ON THE NATURE OF HEMOPROTEIN REACTIONS 



Although the affinities of ferrimyoglobin and ferricyto- 

 chrome c for cyanide are so similar, the heat and entropy data 

 given in Table VI reveal very profound underlying differences. 

 The ferrimyoglobin reaction with CN~ is favored by a very 

 large exothermicity of 18.6 kcal./mole, whereas t^S'^ is very un- 

 favorable, — 24 e.u. The ferricytochrome c reaction, in great 

 contrast, is a little endothermic, and the complex forms entirely 

 by virtue of a very favorable entropy change, +31.3 e.u. Such 

 differences between A//" and A^"*^ values are just what would be 

 expected if the heme in cytochrome c was bound in a crevice in 

 the protein. Energy would be required to break the bond to 

 the more weakly held amino acid residue, so diminishing the 

 net exothermicity of complex formation, but the entropy change 

 would be favorable because part of the protein would gain more 

 freedom of movement. A comparison of (-S^l — -^m) values, 

 +4.2 e.u. for ferrimyoglobin and +59.5 e.u. for ferricytochrome 

 c, shows that this structural difference between the two hemo- 

 proteins favors cyanide complex formation by ferricytochrome 

 c relative to ferrimyoglobin by about 55 e.u.; and in the con- 

 trary sense this is the entropy loss suffered by cytochrome c 

 itself because of its peculiar structure. 



Electron Transfer in Myoglobin and Hemoglobin Systems 



The reversible oxidation and reduction between the ferrous 

 and ferric states is one of the least specific of all hemoprotein 

 reactions. Catalase alone is the exception; it can only be 

 reduced directly in solutions made up from the freeze-dried 

 enzyme. Provisional values (18) have been obtained for the 

 heat and entropy changes associated with the cell reaction 



Fe+(H,0) + V2H2 ;=^ Fe(H20) + H + 



of both myoglobin and hemoglobin from measurements of the 

 oxidation-reduction potentials at pH 6.0 and ionic strength 

 0.22 over a range of temperatures. Since A//" for the reaction 

 H+ + e~ ;^ V2 Ho is 82.5 kcal./mole, the ionization potentials 



359 



