Now, another important thing to be transport in the body is that of oxygen,
oxygen of course is necessary for respiration and as will be discussed in another module,
oxygen is necessary for efficient production of ATP.
Muscles are very, very dependent upon oxygen, when you're exercising for example.
Well, if we think about what happens in the body, the lungs are very well-prepared to handle oxygen there
but leaving the lungs and going to tissues that are far away, it's important to have a system,
the system that we use as hemoglobin to carry the oxygen to those tissues.
Now, there's some important consideration about what hemoglobin has to do
in that process and so part of that is illustrated in this slide here.
What you see on the screen is a plot of the oxygen saturation
or you can think of a oxygen carrying capacity of a protein.
The protein Myoglobin is showing in blue and Hemoglobin is showing in green.
Now, Myoglobin is also an oxygen carrying protein.
It's not really used much for carrying though, it's used mostly for storing oxygen in muscle cells.
So in muscle cells, what Myoglobin does, is it grabs oxygen when it's available and it holds on to it
and when the oxygen concentration gets really, really low,
Myoglobin lets go of it and gives it to the muscle.
So Myoglobin acts a bit like what we described is an oxygen battery.
A battery of course provides electricity when the electricity isn’t there.
The Myoglobin provides oxygen when the oxygen isn't there.
If we look at the oxygen binding tendencies for a Myoglobin,
we see the curve rising very sharply to begin with.
And at about two millimeters of mercury, we see that Myoglobin is 50% saturated with oxygen.
Meaning it takes very little to get saturated but that also means the flip side
is not gonna let go very much until the oxygen is almost completely gone.
Well, that's good if the cell is desperate but cells don’t like to live in a desperate state all the time.
Hemoglobin by contrast has a different profile for binding oxygen than Myoglobin does.
We can see that hemoglobin curve is shifted considerably to the right and instead of being a hyperbola as we see from Myoglobin.
The Hemoglobin curve is what we call sigmoidal, it looks like an S-shape.
The S-shape curve for Hemoglobin indicates that it actually has two different binding affinities for oxygen.
At low oxygen concentrations such as we find in tissues, the Affinity -
the Hemoglobin has for oxygen is low, that means that when hemoglobin is traveling through tissues,
it's letting go of oxygen much more easily than Myoglobin was doing,
that's the gap between the two curves near the bottom.
As we move to the upper right, we see that hemoglobin’s affinity for Oxygen is almost up to a hundred percent.
Meaning that it's almost the same as Myoglobins and this makes very good sense as well
because when hemoglobin is in an environment when there’s a lot of oxygen,
you want it to bind to oxygen tightly and that’s what happens as hemoglobin is traveling through the lungs.
So hemoglobin changes depending upon the environment in which it finds itself.
How does this change actually happen? Well, it turns out that hemoglobin can do something that myoglobin can't do.
Myoglobin has one protein sub-unit and hemoglobin has four protein sub-units
and because of those four protein sub-units.
The sub-units can affect each other after one oxygen has been bound.
Now this is a phenomenon known as Cooperativity and I'll show this in the next slide.
In the next slide, we can see hemoglobin starting on the left where it's already dumped off its oxygen,
this might be a hemoglobin for example that is passed through the tissues and giving the oxygen away.
It's drawn in squares because as squares, hemoglobin has very little affinity for oxygen.
Moving to the right, we see first of all that one of the squares has converted to a blue circle that now contains oxygen.
This could happen for example as the hemoglobin is approaching the lungs.
The square has converted to a circle and the circle you can see is interacting
with two other portions of the hemoglobin that were previously squares.
They’re starting to become rounded.
The binding of one oxygen has favored changes in the adjacent proteins such as -
they are much more likely to bind oxygen than they were before.
So the binding of one oxygen favors the binding of a second oxygen and the binding of a second favors the third
and the binding the third favors the fourth.
So because of this hemoglobin gets loaded up with oxygen as its passing though the lungs
and then the process reverses as it goes back out through the tissues.
It's a remarkable flexibility and remarkable ability of hemoglobin to make this process happen.
Myoglobin can't do it because it has a single unit and there's no communicating that it can possibly do.