296 UNITY AND DIVERSITY IN BIOCHEMISTRY 



The behaviour of the various haemoglobins in the course of oxygenation 

 gives us a further means of characterizing them. This is done by plotting 

 a graph showing the degree of oxygenation as a function of the partial 

 pressure of oxygen and is commonly known as the dissociation curve of 

 oxyhaemoglobin. If we represent by T the portion of the haemoglobin 

 molecule corresponding to a group capable of being oxygenated, i.e. an 

 iron atom, then the equilibrium will be represented by 



TO^^T + Oa (1) 



K being the equilibrium constant for the oxygenation. Since the concen- 

 tration of oxygen in the solution, [Og], is proportional to the partial pressure 

 of oxygen in accordance with Henry's Law, we can replace it in equation (2) 

 by the partial pressure p. 



If we represent the concentration of oxygenated haemoglobin by 

 [HbO,^ and that of the non-oxygenated haemoglobin by [Hh\ the equation 

 becomes 



log i-j^-" = log ^ + log K (4) 



The logarithmic form is particularly useful since if we plot the values of 

 log [//6O2] / [//6] as ordinates and the values of log p as abscissae, then 

 since i^ is a constant, we shall obtain a straight line inclined at 45° and 

 cutting the ordinate axis at the value of log K. In the usual form where the 

 percentage saturation is plotted on the y axis against partial pressures on 

 the X axis, one obtains a hyperbola. This is the case for the muscle haemo- 

 globin of mammals and the larval haemoglobin of Gastrophilus. In the most 

 general case the dissociation curve is not a hyperbola but a sigmoid-shaped 

 curve whose form can be empirically expressed by the well-known 

 equation : 



[HbO;\ ,^ 

 [Hb] ^ 



in which the exponent represents, although rather vaguely, the degree of 

 interdependance or difference of the various groups which are oxygenated. 

 In fact, in solutions of the muscle haemoglobin of Vertebrates and the 

 haemoglobin of Gastrophilus there is only one type of oxygen-binding 

 group. Myoglobin contains only one iron atom, and the larval haemoglobin 

 contains two atoms which are identical in character. In haemoglobins 

 giving a sigmoid curve, either the oxygen-binding groups in the same 



