a look at amino acids and primary structure.0008

We have taken a look at a secondary structure, tertiary structure and quaternary structure.0013

Now, we are going to talk about protein function.0018

In particular, we are going to be talking about ligand binding, and we are going to be spending a fair amount of time talking about this protein called myoglobin.0022

OK, let’s see what we can do.0030

Proteins, they interact with other molecules.0034

That is our beginning.0038

You know what, let me...I think I want to go to a different color to start off with.0043

Proteins interact with other molecules.0054

Let’s define something called a ligand or a ligand.0066

It depends; a lot of people pronounce it ligand.0071

I prefer to pronounce it ligand; any molecule - excuse me - that binds - here is the key word - reversibly to a protein, now, this can be any molecule even another protein- that is it.0074

A ligand is any molecule that binds reversibly to a protein.0122

Now, the binding site is exactly what you think it is.0127

The binding site, it is a place on the protein where the ligands bind.0132

OK, now, a protein may bind several ligands at several sites.0152

Again, we are not putting any restrictions on this.0173

We can have 1 binding site, 2 binding sites, 37 binding sites, whatever is necessary for that protein to function and do what it does.0177

Protein may bind several ligands at several binding sites.0184

OK, and binding is specific for that ligand.0192

It is not just any molecule that comes around, and the protein will bind to it.0197

It is very, very specific; binding is specific for that ligand.0202

OK, now, proteins are not static or fixed.0216

They are very, very, very flexible.0233

They are very flexible.0240

Well, I am not going to repeat and say that they definitely move; they definitely move.0247

I mean, we know that; they are very, very flexible and very, very accommodating to different things.0251

OK, now, when a protein changes confirmation to accommodate the binding of a ligand, this is called induced fit.0257

You have often heard this term induced fit used to describe an enzyme from your other bio courses when you talked about enzymes.0298

An enzyme, it is induced fit in order for it to bind its particular substrate.0307

Well it is the same thing; I mean, an enzyme is just a protein molecule.0312

The only difference between an enzyme and a substrate, we do not call a substrate a ligand because the enzyme actually does something to the substrate and changes it.0315

It changes molecular form and spits out another different molecule all together, whereas for a protein that binds the ligand, it does not do anything to the ligand.0326

That is the difference, but essentially it is the same thing.0335

A protein is a protein that behaves a certain way; OK, let's see.0339

Now, in multi-subunit proteins, - excuse me - changes in the conformation of 1 subunit will cause changes in the conformations of 1 or more of the other subunits.0346

OK, now, as we said before, enzymes are a special class of ligand binding protein.0405

Again, not all proteins bind ligands; the proteins that do, they are called ligand binding proteins.0414

Enzymes are a special class of them; in so far as the ligand that they bind, they actually do something to it and then release it, whereas, for a normal ligand binding protein, it interacts with that ligand and then the ligand just leaves.0420

There is no molecular change that is affected on that particular ligand.0436

OK, let’s talk about molecular oxygen as a ligand.0441

Oxygen binding proteins, in our case, we are going to be concerned with myoglobin.0451

OK, that is OK; I will go ahead and mention hemoglobin because I am going to talk about this generally at first, and then, I will go ahead and stick specifically with myoglobin and talk about hemoglobin in a subsequent lesson.0472

OK, now, tissues need oxygen, but O2 is not very soluble in aqueous solution.0484

It cannot just dissolve in the blood, and then, when it gets to wherever it needs to get to in another part of the body, just do what it does.0511

We have to find a way of actually carrying it from one point of the body to the other, so not very soluble from the lungs to the other parts of the body, not very soluble in aqueous solution.0518

Now, the body had to find a way to deliver O2.0530

Now, proteins that can carry O2 make sense but no amino acid residue, no amino acid side chain is available that actually binds molecular oxygen.0548

There is no way just from a protein structure itself with all of these amino acids, there is no place on that protein where molecular oxygen will just stick to and bind to it.0575

Proteins carrying O2 make sense, but no amino acid side chain binds O2 reversibly.0587

It might bind O2, but it is not just a question of binding it.0606

I need to bind it; I need to transport it to where it needs to go, and then, I need to be able to release it.0611

That is the whole idea; there is no point in just holding on to the O2 if I cannot deliver my pay load, but no side chain binds it reversibly.0615

That is what is important; OK, now, iron - I should say iron(2+), the ferrous, the 2+ oxidation state - it binds O2 reversibly, and in higher organisms like we are, the O2 is carried by a structure called heme, which is a large ring-structured – which you will see a picture in just a minute – molecule, which is a prosthetic group incorporated into various proteins.0625

OK, now, recall what a prosthetic group is some molecular species associated with the protein and which a protein requires in order to function properly, requires for proper functioning.0713

Example, well, you guessed it.0775

Our example is going to be...OK, our protein is going to be our myoglobin.0779

Our prosthetic group, that is going to be our heme.0789

Actually, it is going to be the protoporphyrin.0802

Let me use the Roman designation - IX, and the ligand, itself, is going to be oxygen.0813

The heme is the porphyrin ring system plus the iron.0822

I am sorry, let me go ahead.0833

The prosthetic group, that is going to be our heme, which is going to be protoporphyrin IX plus our iron plus Fe2+ and our ligand - I guess I could have just left it - is O2.0834

Again, heme is the protoporphyrin ring plus the iron.0850

OK, now, let’s go ahead and yes, that is fine.0855

Let's go ahead and take a look at some pictures here.0862

The first picture we have is our ring system, this protoporphyrin IX.0866

We have this ring structure right here; this is the prosthetic group that is actually embedded inside of the protein, in this particular case, myoglobin.0871

Notice here, it does not have the iron attached to it.0880

When it is attached to it, that is when it actually becomes heme.0884

This is our protoporphyrin IX.0887

Now, these groups right here, like that and that, that metal group, that metal group, this alkene, this alkene, these, this ring structure is the basic structure.0896

OK, they have different things attached to it.0910

Different proteins have different porphyrin systems.0916

In the case of myoglobin, it is going to be the protoporphyrin IX.0920

These happen to be the functional groups that are attached to this ring system.0924

It is this ring system right here, - let me do this in red - that is going to be your basic structure and to it, these things actually change so you have different porphyrins.0929

Let's go back to black; now, this is our heme.0942

Heme is the iron and the protoporphyrin 9; As you see, you have this functional group.0946

You have the methyl here; you have this alkene here.0954

You have this alkene here; you have another methyl group, and here is your iron.0960

The iron is actually attached, is coordinated to the nitrogens in 4 places.0966

There are 2 additional sites that are going to be coordinated to the iron.0973

One of them, the oxygen is going to bind to; the other one is what actually attaches this thing to the protein, itself, via histidine residue, which you will see in just a minute.0978

I just wanted you to get an idea of what this look like.0987

This is the heme, protoporphyrin system, heme, spiral ring0990

I do not know about the extent to which your teacher actually wants specifics about the structure of this ring system- very, very important.0994

And again, different porphyrin systems have different metals; it does not have to always be iron.1002

It could be magnesium; it could be copper.1006

It could be zinc; it could be whatever.1009

We have actually created porphyrin ring systems synthetically that have different transition metals altogether.1011

It might be cobalt, who knows.1018

OK, let’s go ahead, and now, let’s take a look at myoglobin with its heme group.1021

Here is the myoglobin molecule; you see this ribbon diagram and the heme.1029

This is the heme right in there.1035

OK, I do not know if you can see it; it is attached to a little histidine.1039

In this particular case, it does not look like there is an oxygen attached anywhere.1044

It is just the heme group and the myoglobin, and then, in this particular case, this is the same thing, a slightly twisted view.1049

This time, it does not have the contour mapping on top of it, just the ribbon diagram, and here, you see the heme.1057

This is the heme group, and in this particular case, you do see the oxygen attached to it.1067

Protein is myoglobin; the heme is the prosthetic group.1075

It is associated with the protein, and the ligand that is reversibly attached, comes and goes.1078

That, in this case, is going to be the oxygen molecule; OK, now, let’s go on.1085

OK, the iron(2+), as we said a second ago, has 6 coordination sites.1095

In other words, there are going to be 6 bonds to the iron(2+) metal- 6 coordination sites.1103

When we talk about things being bonded to transition metals, we talk about them being coordinated to it.1111

It is just the language that inorganic chemistry uses when talking about metalloprotein and things like that.1118

OK, 4 of these sites are to the ring system, as we saw, the nitrogens and the inner part of the ring system.1127

One of them is to a histidine residue on the protein, and one is to O2.1141

Now, iron(2+), it binds O2.1164

Iron(3+) does not bind O2 or should I say binds it very badly.1174

OK, now heme, this prosthetic group, this protoporphyrin IX plus its iron center notice, 2+ and 3+.1185

There is oxidation-reduction happening; heme is associated with proteins involved in redox reactions.1198

OK, let's draw a little picture here.1217

We have our enzyme.1220

Let's go CH2; let’s go C-N, double bond, C-N, double bond C and single bond N.1225

Let’s go ahead and put an H there; put an H there.1240

I hope we have no't forgotten anything.1243

Here is our bond; there is our coordination bond.1246

I will do it in red, and I will go back to blue for our iron.1250

That is the iron center; it is in a 2+ oxidation state.1255

This is a side view looking at the protoporphyrin ring system along its edge- something like that.1263

It looks like that, and it is another coordination bound to oxygen, too.1273

That is what it looks like.1282

This, another representation of it, if you want to slightly turn it, you will end up something that looks like this.1286

Instead of looking at straight edge on, let's turn it just a little bit.1298

We end up with something that looks like this: Fe2+.1302

Let’s go to red; it is going to be bound to OO, and from the back, it is going to be attached to histidine 93, which is attached to the - not the enzyme - protein.1310

This is not an enzyme - that is it - I have just taken this and twisted it a little bit, so you are not looking it on the edge; and, of course, you have other coordination sites.1330

You have the nitrogens there, there, there so 1, 2, 3, 4, 5, 6, 6 coordination sites on the iron on this protoporphyrin IX system, on this heme system.1338

OK, now, that is OK.1350

I can go to the next page now.1358

Lets go back to blue; myoglobin, which is often abbreviated as Mb, is a single polypeptide with 153 amino acid residues- that is it.1362

153 amino acids ends up folding into some globular protein.1392

Inside of that, there is this heme; and to that heme, oxygen binds.1396

OK, now, as we said, myoglobin’s function, it depends on its ability to bind oxygen.1402

Now, let's go back to...I think I will go back to black.1411

We can describe this myoglobin oxygen interaction quantitatively.1419

That is what is nice.1447

OK, let's begin with an equilibrium expression.1451

We have protein plus ligand is going to form a protein ligand complex, protein attached to ligand- that is it.1470

Some protein, you have some ligand, and now, you have the protein and the ligand complex- that is it.1485

That is all that is going on here.1496

Let's go ahead and form the equilibrium constant; the equilibrium constant we know is products divided by reactants so we form Keq is equal to the concentration of PL/P x L.1500

Now, this forward reaction is called the association constant because the P and the L are associating to form the PL.1515

The reverse reaction would be called the dissociation constant because this PL complex is dissociating, breaking up into P and L.1523

OK, now, the forward reaction is called association, and this Keq is often symbolized as Ka or Ka.1531

Please do not confuse this with the acid dissociation constant from general chemistry when we talk about acids and bases.1579

When we talked about protein ligand interactions, when you see Ka, it is the association constant.1587

It is the reaction running in that direction.1593

OK, just be aware that that is often how they represent it.1598

It is probably like that in your books; OK, now, this - let's go back to black - Keq or Ka is a measure of the protein’s affinity for its ligand.1602

Now, of course, it is.1640

A high K value means that this numerator is high, and the denominator is low.1644

Well, if the numerator is high that means that most of the protein is going to be found attached to its ligand in this form, the form of the numerator.1649

Because it has a high affinity for its ligand, so, it wants to be attached to its ligand, which makes PL high, which makes free protein and free ligand low, which gives you a very high Keq.1659

That is what an equilibrium constant is; it is a measure of the extent to which a reaction is forward or back here.1669

It is mostly product or mostly reactant; in the case of an association of protein and ligand, your product is your protein ligand complex.1676

It is a measure of the affinity for the 2.1684

Let's write that down.1689

A high Ka means that PL is high which means a protein is mostly in the PL state.1693

This implies high affinity.1725

OK, let's rearrange this to get...we are just going to move the L concentration over to the left.1731

We end up with PL/P.1745

Notice what this is; this is just a ratio of bound protein to its ligand to unbound protein.1749

This is just a ratio; now, we noticed that it is actually a direct function of ligand concentration.1755

OK, all we have done is rearrange the Keq, move the L over here, to get an expression for bound protein over free protein.1765

This ratio here is equal to Ka/L.1772

OK, alright, let's write that again on this page.1777

Ka x L = PL/P- there we go.1784

Now, under most physio conditions/physiological conditions, the concentration of ligand is a lot higher than available binding sites.1793

As ligand binds, the ligand concentration, of course, decreases, doesn't, right?1832

As more ligand binds, the ligand concentration goes down, but because of the ligand concentrations is so high compared to the available binding sites, any reduction in ligand concentration is going to be unnoticeable, which means that its basically constant.1841

For example, if I had a cup of water and in that cup of water which has billions and billions and billions and trillions and quadrillions of molecules, I have a handful of protein molecules in there.1858

Well, if I take, let's say, the top 1 inch of water off of that, well, it is not really going to change the concentration of water relative to the protein.1870

I mean there is still so much water that even if I take a whole bunch of water, the water concentration relative to the protein concentration is going to be constant.1878

This is actually simplifies the math, I do not have to worry too much about L.1888

I can treat the ligand concentration as essentially constant; even though it does decrease relative to the protein concentration, the decrease is so small that it does not matter.1891

It is as if you do not even notice; it helps with the math.1903

As L binds, L decreases but this decrease is negligible, which means that the concentration of L is virtually constant.1912

This can help us out.1934

OK, in other words, we do not have to keep track of the ligand concentration in this expression.1939

We can just forget about it; it is going to stay constant, right?1943

When you do equilibrium constant, remember, the water concentration is essentially constant.1947

You do not put it in the equilibrium expression because it does not really change.1951

For our practical purposes, since the ligand concentration does not change, you can just ignore it from the expression.1956

It is just part of the constant; it is eaten up by the Ka.1962

That is all we are doing here.1968

Now, we want to define something.1971

Now, define this things called alpha, which is a ratio of the binding sites occupied divided by the total binding sites available.1975

I am sorry, divided by the total binding sites, not the total binding sites available.1994

OK, if I take the binding sites that are occupied divided by the total binding sites, I am getting the fraction of binding sites that are occupied, right, the part over the whole.2000

If I have 10 binding sites available and if I have 5 of those binding sites that are occupied, that means that I have 1/2 of the binding sites occupied.2015

That is all this is; this expression here, alpha is the fraction of bindings sites occupied- fraction.2027

This is equal to, well, PL which is the binding sites occupied divided by PL + P, the binding sites that are occupied plus the binding sites that are not occupied free protein, protein ligand complex.2044

This is that, so alpha is equal PL/PL + P.2064

OK, now, Ka is equal to PL/P x L, which we can rearrange to be Ka x P x L is equal to PL.2069

Alpha is equal to...since PL is equal to this, I will just put PL into here, and into here let’s see what I will get.2093

I get Ka PL/Ka PL + P.2104

Well, the Ps cancel, leaving me with Ka L/Ka L + 1, right?2124

That is my alpha; now, I am going to multiply the top and the bottom by 1/alpha, 1 over the Ka - sorry, excuse me - just to, sort of, simplify and get rid of this Ka thing.2140

When I multiply the top and the bottom by 1/Ka/1/Ka, which I am just multiplying by 1, I basically end up: this and that go away.2160

This and that go away, and what you end up with is the following.2171

You get alpha is equal to the ligand concentration, divided by the ligand concentration + 1/Ka.2175

This is just mathematical manipulation- that is all it is.2184

What we have here is the fraction.2191

We started with the fraction of binding sites occupied.2199

We have rearranged and manipulated it mathematically, so the fraction of binding sites occupied is a hyperbolic function of ligand concentration.2204

Anytime you see an equation of this form, where you have Y = X/X + Z, you are going to get a hyperbola.2221

OK, the fraction of the binding sites, we started with this equation; this was the expression for it.2233

We rearranged the Ka; we substituted and did some mathematical manipulation, alpha, which is this, is now, this L/L + 1/Ka.2237

The fraction is a function of ligand concentration.2246

This is really, really great; OK, now, let’s go ahead.2252

Let's rewrite the equation, so we have it on this page: alpha = ligand concentration/ligand concentration + 1/Ka.2258

Again, this equation describes a hyperbola; now, if we plot alpha, the fraction on the Y axis and ligand concentration on the X axis, we end up something like this.2269

Let me do it over here actually.2284

OK, this is ligand concentration L.2291

This is alpha, the fraction; it is really, really great because you can never have a fraction that is bigger than 1.2295

What you end up with is this; when we try different concentrations of L-alpha, you get this behavior.2303

This is our maximum; in other words, you cannot have more than 100% of the binding sites occupied.2312

You get this.2318

You get this hyperbolic behavior; it is really, really, really great.2322

OK, now, 0.5, I will tell you what this means in just a minute2328

Now, for the special case, actually, before I continue, let's just recall what we did.2341

We have this equation.2350

When we take a particular protein, and we measure, we start with different concentrations of ligand, and then, we measure the alpha, the percentage, of sites that are occupied.2354

We get a whole bunch of data points; we plot those data points, and we see that it actually follows a hyperbolic curve.2370

Now, if we decide to take some random numbers - not random, I mean the special case of 0.5 - it is going to be something like this.2378

For the special case of the fraction of alpha equalling 0.5, I get the following.2386

We get, I put 0.5 in for alpha 0.5 = lambda - not lambda - ligand concentration/ligand concentration + 1/Ka.2394

Now, I am going to solve for the ligand concentration.2411

That is what I want; It is going to imply the following.2416

I have got 0.5 x ligand concentration + 0.5/Ka - this is just algebra, I multiplied both sides by the denominator - equals lambda.2420

I end up with 0.5/Ka = 0.5 - why do I keep saying lambda - ligand concentration.2436

The 0.5s cancel, and what I end up is the following: 1/Ka = ligand concentration.2450

At the point where half of the binding sites are occupied, it matches.2458

It happens to be - let me go back here - this is 1/Ka.2467

This gives me a way of actually finding the equilibrium constant Ka.2473

OK, 1/Ka is the concentration.2480

It happens to be units of concentration, the concentration of ligand at which 1/2 of the binding sites are occupied.2488

This is really great; I get great concentration data.2510

I run the experiment; I try a bunch of different ligand concentrations.2515

I get a bunch of different fractions; I make the graph.2519

I go half way; I go over to the graph.2520

I go down, and this number that I hit, that is equal to 1/Ka.2523

I do a little algebra, and I actually solve for the Ka.2527

This is a practical method of actually finding the Ka value; this is fantastic.2531

OK, there you go; there is that.2535

Now, this hyperbolic behavior...well, actually you know what, I do not necessarily need to write this down.2539

Let me just go ahead and redraw the graph here quickly, sort of, a qualitative version of this graph.2555

This is 1; We are going to end up something like this.2561

And again, this is going to be our 1/Ka value.2565

This hyperbolic behavior makes sense.2569

As you start to add more and more and more and more ligand, you are going to occupy more and more and more of your binding sites that are available for binding, but at some point, you are going to add so much ligand that your concentration of ligand is going to exceed the available binding sites.2573

What is going to end up happening is you are going to reach a saturation point.2589

At some point, every single binding site is occupied; you can add as much ligand as you can.2593

You are not going to get any higher fraction than 100%.2598

That is, sort of, a theoretical maximum; you are probably going to be somewhere at...for totally, totally, really, really high concentrations of ligand, you might be up at 97, 98% saturation.2604

There is always going to be some free protein that is not binding anything, but it makes sense.2613

As you add more ligand, you have already occupied all of the binding sites, so the rate at which it is actually going to bind is going to slow down.2617

That makes sense; this is typical, typical behavior for ligand protein interaction.2628

OK, now, this Ka, as we said, is the association constant.2635

Well, you remember what 1/Ka is, right?2649

It is the equilibrium constant for the reaction in the reverse direction.2653

1/Ka is the dissociation constant.2658

Remember, we had protein plus ligand going to protein ligand.2668

That is product/reactant; well, if I take the dissociation constant, protein ligand dissociating into - let's just write it out as a single arrow - free protein and ligand.2675

Now, this is the product, and this is the reactant; it is just the reciprocal of this.2687

OK, 1/Ka is the dissociation constant.2692

Let’s call it Kd; let’s call it Kd.2696

That is going to equal protein concentration, ligand concentration over protein ligand concentration.2703

That is all this says, so let’s rewrite.2713

Alpha is equal to L/L + Kd.2718

At least, this way, we do not have to deal with this 1/Ka thing, and plus, I think it makes a little bit more sense.2724

Now, again, a high affinity means that you are going to find the protein and ligand bound together in the complex form because the protein wants its ligand.2729

It is going to be mostly this; well, it is mostly this.2738

That means there is very little of this, very little of this; when this is high and this is low, this ratio is low.2742

The lower the Kd, the higher the affinity the protein has for its ligand; let me say that again.2756

The lower the Kd, the higher the affinity the protein has for its ligand; it is just a simple equilibrium, mathematical justification for it.2758

That is all that is going on here.2767

OK, A equals that, and Kd has units of molarity; but we know that already.2771

1/Ka is units of molarity because on this axis, it is ligand concentration in moles per liter or millimoles per liter or micromoles per liter.2781

It does not really matter, so 1/Ka equals the Kd.2793

Alright, now, Kd - excuse me - is the ligand concentration, which causes 1/2 of the binding sites to be occupied.2801

Let's see if we can write a little bit better here.2838

OK, the higher the affinity of a protein for its ligand, the lower the Kd.2845

Well, we just said the lower the Kd, the higher the affinity, so either direction is fine.2854

That means that you are going to have behavior; graphically, it is going to be like this.2859

The higher the affinity, it is going to end up being like this.2863

It is going to be more well-defined hyperbola because now, this Kd is going to end up shifting that way because now, a higher affinity protein will demonstrate really characteristic hyperbolic behavior.2869

A low affinity protein is going to end up being like this, something like that- that is it.2887

It is just a measure of affinity; that is all its graph really does for you.2894

OK, now, again, myoglobins binding O2, it follows this pattern, which is...we handle it generally, but now, but the problem is that O2 is a gas.2898

We do not often speak about moles per liter, when it comes to gas.2930

We often talk about partial pressure of the gas; what we are doing is we are measuring the partial pressure of the gas above the aqueous solution because that is going to be proportional to the amount of gas that has actually dissolved the concentration.2934

You remember from PV = nRT; pressure and concentration are actually the same thing.2947

It is like the same 2 sides of a coin, you have the heads, and you have tails.2953

You remember PV = nRT; well, if I rewrite this as P = nRT/V, now, if I just take the n/V part, which equals n/V x RT, well, what is n/V?2959

n is moles, and that is volume; well, moles per volume, that is molarity, so molarity RT, and this RT is just a constant.2974

Concentration and pressure or let me do it this way pressure and concentration are directly proportional.2984

As it turns out, concentration in moles per liter and pressure in atmospheres, or whatever unit you happen to be using for pressure, are just different ways of representing concentration.2994

We can do that with a gas; it is easier to measure pressure.3004

The expression for alpha becomes, well, the concentration of O2 over the concentration of O2 plus its Kd.3008

Well, for gasses, experimentally, again, we tend to work with partial pressures.3019

The equation becomes alpha equals the partial pressure of O2 over the partial pressure of O2 + Kd- that is it.3024

You will see it sometimes like this.3035

If we define Kd as the partial pressure of O2, 0.5 this is, sort of, a symbol, it is also symbolized as this way, p50, you will see it as alpha = PO2/PO2 + p50.3041

I do not like this symbolism myself personally; you will see it in your books.3065

I like this; this actually tells me something or this.3069

This is fine, too, if we are going to deal with partial pressures instead of molarities; partial pressure, I like to see the Kd in the expression.3073

This p50, it has always confused me; it always will confuse me.3080

Again, whichever you like is fine; I personally prefer that one.3084

OK, thank you so much for joining us here at; we will see you next time for a further discussion of protein function, bye-bye.3090