JEFFRIES WYMAN, JR. 



shall be concerned mainly with the oxygen equilibrium of horse haemo- 

 globin in its relation to the constitution of the molecule, but we shall 

 also present some considerations involving oxidation.* 



In approaching this problem we may initially take it for granted 

 that the power of oxygen to combine with haemoglobin is due to the 

 presence of protohaem in the molecule, for in solutions of haemo- 

 globin as well as of all other haem proteins capable of uniting with 

 oxygen, e.g. myoglobins and erythrocruorins, the oxygen capacity, in 

 moles, is equal to the number of haems present, and when the haem 

 is split from the protein the power to take up oxygen is lost. In 

 vertebrate haemoglobin there are four haems per molecule, and there- 

 fore four oxygen combining centres. As far as the haem is concerned, 

 these must all be regarded as identical, but this need not be true as 

 regards the local configuration in the globin to which the haem is 

 attached. It cannot be assumed, therefore, without further evidence 

 that the four oxygen combining centres are all alike. Apart from this, 

 the question arises whether or not the four centres are far enough apart 

 in the molecule to be independent of one another, or whether they 

 interact so that the reaction of one affects that of another, and if so, 

 how. Also there is the question whether all the haemoglobin molecules 

 are alike or whether there may be more than one kind, though if so 

 the different kinds would have to be of essentially the same molecular 

 weight (~ 68,000), in view of the fact that all the experiments show 

 that solutions of native vertebrate haemoglobin are monodisperse. 

 These are all considerations which are significant in the problem of 

 interpreting the oxygen-haemoglobin equilibrium. 



Before we proceed to haemoglobin itself, it is instructive to consider 

 the case of myoglobin, which is a closely related but simpler molecule, 

 having a molecular weight of only about 16,000, or approximately one 

 quarter that of haemoglobin, and containing only a single haem. It 

 is satisfactory to find that for myoglobin the oxygen equilibrium, which, 

 unlike that of haemoglobin, is essentially independent of pH, is just 

 what would be predicted for the reaction 



Mb + 2 = Mb0 2 ....(1) 



That is, a plot of Y, the fractional saturation of the protein with 

 oxygen, against p, the activity or partial pressure of oxygen, gives a 

 rectangluar hyperbola, in agreement with the mass law equation 



Y= X? ....(2) 



1 +Kp 



* A more complete general account of thesa and related matters is contained in a recent 

 review by the author, Advances in Protein Chemistry, Vol. IV, New York, 1948, Academic 

 Press. References and detailed arguments and derivations are given there. 



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