ON THE NATURE OF HEMOPROTEIN REACTIONS 



depends not only on the free-energy change for the binding of 

 oxygen being favorable, but also upon the oxidation to ferri- 

 hemoglobin and ferrimyoglobin being a slow reaction in con- 

 trast to the rapid oxidation of other coordination complexes of 

 ferrous iron that possess comparable structures and similar 

 oxidation-reduction potentials. For instance, heme itself and 

 the hemochromogens are very readily oxidized by molecular 

 oxygen. 



Kinetic studies of the oxidation of hemoglobin and myo- 

 globin by oxygen have shown it to be an extremely complicated 

 process (1,25) with other groups in the molecule becoming 

 oxidized as well as the heme iron. Nevertheless, two factors can 

 be distinguished which contribute greatly to the resistance 

 towards oxidation. Whatever the detailed mechanism of the 

 oxidation process, the fundamental electron transfer reaction 



Fcl+ + O2 > Fcl+ + O- (5) 



will be among the most important governing its speed. If this 

 reaction is fast the net oxidation process will also be fast unless 

 there are significant back reactions regenerating the ferrous 

 state. Using the provisional values of 96 and 97 kcal./mole for 

 the ionization potentials of ferromyoglobin and ferrohemoglobin 

 obtained from the oxidation-reduction potential study and other 

 thermochemical data, reaction (5) can be shown to be endo- 

 thermic with AH^ values of 20 and 21 kcal./mole, respectively. 

 Reactions which are endothermic to this extent are usually very 

 slow. In fact the ionization potential of the simple aquated 

 ferrous ion is a little less, 95 kcal./mole, which means that on this 

 criterion these ferro-hemoproteins are a little less susceptible to 

 oxidation than ferrous salts. These remarks refer to the bi- 

 molecular electron transfer between the ferrous state and oxygen. 

 For the fission of the oxygen complex itself with electron transfer, 

 reaction (6), an additional factor must be considered, namely, 

 its heat of formation. 



Fe2/02 > Fel+ + O" (6) 



367 



