Some Physico-chemical Evidence Regard- 

 ing the Structure of Haemoglobin 



JEFFRIES WYMAN, JR. 



From evidence provided by studies on the effects ofpH and tempera- 

 ture on the oxygen equilibrium, it is argued that the four oxygen 

 combining groups of vertebrate haemoglobin, one for each haem, 

 are all identical. Each haem is linked with two acid groups, one of 

 which is strengthened, the other weakened, when oxygenation occurs. 

 Thermodynamic studies indicate that the heat of dissociation of each 

 of these groups is 6,000-6,500 cals, which suggests that they are 

 both imino groups ofhistidine. The value ofn — 2-5 — 3-0 required 

 to describe the oxygen equilibrium in HilVs empirical equation 

 requires the existence of a stabilizing interaction between some, at 

 least, of the four oxygen combining centres. From experiments on the 

 oxygen equilibrium of haemoglobin split by dissolving it in urea 

 solutions, it appears that the four haems occur, not at the corners of 

 a square as originally suggested by Pauling, but in pairs, with strong 

 interaction between members of the same pair and weaker interaction 

 between members of different pairs. The latter, and possibly also 

 the former, interaction seems to involve primarily the un-oxygenated 

 haems. This picture accords with deductions from a recent x-ray 

 study on crystalline vertebrate haemoglobin. 



There is perhaps no better instance of biological adaptation at a 

 molecular level than the way in which vertebrate haemoglobin reacts 

 with oxygen. The physiological significance of the familiar curve which 

 describes the equilibrium of these two substances has been pointed out 

 repeatedly : its sigmoid character is such as to increase the working 

 range of this molecular mechanism of oxygen transport and at the 

 same time the dependence of the position of the curve on pH, reflecting 

 a linkage between the oxygen and acid-base functions of the haemo- 

 globin molecule, adds further to the efficiency of the respiratory 

 exchanges. To provide a physico-chemical explanation of these 

 phenomena has long been a major concern of physiologists and bio- 

 chemists, but the problem still lacks a complete solution. Closely 

 related problems also occur in several other reactions of haemoglobin, 

 such as oxidation and combination with carbon monoxide. To 

 provide a solution of any of them would mean a substantial step in 

 the understanding of the haemoglobin molecule. In this paper we 



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