SOME ASPECTS OF BIOCHEMICAL DIVERSITY 



297 



molecule have different oxygenation constants or the oxygenation of one 

 modifies the constants of its neighbours. In general, the dissociation curves 

 of Vertebrate haemoglobins have a sigmoid form but the degree of flatten- 

 ing varies from one class to another (Fig. 86). 



Fig. 85 (Keilin and Wang) — Dissociation curve of a concentrated solution (1 X 10~' g- 

 mols haematin per litre) of a Gastrophilus haemoglobin. Temperature 39°C. 



One can obtain an idea of the affinity of a particular haemoglobin for 

 oxygen by noting the position of its dissociation curve at the p^Q value, i.e. 

 the partial pressure of oxygen corresponding to 50% oxygenation. 



However it is necessary to compare values oi p^o obtained under com- 

 parable conditions. Changes in temperature and pH displace the dissocia- 

 tion curve. The combination of molecular oxygen with a carrier being an 

 exothermic process, an increase in temperature will lower the affinity, and 

 a decrease in temperature will increase it. Figure 87 illustrates the shift 

 in the dissociation curve of the oxyhaemoglobin of the ray Rata ocellata 

 with temperature. 



The influence which changes in the partial pressure of carbon dioxide 

 (pco,) have on the affinity of oxygen for haemoglobin is known as the 



