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HAEMOGLOBIN

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Haemoglobin (or Hemoglobin) = Haem group + Globin protein

Haem groups are iron containing porphyrin ring structures, also found in other pigments (e.g. cytochromes and the enzyme catalase). In chlorophyll the porphyrin ring contains magnesium although the molecule is first constructed around an iron atom.

 

The Polypeptide Subunits

The haemoglobin molecule is made of four polypeptide subunits each with a haem group. One molecule can therefore combine with four oxygen molecules. In the adult haemoglobin there are 2 α and 2 β subunits. In the foetal haemoglobin there are 2 α and 2 γ subunits. (Also in embryonic haemoglobin 2 α and 2 ε subunits).

α subunit = 141 amino acids

β subunit = 146 amino acids

γ subunit = 146 amino acids (37 different from the b chain)

 

Haemoglobin and Red Blood Corpuscles

Haemoglobin has a relatively small molecular weight for a respiratory pigment. It is advantageous to package it in red blood corpuscles for at least two reasons. The haemoglobin floating free in the plasma would increase the colloidal osmotic pressure of the plasma making it very viscous. Also the chemical environment of haemoglobin can be more carefully controlled if it is found inside a red blood corpuscle.

 

Oxygenation of the haem group

Haem group

NOTE: The iron remains as Fe2+ THIS IS NOT AN OXIDATION BUT AN OXYGENATION.

Oxygenation causes the two β subunits to move together which makes successive additions of oxygen easier and easier. The opposite happens when the oxygen molecules dissociate from the haemoglobin. This gives rise to the sigmoid dissociation curve (opposite).

Oxygen and haemoglobin

Oxidation of the iron ion can occur: Fe2+ Arrow right Fe3+ giving methaemoglobin (a dark coloured form of haemoglobin). This reaction takes place in the presence of nitrites. The reaction is made reversible by the enzyme NADH methemoglobin reductase present in the red blood cells.

NOTE: The iron ion in the cytochrome haem group is oxidised and reduced under normal functioning in the electron transport chains.

 

Carboxyhaemoglobon

Carbon monoxide has a has an affinity for haemoglobin which is 210 times greater than that of oxygen.

CO + Hb Arrow right COHb Carboxyhaemoglobin (cherry red coloured)

Car exhaust contains up to 6% CO. Death occurs when 70% to 80% of the haemoglobin is converted to COHb.

COHb will dissociate very slowly so the treatment of carbon monoxide poisoning is adequate ventilation preferably with pure oxygen.

 

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