So, we've seen hemoglobin does
some really remarkable things.
One of the things that I have not talked about
is how hemoglobin is affected by carbon
dioxide, and that's what I want to spend just
a little bit of time talking about here.
Carbon dioxide is moved in the
body by a couple of processes,
one of which
Now, I want to emphasize first of all, that
carbon monoxide can bind to the heme unit
and compete with oxygen because it has a
shape very much like an oxygen molecule.
Carbon dioxide cannot do that.
It binds to a different
portion of hemoglobin.
So what I'm talking about here
is not competing with oxygen,
but rather binding to another
part of the hemoglobin molecule.
That binding the other part of the
hemoglobin also causes some changes
in hemoglobin which is why the
hemoglobin releases the oxygen molecule.
Well, let's look at the path that a carbon
dioxide molecule takes in going from a
rapidly metabolizing tissue to get back
to the lungs so that it can be exhaled.
We see carbon dioxide
at the very top
and we see the carbon dioxide
can combine with hemoglobin.
And when this happens in
the presence of protons,
oxygen is released and
the carbon dioxide is
bound to the hemoglobin
as you can see here.
That only accounts for a
relatively small portion
of the carbon dioxide that needs
to travel back to the lungs.
The remainder of the carbon dioxide gets
solubilized and it gets solubilized by
an enzyme that converts carbon dioxide
into carbonic acid as you can see here.
The enzyme is known as carbonic anhydrase
and it's a very, very efficient
enzyme producing carbonic acid the
bottom molecule shown right here.
Well, carbonic acid is an acid and at the
pH of the blood readily loses its proton.
When that happens, it becomes the ion
at the bottom known as bicarbonate.
So bicarbonate is a traveling form of
carbon dioxide solubilized in the blood.
A longer travel back to the lungs, the
blood, if it's still carrying oxygen,
as you can see here, can exchange that oxygen
for carbon dioxide through the carbonic
acid reversal pathway to give
additional CO2 and allow
the hemoglobin to pick it
up as we can see here.
When we get back to the lungs, this
hemoglobin that's bound to carbon dioxide,
on the other hand, has a very different
situation that existed inside of the body.
Remember in the lungs that the oxygen
concentration is very high and oxygen
forces its way on to the hemoglobin
that's bound to that carbon dioxide.
The hemoglobin, as you can
see here, gains an oxygen.
And in the process, loses the
carbon dioxide that it had.
So we see the oxygen coming on.
We see the water and we see the
carbon dioxide being released here.
And we also see the carbonic
acid, which is in the form of
bicarbonate, being converted
into oxygen and carbon dioxide.
All of these help favor
the exhaling of CO2.
So hemoglobin carrying carbon dioxide
and bicarbonate dissolved in the blood,
both come together at the very
end as they reach the lungs
and cause carbon
dioxide to be exhaled.
That's critical because carbon dioxide
in high concentrations will kill you.
As I mentioned earlier,
carbon monoxide competes with
oxygen for binding to the
iron atom of the heme group.
Now, this is important because
obviously we want to be
carrying oxygen not carbon
monoxide in our hemoglobin.
And it turns out that there's yet
another histidine that's present
in the environment of the iron as
you can see in the figure here.
In addition to having
histidine below the heme,
which is physically
attached to the iron atom,
there's another histidine above the heme
group that is not attached to the iron.
This above histidine causes
the carbon monoxide to
actually have a lesser
affinity for the heme group.
It doesn’t affect oxygen in the same way
that it affects the carbon monoxide.
Now, this means that because of the
presence of this histidine atom above the
iron atom that the oxygen is much more
likely to bind than carbon monoxide is.
High enough concentrations carbon
monoxide is going to be a problem,
but not as much as it would be
if that histidine wasn't there.
So, even though carbon monoxide is present
and it's present in the lungs of smokers,
for example, it doesn’t reach toxic
enough concentrations unless
you get a higher concentration that
is present in cigarette smoke.
Nonetheless, you don’t want to have
anymore carbon monoxide than necessary.