• The human body contains about 4g of iron. About
70% of this is found as hemoglobin , the red
pigment in the erythrocytes (red blood cells).
• Each hemoglobin molecules consists of four iron
containing parts(heme) and four protein
• The fact that blood contained iron was discovered
in 1747 by Menghini who showed that if blood was
burnt to an ash,iron like particle could be
extractred by a magnet
Structural feature of hemoglobin
• Hemoglobin has a molecular weight of nearly
65000, and is made up of four subunits. Each
subunit comprises a porphyrin complex heme
which contains Fe2+ bonded to four N atoms, and a
globular protein called globin.
• Each hemoglobin molecule is made up of four
globin proteins subunits, two α and two β.The
globular protein is coordinated to the Fe2+ in heme
through a fifth N atom from a histidine molecule in
• The sixth position around the Fe2+ is occupied
either by a dioxygen molecule or a water molecule.
Fe(II) Porphyrins as dioxygen carriers
Fe(II) porphyrins without the protein chain (also called
naked heme) cannot act as oxygen carriers. They react
irreversibly with the dioxygen in aqueous environment
forming the µ-peroxo dimer having Fe(III).This peroxo
dimer reacts with another heme to give the oxo-
bridged dimer called hematin. Hematin is unable to
Role of the Polypeptide chain in
The globin besides acting as a carrier of heme in hemoglobin
also plays a role in stabilizing the heme-O2 complex so that
oxidation of Fe(II) of heme does not take place and it can act
as an effective dioxygen carrier. The polypeptide chains α and
β are folded in such a manner that hydrophobic residues are
inside and most polar residues are on the exterior. The
polypeptide chain prevents the irreversible dioxygen oxidation
of Fe(II) in two ways.
• The hydrocarbon environment around the iron has a low
dielectric constant and is hydrophobic and therefore act as a
non-polar and provides non-aqueous environment.
• It provides steric hindrance and does not allow the
formation of hematin.
Hemoglobin is made up of four subunits, and when one subunit
picks up an O2 molecule, the Fe2+ contracts and moves into the
the plane of ring. In doing so, it moves the histidine molecule
attached to it, and causes conformational changes in the globin
chain. Since this chain is hydrogen bonded to the other three
units, it changes their conformations too,and enhances their
ability to attract O2. This phenomenon is called the cooperative
The Four irons can each carry one O2, with generally increasing
CO2 is the end product from the
breakdown of glucose to release energy.
There is an appreciable build up of CO2 in
the muscles. This is removed from the
tissue and converted into soluble HCO3
As the concentration of CO2 increases,
formation of biocarbonate causes the pH
to decrease and the increased acidity
favours release of O2 from the
oxyhemoglobin, called the Bohr effect.
Oxygen Dissociation Curve for
• Oxygen Dissociation Curve (or
oxygen saturation curve) is a plot of
percentage saturation with
dioxygen versus the partial
pressure of dioxygen. For
hemoglobin the dissociation curve
is sigmoidal in shape. As seen in
the figure at low partial pressures
of dioxygen, hemoglobin does not
bind dioxygen effectively. At high
partial pressures of dioxygen,
hemoglobin has high percentage
saturation with dioxygen. The
oxyhemoglobin dissociation curve
helps in understanding of how
blood carries dioxygen from the
lungs to the tissues.
Functions of hemoglobin
The Function of hemoglobin is to pick up dioxygen at the
lungs. The arteries carry blood to parts of the body where
oxygen is required such as the muscles. Here the oxygen is
transferred to a myoglobin molecule, and stored until the
oxygen is required to release energy from glucose sugar.
When O2 is removed from hemoglobin it is replaced by a
water molecule. Next the protein part of hemoglobin absorbs
H+. Indirectly this helps remove CO2 from the tissues, since
CO2 is converted to HCO3
- and H+. The blood removes the
more soluble HCO3
- ions and the reduced hemoglobin
removes H+. The blood returns to the heart through the veins.
It is then pumped to lungs, where the HCO3
- ions are
converted back to CO2. This is excreted into the air in the
lungs and exhaled. The blood picks up dioxygen again, and the
process is repeated.