Question

Which is the oxidation state of iron in haemoglobin?
A) $Fe^{-}$
B) $Fe^{2+}$
C) $Fe^{3+}$
D) $Fe^{4+}$

Answer 1

Haemoglobin is a protein found in red blood cells or erythrocytes. It is essential for the transport of oxygen through the blood to the tissues, and carbon dioxide away from the tissues back to the lungs for excretion.

Complete answer:
The oxygen carrying capacity of haemoglobin depends on the oxidation state of an atom of iron found in the protein. Each globin unit of the haemoglobin subunit carries an atom of iron with a heme group which can bind one atom of oxygen. The protein can therefore bind four atoms of oxygen at a time.

The oxygen anion exists in the charged state of $O^{2-}$ . In order to bind with this, the iron cation has to correspondingly be in the oxidative state of $Fe^{2+}$ or ferrous Fe (II) iron.The correct answer is option B.
Iron ions are positively charged, or cations. Option A is incorrect.

Option C is ferric or Fe (III) iron. Iron does exist in this oxidation state as well, but not in the haemoglobin molecule. Option C is incorrect.

Iron does not take the valency of 4+ therefore option D does not apply at all and is incorrect.
The binding of oxygen to haemoglobin is cooperative. That is, as each ion binds, it becomes easier to bind the next one as the shape of the protein alters. The more oxygenated haemoglobin is, the brighter the shade of red.

Hence the correct answer is option ‘C'.

Note: Haemoglobin can also bind carbon monoxide very efficiently. If the levels of CO in the environment rise, carboxyhemoglobin is formed, and oxygen cannot bind. If not reversed, this state can lead to death by asphyxiation.