Now, I'm going to show you schematically
here what actually happens
with that process I've
just described to you.
Hemoglobin can exist in
what are called "two states".
A T-state, which is described
as the tight state
and R-state, which is
described as the relax state.
And on this figure, the
T-state is shown in squares
and the R-state is
shown as circles.
Well, what's the difference
between the two?
The difference is that the T-state does
not tend to bind oxygen very well at all.
Whereas the R-state binds
oxygen really well.
What happens when hemoglobin gets
to the lungs if it's in the T-state?
It's in state that
we see on the left.
Four units, none of which like
to bind oxygen very well.
Well, the oxygen concentration
in the lungs is relatively high.
In fact, it's very high compared to
what it was in the rest of the body.
It's so high, in fact, that oxygen
forces its way onto one of those units
that doesn’t want
to bind oxygen.
Well, that's depicted in
the second figure over.
We see the blue circular form.
That is that molecule or
that protein of globin
that got changed in structure by
the binding of the first oxygen.
So that change in structure changed
it from a T-state into an R-state.
And we can see that as a
result of that change,
that blue circle is interacting
with two other subunits.
And those two other subunits have
started to change their shape,
meaning that the change in shape of
the first one has been communicated
to the other two
subunits touching it.
Well, because they have had
their shape changed slightly,
each of those subunits is more
like an R than it was like a T.
The result being that they will tend
to bind oxygen much more readily.
And we can see that happening
in the third, fourth and fifth
that this change in shape is being
communicated across the overall molecule.
So this process I have just described
to you is a very important process
that's happening inside of our lungs.
We're going from a non-oxygenated
state to a fully oxygenated state
in very short amount of time.
And the binding of the first
oxygen was difficult.
The binding of the second
was less difficult.
And the binding of the
third was less difficult.
It got easier the further we went along.
That process that I just described
to you is known as cooperativity.
And cooperativity is important because
hemoglobin has to get that oxygen quickly
and it has to fill itself up when it
encounters the oxygen in the lungs.
As hemoglobin is leaving the
lungs, it's fully in the R-state
and it's full of oxygen.
Now, this graph, I'm a biochemist
so I have to show graphs.
This graph shows the binding of oxygen
for hemoglobin compared to myoglobin.
Remember that myoglobin is like
hemoglobin but it only has one subunit,
whereas hemoglobin has four,
two alphas and two betas.
If we look at the graph,
we see on the Y axis
that the oxygen saturation of the
molecule being studied is plotted.
On the X axis is plotted the
concentration of oxygen
that each of these units is placed in.
And we can see that myoglobin has
a very different binding curve
for oxygen than hemoglobin does.
We see that very low
concentrations of oxygen
as shown in the left
part of the graph.
Myoglobin has a pretty high
percentage of oxygen bound to it.
If we compare that to hemoglobin,
we see that at the same
where myoglobin has 50%,
hemoglobin has much less oxygen bound.
Maybe 10% or less of its oxygen is bound
When we compare that to the high oxygen
concentrations seen on the right,
we see that both curves have about 100%.
Now, what this tells us is that
myoglobin grabs oxygen really
readily and holds on to it,
and hemoglobin's affinity for
oxygen changes as a function
of the concentration of oxygen
that's found in the cell.
Now, that's important because
at high oxygen concentrations,
you want hemoglobin to load
up such as found in the lungs.
At lower oxygen concentrations such
as found in the rest of the cell,
we want to oxygen to let go of that
oxygen and give that oxygen to cells.
This graph shows very
clearly that hemoglobin
is much better suited for delivering
oxygen than myoglobin is.
So what's myoglobin good for?
Well, myoglobin is what we
call an oxygen storage protein
and it's found in our muscles.
That’s the myo- part of the myoglobin.
Myoglobin is really good at
grabbing oxygen when it's available
and holding onto it for muscles when
the muscles really need oxygen.
So, when you're out running that marathon
or you're doing that really fast race,
your muscles very quickly
run out of oxygen
and your muscles can --
need oxygen faster than
your blood can supply it.
Well, that's where you want to
have a backup supply of oxygen
and that's where myoglobin comes in.
Because as the muscles get to
very low oxygen concentration,
that's when myoglobin
gives up its oxygen.
It's good for those emergency situations,
but it's not good for regular
Hemoglobin is much better.
Now, myoglobin's curve has a
shape that we call hyperbolic
as you can see on the
label on the graph,
whereas hemoglobin's curve is called
sigmoidal, and that is sort of S shaped.
And these distinct shapes
really tell us a lot as I said
about the differences of the binding of
myoglobin and hemoglobin for oxygen.
Now, I want to show you a principle
that shows in the graphs I'm
going to show you in a minute
that I want you to keep in mind.
As we see graphs like this one
moving from the left, myoglobin,
to the right for the same
thing for hemoglobin,
what this means is that the molecule that
corresponds to the graph on the right
is exhibiting less affinity for oxygen than
the graph for the molecule on the left.
So, at higher oxygen concentrations,
it takes more oxygen to get hemoglobin
bound with oxygen then it does myoglobin.
And at low oxygen concentrations,
myoglobin is great at holding on to it.