ON THE NATURE OF HEMOPROTEIN REACTIONS 



B on the basis of certain kinetic results (25). Reaction (5) thus 

 involves passing from state A to B, from B to C, and finally from 

 C to D, whereas reaction (6) requires only the change from B to 

 C and C to D. It is clear from the diagram that because of the 

 energy diff'erence between the two initial states, reaction (5) is 

 less unfavorable than reaction (6), because its endothermicity is 

 less by about 16 kcal./mole, provided of course that the exother- 

 mic heat of formation of Fep'''02 (the 16 kcal./mole) is not dissi- 

 pated before electron transfer to state C has occurred. If it is dis- 

 sipated, then no distinction is possible, and only reaction (6) re- 

 mains. 



It thus appears that in contrast to other classes of hemo- 

 proteins and heme derivatives, the exothermic formation of an 

 oxygen complex by myoglobin and hemoglobin is in itself a 

 very significant factor in "protecting" the heme from oxidation. 



Higher Oxidation States of Ferrimyoglobin, Ferriperoxidase, and 



Ferricatalase^ 



The idea that catalysts forin intermediate compounds with 

 one or other reactant antedates the study of enzyme action by 

 many years. In the oxidation of various reducing agents by 

 hydrogen peroxide or alkyl hydroperoxides catalyzed by these 

 hemoproteins, intermediate compounds of this kind are formed, 

 and their nature will now be discussed. 



They have often been regarded as "enzyme-substrate 

 complexes," as postulated in the Michaelis-Menten theory of 

 enzyme action, and their structures accordingly have been 

 formulated with, for example, H2O2 or its anion 02H~ co- 

 ordinated to the iron just as O2, CO, CN~, F~, etc., are co- 

 ordinated in the other hemoprotein complexes. With con- 

 centration ratios of about unity, ferrimyoglobin gives only one 

 intermediate compound — reddish-violet in color — whereas per- 

 oxidase and catalase give two, compound I, which is green and 

 which changes spontaneously into compound II, which is also 



* See footnote regarding formulas on page 366. 



369 



