CHAPTER XI 



THE DISSOCIATION CURVE OF HAEMOGLOBIN 



{continued) 



X HE further consideration of the graphic representations defining 

 the equilibrium which exists between oxygen, reduced haemoglobin 

 and oxyhaemoglobin must start, in the present state of knowledge, 

 from two assumptions : ( 1 ) that the molecular weight of haemoglobin 

 is of the order of 70,000, each molecule uniting reversibly with four 

 oxygen or CO molecules, and (2) that the basal curve from which 

 others are derived is the hyperbola ; even though we are driven back 

 to CO-haemochromogen in order to be certain of finding it. 



What then is the shape of the curve which represents a reversible 

 reaction between one molecule of haemoglobin and four of oxygen? 

 The answer is not quite simple, because the form of the curve cannot 

 be stated until another matter is settled. Do the molecules of oxygen 

 unite with the haemoglobin molecule one by one, or do they join in 

 the " all-or-none " principle? In the first case the reaction would be 



Hb4i +02:^Hb402, 

 Hb402 + 02^^Hb404, 



Hb404+02 5^Hb406, 



Hb406 + 02 5=?:Hb408. 

 In the second case the reaction would simply be 

 Hb4 + 402:5^Hb408. 



In the last case any molecule which is stable must be either Hb4 or 

 Hb408; the intermediate molecules may be formed, but if so they 

 are less stable than those in reduced and fully oxidised molecules 

 and therefore cannot remain in their presence. 



On general grounds it is much more likely that the oxygen molecules 

 join seriatim. On the other hand no one has ever isolated, or even 

 discovered cogent evidence of the existence of, the bodies Hb40a, 

 Hb404andHb406. • 



But to return to the curves, what sort of curves depict the mathe- 

 matical representation of these two alternative forms of reaction? 



^ By Hb^ is meant four hsematins attached probably to one globin. 



