MACROMOLECULES 



117 



6 coordinate covalent bonds {d~sp^) and the appearance of the octahedral 

 structure. Oxyhaemoglobin and carboxyhaemoglobin are diamagnetic 

 (same structure as that of the ferrocyanide ion). The detachment of one of 

 the imidazole groups from the complex and the transformation of the ionic 

 bond of the other group into a covalent bond, brings about a change in the 

 pK of the two imidazole groups thus explaining the Haldane effect (change 

 in the isoelectric point of haemoglobin at the moment of its oxygenation). 



I 

 I 



;Fe-' 



N globin 

 Oxyhaemoglobin 



N globin 

 Carboxyhaemoglobin 



Haemoglobin can be oxidized to methaemoglobin or ferrihaemoglobin 

 by oxidizing agents other than oxygen. Methaemoglobin is a parahaematin 

 of a special type since it contains 5 unpaired electrons, therefore all the 

 bonds in the complex are electro valent. 



Haemoglobin possesses the unique property of complexing reversibly 

 with molecular oxygen instead of being oxidized by it to ferrihaemoglobin. 

 It is endowed with this property by the globin which forms with ferro- 

 porphyrin a complex, which is rather unusual since, unlike other haemo- 

 chromogens, it is paramagnetic. 



The characteristics of the binding of protohaem with a special type of 

 protein confers on haemoglobin the property used by organisms in many 

 ways — that of being reversibly oxygenated and deoxygenated without 

 change in the valency of the iron, which remains in the ferrous state. 



Oxyhaemoglobin, like the other haemochromogens, has two absorption 

 bands in the visible range, in addition to the Soret band which is situated 

 in the ultra-violet (Fig. 16). Deoxygenation transforms the two-band 

 spectrum into a one-band spectrum. 



5. Chlorocriiorin 



Chlorocruorin, which acts as an oxygen carrier in certain types of Anne- 

 lids, is a derivative of chlorocruorohaem. This latter substance is the haem 

 of chlorocruoroporphyrin or Spirographis-porphyrin (from the name of the 

 worm {Spirographis) whose blood is most commonly used as a source of 

 chlorocruorin) (porphin- 1,3,5, 8-tetramethy 1-2-f ormyl-4- viny 1-6, 7- propi- 

 onic acid). In fact, it is derived from protoporphyrin by oxidation of the 

 vinyl group at position 2. 



