One of the most important and most interesting
proteins in our body is hemoglobin,
the molecule that carries
oxygen throughout our tissues
and supplies the oxygen that
our cells need to be alive.
Now, animals have very widely
varying needs for oxygen.
They're very different
from plants for example.
The demand for oxygen
can change in seconds.
The basal needs of oxygen that
are cells have are significant,
and so we can't rely on
oxygen that gets through
our tissues by diffusion
like insects can do.
issues and other things like
exercise add to the need that
our body has for oxygen.
Now, the reason oxygen is important is because
ATP, the sort of gasoline of our body,
ATP energy is produce 15
times more efficiently
when oxygen is present than
when oxygen is absent.
That is anaerobically.
So respiration using oxygen is much
more efficient that fermentation
which is what happens during the
anabolic or the anaerobic process.
Efficient, adaptable oxygen
deliver is therefore essential
and fortunately we have that
in the form of hemoglobin.
Now, hemoglobin has a structure
like you can see on the left.
It has four individual polypeptide
chains: two identical
units shown in blue and two
identical units shown in red.
These are known as alpha 2 beta 2:
two alpha units and two beta units.
And they're very, very similar in
structure, but they're not identical.
Each of those individual units of hemoglobin
contains a molecule known as heme.
And heme is the portion of the hemoglobin
molecule that carries the oxygen.
It actually physically holds
onto it using an atom of iron.
Myoglobin is a related compound.
Structurally, it's very similar to the
one of the four units of hemoglobin.
In fact, it's similar to
all four because the betas
and the alphas are very
similar to each other.
But myoglobin and only having
one unit, only has one heme.
And that gives myoglobin some
very different properties than
hemoglobin even though the proteins
themselves are very similar.
Now, each heme unit in both hemoglobin and
myoglobin binds one molecule of oxygen.
Hemoglobin can therefore bind
a total of four and myoglobin
can only bind a total of
one molecule of oxygen.
The heme unit that I've just described is
what we describe as a prosthetic group.
A prosthetic group in a
protein is non-amino acid
that is part of a protein that helps
expand the function of a protein.
Now in the case of
the heme unit contains at its
very center an atom of iron.
This is known as ferrous iron
or what we call iron plus two.
If we have iron plus three,
which is known as ferric iron,
the heme will not be functional
and will not carry oxygen.
So it's very important that we
maintain our heme with the iron in the
ferrous state because only the
ferrous form will bind that oxygen.
Now I want to spend a little
bit of time talking about
the structure of that heme
group because it turns out
that the structure of that heme
group is essential for the
really interesting function
that hemoglobin performs.
Now on the last
slide I showed you,
the hemoglobin face forward looking
like this with the iron in the center.
This slide shows the same molecule of heme
but now has rotated so that it's flat.
And we see the heme iron in the
very center of that structure.
Beneath that, we see a molecule labeled His,
H-I-S and that His stands for histidine.
It's one of the amino acids
in the hemoglobin protein.
And by the way, the protein part
of hemoglobin is called globin.
So the globin contains
And this histidine is covalently linked to
the iron atom in the middle of that heme.
Now this slide shows what
happens when oxygen comes
in and binds to that iron
in the center of the heme.
We can see oxygen coming
in with the arrow.
And the oxygen is shown in the figure
on the right as two green balls.
That's molecular oxygen or two
atoms of oxygen held together.
The binding of oxygen to that iron
atom in the middle of the heme
causes a very slight change in
the position of that iron atom.
As you can see in the figure, the iron
atom is moved upwards very slightly.
Well, since the iron atom is attached
to the histidine amino acid below it,
that means that the histidine is
also lifted a very small distance.
Well, histidine is
not there by itself.
It's attached to the rest of the
globin protein part of hemoglobin.
That means that globin subunit,
either the alpha or the beta,
and this happens for both, so
it doesn’t happen for each one.
What happens is that globin subunit
gets changed very slightly in shape.
And that change in shape has dramatic effects
on the hemoglobin molecule as a whole.