Question
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Haemoglobin shows maximum affinity with
a. Carbon monoxide
b. Carbon dioxide
c. Oxygen
d. Ammonia

Answer
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Hint: Haemoglobin is roughly spherical, tetrameric protein containing four heme prosthetic groups. It has higher affinity for smaller molecules.

Complete answer:
Haemoglobin is a globular protein consisting of two types of globin as two alpha chains and two beta chains. It is bound with a prosthetic group as heme. Heme consists of a complex organic ring structure which is bound with an iron atom. Iron has six coordination bonds, four with nitrogen atoms as a part of a porphyrin ring and two perpendiculars to porphyrin. Haemoglobin has higher affinity for smaller molecules like Carbon Monoxide, Nitrogen Oxide etc.

Carbon monoxide has a higher affinity to bind with haemoglobin. The reason is the electron coordination between the heme iron and Carbon monoxide is perpendicular to the heme plane. It forms carboxyhemoglobin when combined with Carbon monoxide.

When Oxygen binds to haemoglobin, the coordination of Oxygen to the iron is not perpendicular to the heme plane and so is significantly weaker. The reason behind this is the steric hindrance between two oxygen atoms. This phenomenon is not seen in Carbon monoxide.

Haemoglobin has less affinity for Carbon dioxide as compared to oxygen because Carbon dioxide is a larger molecule than oxygen. After binding, structure will not stabilize as oxygen atoms will show strong steric hindrance.

Ammonia is also having less affinity because it is also a larger molecule. Ammonia in blood is toxic for blood cells as it increases formation of methaemoglobin in blood which reduces oxygen carrying capacity of blood.

Thus Carbon monoxide has the highest affinity with haemoglobin as compared with oxygen, carbon dioxide and ammonia.

Hence, The correct answer is, option (A).

Note: Haemoglobin has higher affinity for Carbon monoxide because the coordinate bonds formed with carbon monoxide are perpendicular to the porphyrin ring in the heme structure. This structure is favourable for haemoglobin molecules. Thus, it has higher affinity with Carbon monoxide. When haemoglobin combines with Carbon monoxide, its oxygen carrying capacity is reduced.