The Haem-Globin Linkage 



191 



proton addition to the latter groups. There is no reason why A// for metal-ligand 

 bonding should be the same as for proton addition to the ligand. 



Phillips: The conclusion reached in part 4 of O'Hagan's summary seems not only un- 

 justified but also contrary to the facts. One cannot deduce the ^K of a ligand from 

 the pH at which the co-ordination complex is half dissociated without knowing the 

 stability constant of the complex and the concentration of the various species: One 

 can be certain, however, that the pA" will be greater than the pH of half-dissociation 

 and all one can deduce from O'Hagan's results is that the pAT of the co-ordinating 

 group is > 5 which could well be histidine (pAT — 7) or even lysine (pAT '~ 9) but is 

 unlikely to be a carboxyl group (pAT — 5). 



O'Hagan: It is true that the pAT values I have suggested are not exact; they were not 

 intended to be. I was not attempting to obtain an absolute value under these con- 

 ditions, but one which would at least indicate the most likely group ionizing. In 

 regard to Phillips' point, curve (a) shows some preliminary results on the dissociation 

 of horse oxy haemoglobin in acid buffers. There is evidence that at pH 50 groups are 

 dissociating. These cannot be the haematin side-chain carboxyls, the evidence points 

 to the groups linked to the iron. At pH 4-9, Ferry and Green (/. biol. Chem. 81, 175, 

 1929) found horse haemoglobin stable enough to obtain an oxygen dissociation 

 curve. If we examine the curve for the apparent heat of dissociation of horse oxy- 

 haemoglobin of German and Wyman {J. biol. Chem. 117, 533, 1937), we can see that 

 at pH 5-0 the apparent heat of dissociation is about — I kcal. If there were imidazole 

 groups ionizing we would expect a value of about 6 kcal. The value found would 

 appear to indicate carboxyl groups are linked to the iron atoms. 



4- 



3- 



. 



The Haem-Globin Linkage 



(a) Change in the Soret peak absorbance at 410 m/< of horse oxyhaemoglobin 

 in phthalate buffers (pH 4-0-6-2, I = 005). 



(b) Curve for apparent heat of dissociation of horse oxyhaemoglobin from 

 German and Wyman {J. biol. Chem. Ill, 533, 1937). 



