at Oregon State University.

Kevin Ahern: Okay, folks, let's get started!

I can't hear, there I go!

You guys are the quickest-to-quiet-down class

I've ever had, and that's good.

Just think how much more biochemistry we can squeeze in

now when you quiet down quickly.

I hope everybody's doing well and ready for a big weekend.

We will have an exam in here a week from Monday, not today.

For those of you who thought we were having it today,

I know you'll be disappointed.

I will announce later where that stopping point will be.

I sort of decide on, it depends on where I get in my lectures,

and it varies a little bit from term to term.

Today I'm going to finish up hemoglobin

and start talking about enzymes.

I hope I gave you the impression

or the understanding that hemoglobin is a remarkable protein.

It has a remarkable number of functions built into it,

and those functions are directly related

to the structure of the protein.

We saw structural things about proteins,

in general, when I talked about primary,

secondary, tertiary, et cetera.

But here's a protein where you get a real live,

up close and personal look

at how those structures that we see in proteins

give proteins specific functions.

We'll see more of that when we talk about enzymes.

I want to emphasize, speaking of enzymes,

that hemoglobin is not an enzyme.

People commonly think that, but it's not.

Enzymes catalyze reactions and hemoglobin

isn't catalyzing anything.

So it's an oxygen-carrying protein.

That's really its only function.

We'll see that the way that it binds oxygen

is not unlike the way that enzymes bind their substrates,

but hemoglobin is not an enzyme.

Last time I finished by talking about fetal hemoglobin

and fetal hemoglobin has the very interesting property

of having the slightly different subunits.

It has the two gammas instead of the two betas,

and that sort of makes a structure

that doesn't have that doughnut hole

that fits the 2,3BPG in the same way.

As a result of that, as I had noted,

the fetal hemoglobin stays in the R state almost all the time,

and that's why the fetal hemoglobin

has greater affinity for oxygen than adult hemoglobin.

There are yet other things

that we need to understand about hemoglobin.

I always like to think about hemoglobin as having,

obviously, structures that give it the functions that it need,

that those structures correlate very well

with the needs of the body.

They correlate very, very well with the needs of the body.

You saw 2,3BPG was being produced by cells

that were actively metabolizing,

and it was providing a signal that,

"Hey, here's the place where I need the oxygen.

Let go of the oxygen."

And they cause hemoglobin to let go of the oxygen.

There are other signals that hemoglobin can respond to

with respect to actively respiring cells.

One of these is pH.

When we talk later about,

it'll actually be next term,

but when we talk about actively respiring cells next term,

one of the things we'll discover

is that actively respiring cells

have a higher concentration of protons around them

than non-actively respiring cells,

which means that the pH around an actively respiring cell

is lower than that of a non-actively respiring cell.

Again, when I think of an actively respiring cell,

you can almost always think of muscle.

Muscles really change a lot.

When muscles are contracting,

they're really needing energy a lot,

and things that need energy need oxygen.

They go hand in hand.

A scientist named Bohróand no, that's not B-O-R-E,

that's B-O-H-Rómade a very interesting observation

about hemoglobin many, many years ago.

The observation was, if he took hemoglobin

and he did this oxygen-binding curve that we did before,

where we see the percent of the hemoglobin

saturated with oxygen and the concentration

of the oxygen on the x-axis,

what he saw was, he did the plot

and he saw that nice sigmoidal plot that we did before

and if he dropped the pH the curve actually shifted downwards.

Well, that shift downwards corresponds to less affinity,

meaning that the hemoglobin is releasing oxygen,

and it's releasing oxygen as a function

of the pH environment in which it finds itself.

Now, again, this is a functionality

built into hemoglobin that is directly responding

to the body's needs: pH drops around actively respiring cells,

hemoglobin will tend to give up more oxygen

around actively respiring cells.

It's a very cool phenomenon.

Now, the chemical basis of the effect is not too surprising.

When we look inside of the hemoglobin molecule,

we see that there are various amino acid side chains

that can be charged.

In this case, we see a lysine up here

that is attracted to a portion of a histidine,

and here's the functional part of the histidine

where it can gain or lose a proton.

If it gains a proton, it is positively charged

and it will attract a negatively charged side chain,

in this case, of an aspartic acid.

If that proton is off of there,

then it will not attract that,

and we could imagine there would be

some slight shape changes that would happen,

whether it's being attracted or not being attracted.

And that, actually, that very subtle difference there,

is the molecular basis for the change

in affinity of hemoglobin for oxygen.

So a slight shape change happening

according to whether or not we put a proton onto a histidine,

and that histidine changes its interaction

with another side chain of aspartic acid,

and, a result of that,

causes the protein to actually change its configuration

and its affinity for oxygen.

Another thing that we see around actively respiring cells,

and it's actually one of the causes of the drop in pH,

is carbon dioxide.

Carbon dioxide is the final oxidative product of metabolism.

When we go and we burn sugar,

or we go and we burn fatóI'm on a diet right now,

so that burning fat is really on my mind.

If you see me exhaling a lot of carbon dioxide, that's good.

I wish I could do that.

Carbon dioxide is an end product of those kinds of processes.

So, not surprisingly, if we examine the environment

around actively respiring cells,

we discover there's more carbon dioxide there.

It turns out that carbon dioxide also affects hemoglobin.

I thought I had a graph there.

That's not the thing.

If we look at this now, what we see is,

now we're going back to these plots that we did before,

the first plot was the, this is not getting my voice.

The first plot was the pH 7.4, no CO2.

Then if we take no CO2 and that same hemoglobin

and we drop the pH, this is what we saw before,

the affinity for oxygen drops.

But now look at the bottom line.

If we have a pH 7.2 and we add carbon dioxide,

the affinity drops even more.

That means, therefore, that hemoglobin is releasing oxygen

in response to both protons and to carbon dioxide.

Again, these are things that are both present

in higher concentrations around cells

that are actively respiring.

Questions about that?

Question over here?

Student: Could you go back to the figure real quick with the,

where you had the histidine structure?

Kevin Ahern: Okay.

Student: I had a question about that.

Kevin Ahern: Okay.

Here, yeah.

Student: When you say "add a proton,"

Kevin Ahern: So, as the pH changes,

protons will come on or come off.

So that's the variable that's there.

That's always true, yeah.

Student: So it's the pH drop, then?

Kevin Ahern: A pH drop, so a pH drop would be more likely

to put a proton on there.

Exactly.

Okay?

Yes, sir?

Student: When we're talking about CO2

contributing in the same way

that a more acidic environment

around the respiring cells contributes,

is that actually in the form of CO2 or as carbonic acid?

Kevin Ahern: Good question.

His question is, "How does CO2 manifest its effect?"

I'm going to show you that in just a second.

Is that your question?

Student: My question was similar.

Kevin Ahern: Okay, so CO2 exerts its effect.

How does CO2 exert its effect?

Well, one of the ways it exerts its effect

is by forming a covalent bond with amine side chains.

CO2 can be carried in the blood in two ways.

One is it can actually be dissolved in the blood,

and we'll talk about that later.

The other way it can be carried

is by this covalent bond to hemoglobin.

We could imagine, looking at this structure here,

that we have a carbon dioxide.

We've got an amine,

in this case that is shown with no charge on it.

We put a CO2 on it and we develop something

that has a negative charge.

Again, we're introducing a charge

where there wasn't one before.

We could expect that we would, in fact,

see some changes that would happen in structure of hemoglobin.

Again, that's the basis for the change in affinity.

Very, very subtle changes that are happening to the protein,

but they're having big effects on its affinity for oxygen.

In this case, it actually, you'll notice it releases a proton,

and that actually enhances the effect that we saw before,

because, more protons, of course,

now we're going to affect hemoglobin in its own way, as well.

So these really work together

to make hemoglobin give up oxygen

at the places where it's needed.

So that's what I want to say about the Bohr effect.

The last thing I want to talk about

are some genetic considerations.

It's a disease that we hear a lot about,

and there are some interesting,

or at least one very interesting aspect of it,

and that disease is known as sickle cell anemia.

Sickle cell anemia is a disease.

It's a genetic disease where there are mutations

in one or more of the subunits of hemoglobin,

and there are different forms of sickle cell anemia

that correspond to, of course, different mutations.

Some may affect alpha, some may affect beta, et cetera.

So sickle cell anemia is not just one genetic mutation.

What happens when an individual has sickle cell anemia

is that the hemoglobin,

and by the way,

the changes that can happen,

the mutations that happen,

can change a single amino acid,

and if that single amino acid is in the appropriate place,

it will cause the disease.

So why do the cells get sickle shaped?

The reason that the cells get sickle shaped is,

under low concentrations of oxygen,

the hemoglobin inside the cells will actually form a polymer.

Multiple subunits will start joining,

joining, joining, joining together.

Now, normally hemoglobin doesn't do that,

and, in fact, regular hemoglobin,