Question

What is the oxidation state of iron in haemoglobin?
(A) $F{e^ - }$
(B) $F{e^{2 + }}$
(C) $F{e^{3 + }}$
(D) $F{e^{4 + }}$

Answer 1

Haemoglobin is recognized as a protein that is present in the red blood cells or erythrocytes. It is required for the transport of oxygen from the blood to the tissues, and carbon dioxide away from the tissues back to the lungs for excretion. It is an iron containing oxygen transporting pigment.

Complete answer:
Haemoglobin is the important pigment present in the blood which is responsible for carrying oxygen and carbon dioxide. It imparts bright red colour to the blood due to the presence of iron atoms in it. The oxygen carrying capacity of haemoglobin relies on the oxidation number of an atom existing in the protein. Each globin unit of the haemoglobin subunit possesses an atom of iron with a heme crew which can bind one atom of oxygen.

The protein can consequently bind and attach 4 atoms of oxygen at a time. The attachment of oxygen to haemoglobin is cooperative. That is, as every ion binds, it now results less difficult to bind the next one as the form of the protein alters. At complete oxidation, the blood is bright red in colour.
Now let us see the oxidation state of iron in haemoglobin.

A) $F{e^ - }$ : the iron ions usually donate the electrons as it has two electrons in its outer orbit. So it does not exist in a negative oxidation state.
B) $F{e^{2 + }}$ : The oxygen anion exists in the charged state of $O_2^ -$ that includes the negative charge. In order to connect with this, the iron cation has to correspondingly be in the oxidative state of ferrous $Fe(II)$ iron to make the compound neutral. There are four atoms of iron in a molecule of haemoglobin which can bind with 4 molecules of oxygen. So the right answer is choice B.
C) $F{e^{3 + }}$ : Iron does exist in this oxidation state of three as well, but no longer in the case of haemoglobin as it cannot coordinate with oxygen atoms.
D) $F{e^{4 + }}$ : Iron does not exist in the valency of 4.

Thus the correct answer is option ‘A’.

Note: The haemoglobin has high affinity for carbon dioxide as well. Haemoglobin can also get attached to carbon monoxide very efficiently. If the stages of $CO$ in the environment rise, carboxyhemoglobin is fashioned as a result, and oxygen cannot bind.