THE DISSOCIATION CURVE OF HEMOGLOBIN 115 



in equilibrium with the atmosphere, COg will be driven off. If COg 

 be put into the solution the whole chain of events will shift in the 

 opposite direction. 



It now remains to discuss more completely the possible mechanism 

 by which increased hydrogen-ion concentration affects the oxygen- 

 binding power of the haemoglobin. No doubt in a complex agglomera- 

 tion of amino-acids such as make up the protein molecule, there are 

 many hydrogen ions which can be replaced by sodium — just how 

 many, has been a matter of some dispute : but Henderson and Hill 

 are in agreement in supposing that not all these are of equal value 

 in affecting the affinity of haemoglobin for oxygen. Henderson (16) 

 supposes that for each molecule of oxygen there is one particular 

 hydrogen ion, situated in a special position in the protein molecule, 

 near to where the haematin and the protein join; which ion is the 

 only one affecting the oxygen-binding power. Hill(i4), (15) has sug- 

 gested as an expansion of his theory that in the molecule of haemo- 

 globin, (Hb)„, there is only one relevant hydrogen ion for n 

 molecules of dissociable oxygen. If that atom be replaced by base, 

 the haemoglobin unites with oxygen much more readily than if the 

 critical hydrogen ion is itself present. 



The theories of Hill and Henderson may account for the fact 

 that the curves for haemoglobin which Means and I obtained are much 

 less spread out than those for blood; it seems doubtful whether in 

 the absence of base, hydrogen-ion concentration would have any 

 effect at all on the affinity of haemoglobin for oxygen. The solution 

 used by Means and myself (9) certainly contained very little base as 

 compared with blood. 



It is doubtful whether we can speculate further on this subject 

 with profit. It will be shown later that in any case it is only in a 

 certain range of hydrogen-ion concentration that the characteristic 

 effect is found. In solutions more acid than pH =5-5 and more 

 alkaline than pK = 7-6 it does not exist. 



The mention of one particular hydrogen ion being on a different 

 level of importance from the rest raises the question of counting the 

 number of free hydrogen ions which exist in the haemoglobin molecule. 

 This estimation has been carried out by various observers, Straub(22), 

 Parsons (23), A. V. Hill(i4), Adair (24) and others, but most recently 

 the matter has been gone into with great detail and thoroughness 

 at the Rockefeller Institute in New York. The following calculations, 

 which are based by Adair on the data of Hastings, Sendroy, Murray 



8-2 



