ON THE NATURE OF HEMOPROTEIN REACTIONS 



lesser degree, or that appears to be a unique feature, kinetic and 

 equilibrium studies have revealed that the underlying thermo- 

 dynamic and kinetic quantities. A//", A^", etc., have magnitudes 

 which justify treating these reactions on the same basis as those 

 of simpler coordination compounds and aquated ions. A com- 

 parison of data has led to certain tentative conclusions about 

 the way the porphyrin prosthetic group and the protein contrib- 

 ute to the reactivity. 



In the ligand replacement reactions of myoglobin, the 

 magnitudes of the entropy changes suggest that the protein 

 participates through a tightening of its structure as the water 

 molecule coordinated to the iron is replaced by Oo, CO, CN~, 

 F~, and OH~. A similar effect in hemoglobin could be respon- 

 sible for heme-heme interaction. In the reduction of ferrimyo- 

 globin to ferromyoglobin the sign and magnitude of the entropy 

 change are characteristic of a reaction in which the over-all 

 charge on the complex increases. One interpretation is that the 

 prosthetic group as a whole, with its two negatively charged 

 propionate side chains, cooperates in determining the entropy 

 change. 



There is strong evidence at present that the higher oxidation 

 state of ferrimyoglobin is a true quadrivalent iron compound 

 possessing the "ferryl ion" structure, Fe^bO^*", and thermo- 

 dynamic data for its reactions are now very desirable so that a 

 comparison can be made with those for well-established bonded- 

 oxygen structures. 



The acquisition of similar data for peroxidase, catalase, and 

 other hemoproteins, in particular the elucidation of the part 

 played by heme-linked ionizations, would now contribute 

 greatly to a better understanding of the origin and nature of 

 their specific reactions. 



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