ON THE NATURE OF HEMOPROTEIN REACTIONS 



The pH dependence of velocity constants and equilibrium 

 constants arising from their ionization is superimposed on the pH 

 dependence characteristic of the bonding of the ligand. The 

 interpretation of results can therefore be very complicated, and 

 it may be very difficult to decide from such results alone whether 

 the undissociated acid or the anion is bonded to the iron. 



This difficulty can be resolved if the acid strengths of the 

 linked ionizing groups are first derived from data on reactions 

 where the effect of /?H comes entirely from the ionization of these 

 groups, for instance, in the oxygenation reaction — 



Fe(H20) + O, ^==± Fe(02) + HjO (1) 



and in the cell reaction involving the standard hydrogen elec- 

 trode, 



Fe+(H20) + V2H, ;==± Fe(H,0) + H+ (2) 



In the case of hemoglobin these two reactions are affected quite 

 markedly by changes in pH and, within experimental error, to 



100 



Fig. 1 . The logarithm of the equilibrium constant plotted against pH 

 for A, the myoglobin-oxygen reaction (43), for B, the cell reaction involving 

 the standard hydrogen electrode (42), and for C, the formation of ferrimyo- 

 globin fluoride (18). In A and B, 1.00 and 0.60, respectively, have been 

 added to the actual values of logic K to bring them closer to curve C. 



345 



