Hemoglobin (Hb) and myoglobin
oxygen binding proteins.
is found in the red blood
It carries oxygen from the lungs to
found in muscle, primarily
It stores oxygen for use during exercise.
Properties of gasses.
The concentration of gasses is
partial pressure (Torr = mm Hg).
of a mixture of gasses
is equal to the sum of the partial
pressures of each gas. (Dalton’s Law).
The atmospheric pressure is 760 Torr.
oxygen, and therefore the
partial pressure of oxygen in the air
is 152 Torr.
pressure of oxygen in the lungs
is 100 Torr due to mixing of inspired
air with residual air in the lungs.
pressure of oxygen in the tissues
varies with the tissue, but in skeletal
muscle it averages about 20 Torr.
Structure of Hemoglobin
Being an oligomeric protein, Hb has
Myglobin consists of a single amino
acid chain, similar in structure
to both subunits of Hb.
of Hb (and Mb) has a heme
ring and an iron ion (Fe+2) as a
The heme ring is a tetrapyrole.
Thus, each Hb can bind 4 O2.
Oxygen binding by Hemoglobin.
If we plot the oxygen binding
the x-axis goes from zero, through 20
to 100 Torr.
Conceptually we are dealing with a large
number of hemoglobin molecules
inside a single red blood cell.
If we were dealing with a single Hb
molecule, there would only be five
discreet values possible, 0, 25, 50,
75, and 100 %.
Hemoglobin/Oxygen dissociation curve:
We can define the P50, which is the
partial pressure of oxygen at
which Hb is 50% saturated.
The P50 resembles the Km in that
it is inversely related to the
affinity of Hb for oxygen.
curve has an S-shape,
implying that Hb has properties
similar to allosteric enzymes.
One such property is cooperativity of
binding. This means that as one oxygen
binds it makes it easier for the next, until
the molecule is saturated with oxygen.
curve is not sigmoidal, indicating that
there is no cooperativity of binding.
The Hill Coefficient quantifies the
degree of cooperativity.
Hb has a Hill Coefficient of about
3.0, indicating cooperativity.
Mb has a Hill Coefficient of exactly
1.0, indicating no cooperativity.
In the lungs, at 100 Torr, all Hb
RBC is saturated with oxygen.
As the RBC leaves the lungs, and
lower partial pressures of oxygen, the Hb
molecules release their oxygen, and
thus become less saturated.
in the delivery of oxgen to the
tissues. In the tissues, the Hb in the
RBC is about 20 % saturated,
corresponding to an 80% oxygen delivery.
Note that at 20 Torr. Mb is
saturated, so as Hb releases its oxgen
the oxygen is taken up by Mb.
Role of Bisphosphoglycerate (BPG).
The cooperativity of Hb is
the presence of BPG. If Hb is stripped
of its BPG, its dissociation curve
resembles that of Mb.
Note that under these conditions,
be useless as an oxygen transport protein in
the body, because it would deliver
almost no O2.
Effect of pH.
Like allosteric enzymes, Hb is
its environment. As the pH decreases,
the Hb/Oxygen dissociation curve shifts to
the right, i.e. the P50 increases.
This is sometimes called the Bohr effect.
This reduces the affinity of Hb for O2.
is to increase oxygen delivery
to the tissues.
Increasing the concentration of CO2
cause the same shift (Root effect).
CO2 + H2O ßà H2CO3 ßà H+ + HCO3-
can thus be seen as just
an extention of the Bohr effect.
Increasing the BPG concentration
the same shift, and this is important in
acclimatization to high altitude.
Fetal hemoglobin consists of two
subunits and 2 gamma subunits.
Its Hb/O2 dissociation
curve is just
left of the adult Hb curve.
Hb has a slightly higher affinity
for oxygen, and oxygen is pulled from
the maternal circulation across the
placenta into the fetal circulation.