336 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Biochemical Convergence 



When biochemical constituents are analogues without being isologues, 

 as is the case for haemoglobin and haemocyanin, we say that there is bio- 

 chemical convergence. 



Heterotnorphic Evolution 



The phylogenic isology shown by the chlorocruorins and the haemo- 

 globins of Annelids gives us an example of what can be called hetero- 

 morphic evolution, revealed by the acquisition of a modified constituent 

 of a less complete isology. The haemoglobins with a strong affinity for 

 oxygen appear to be more primitive than haemoglobins of weak affinity, 

 and here we are dealing with a heteromorphic evolution of their protein 

 component (molecule evolution). 



In the higher animals, the dissociation of haemoglobin varies with its 

 degree of oxygenation. The oxygenation of haemoglobin displaces the 

 isoelectric point : in the horse for example, the isoelectric point of reduced 

 haemoglobin is pH 6-78, and that of oxyhaemoglobin is pH 6-65. At the 

 isoelectric pH, haemoglobin fixes small but equivalent amounts of acids and 

 bases, but since the pH in red blood cells is on the alkaline side of the 

 isoelectric point, the haemoglobin is present in combination with bases 

 as the salt. 



At the pH of blood, haemoglobin behaves like a polyvalent acid with at 

 least five acid groups per atom of iron. 



Horse oxyhaemoglobin whose isoelectric point is pH 6-65, contains, 

 among others, an acid function (or groups of functions) which is weakly 

 dissociated with a pK of 6-16. In the corpuscles of oxygenated horse 

 blood the pH is 7-1 and this function for the most part is saturated with 

 bases. The haemoglobin has a less acid character since its isoelectric point 

 is at pH 6-8. This modification of the isoelectric point by a change in the 

 oxygenation is due to a considerable decrease in the dissociation of the 

 acid groups in the region of the oxygen-binding group, as we have des- 

 cribed above. Their pK changes from 6-16 to 7-80. When such a change 

 occurs in a medium whose pH varies hardly at all, there is a resulting 

 liberation of the bases fixed to the acid function. The observation that 

 reduction of blood raises the curve of CO2 absorption is known as the 

 "Haldane effect". It appears to be a character in the evolution of haemo- 

 globin due to a modification of the protein moiety and consequently is an 

 example of heteromorphic evolution. In fact the effect is not observed in 

 the blood of the ray, of Mustelus cants or of Urechis all of which contain 

 haemoglobins the dissociation of which does not vary with the degree of 

 oxygenation. 



The molecular evolution of proteins, as the above examples indicate, 

 is evidently an essential facet of the evolution of organisms. Recent work 



