JEFFRIES WYMAN, JR. 



groups, as distinct from the overall heat of oxygenation. The equation 

 involved is 



(Sf)r — *»«■(¥)« ....(5) 



where AX denotes the number of protons dissociated as a result of 

 introducing one molecule of oxygen into the haemoglobin molecule 

 at constant pH. 



The left-hand member of this equation gives the change in the heat 

 of oxygenation with the change in the number of protons dissociated 

 as a result of oxygenation, and may therefore be interpreted as the heat 

 of dissociation of those protons per equivalent. Now the experiments 

 show that the curves of AX vs pH are simply displaced along the pH 

 axis, without change of shape, by changing the temperature. This 

 means that the right-hand member is constant. Its value, the same 

 over the whole pH range, is obtained from the size of the displacement 

 as 6,500 calories, though this is subject to considerable uncertainty due 

 to the fact that it is based on second order effects. It has been inter- 

 preted however, in connection with other evidence, to mean that both 

 the acid groups associated with each haem are imino groups of histidine, 

 6,500 being a characteristic value for the heat of ionization of that 

 group. The more essential thing however in the present connection 

 is not the identification of the two oxygen linked groups, but the 

 evidence that they are the same for each haem. The two lines of 

 evidence leading to this conclusion, that based on the pH effect and that 

 on the temperature effect, are, taken together, fairly convincing.* 



A historically important step in the theory of the oxygen equilibrium 

 of haemoglobin was taken by A. V. Hill when he pointed out that 

 the sigmoid curve of Y vsp could be fitted, approximately at least, by 

 the equation 



Kp n 



Y = 



l+Kp* ....(6) 



* While the reasoning involved in this argument is rigorous, it must not be forgotten 

 that the data to which it has been applied are subject to experimental error, and the 

 possibility of slight changes of shape of the oxygen equilibrium curve (Y vs. log p) with 

 pH and temperature, reflecting real differences between the acid groups linked with different 

 haems, cannot be categorically ruled out. This point was emphasized by Professor 

 Roughton in discussion at the conference. Considerable differences in the heat of dis- 

 sociation between different haem linked acid groups need involve only a slight temperature 

 dependence of the shape of the equilibrium curve. All we can say is that there has been 

 so far no indication of any change of shape of the curve with either pH or temperature, 

 and that if there is any such change it lies within the limits of error of the extensive 

 equilibrium measurements. 



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