294 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



The term haemoglobin applied to certain molecules implies that they 

 have a number of common characteristics and among them : 



(a) that they are derived from protohaem; the prosthetic groups of 

 different haemoglobins when combined with a given basic nitrogen com- 

 pound, in general give rise to the same haemochromogen. 



(b) that of being heteroprotein in nature, the protohaem being united to 

 a holoprotein to which the name globin has been given ; 



(c) that of giving a characteristic spectrum, two-banded in the oxygen- 

 ated state and one-banded in the reduced state. 



{d) that of complexing reversibly with molecular oxygen instead of being 

 oxidized by it to a ferrihaemoglobin. (As previously stated (p. 117) this 

 property is due to the globin forming a paramagnetic complex with the 

 ferroporphyrin, whilst the other haemochromogens are diamagnetic). 



(e) that of combining with carbon monoxide to give carboxyhaemo- 

 globin, whose visible spectrum has two bands in positions different from 

 those of the corresponding oxyhaemoglobin ; 



(/) that of being, when oxygenated and in pure solution, transformed 

 more or less slowly into methaemoglobin, containing trivalent iron and no 

 longer capable of being oxygenated, the solution of this compound giving 

 an absorption band in the visible region differing from that given by 

 reduced haemoglobin. 



The relationship implied by use of the term haemoglobin applied to 

 haem-proteins which can be oxygenated and contain the base protohaem, 

 does not preclude them possessing differences which is revealed when a 

 comparison is made. 



Besides the fact that haemoglobin crystals differ from one animal species 

 to another, the amino acid composition of different haemoglobins show 

 clear-cut differences as the examples in Table XIV indicate. 



Table XIV 



(Roche and Jean; Roche and Mourgue) 



Mean amino acid composition of various haemoglobins 



The haemoglobin of Vertebrates is clearly different from other haemo- 

 globins in containing less arginine and cystine and more histidine, lysine 

 and leucine. 



