Haemoglobin belongs to the class of conjugated protein. The iron-containing pigment portion haem constitutes only 4% and the rest colourless protein of the histone class globin. Hemoglobin has a molecular weight of 68,000 and contains four atoms of iron, each of which can combine with a molecule of oxygen.
Methaemoglobin If the iron component of the haem moieties is in the ferric state, then the normal ferrous state, is called methemoglobin. Methaemoglobin does not bind O2. Normally RBC contains less than 1% methemoglobin.