ON THE NATURE OF HEMOPROTEIN REACTIONS 



only at certain finite ionic strengths; at zero ionic strength the 

 factor is as low as 2 for the OH~ complexes (15), corresponding 

 to a difference in AF° of —0.25 kcal./mole (see Table V). But 

 although the two hemoproteins have such a similar affinity for 

 OH~ there are rather greater differences in the AH° and A6'° 

 values. As in the case of the previous comparison with ferric 

 ion complexes these values again reflect opposing effects. For 

 the ferrihemoglobin reaction A//° is more favorable by 1.85 

 kcal./mole, whereas A6'° is unfavorable to the extent of —5.3 e.u. 

 The data for the F~ complexes show just the same trends, 

 although in the case of ferrihemoglobin small corrections may be 

 necessary to allow for the participation of linked ionizations. 

 The differences in AF°, A//°, and A^'", are, respectively, —0.4 

 and —1.0 kcal./mole, and —2.4 e.u. 



The Reactivity of Cytochrome c in Complex Formation 



In cytochrome c the heme is bound very differently from 

 that in hemoglobin and myoglobin. A comparison of the data 

 for complex formation should therefore show the extent to which 

 a difference in structure can affect the magnitudes of k, K, 

 AF", AT^o, and A^""^, and so give a clue to the kind of correlation 

 to be expected between structure and reactivity in hemoprotein 

 systems. 



In the /?H range 4 to 1 1 ferrocytochrome c does not combine 

 readily with carbon monoxide, nor does ferricytochrome c 

 readily form fluoride and cyanide complexes like the ferric 

 forms of other hemoproteins (44) . The absorption spectrum of 

 ferrocytochrome c closely resembles that of a hemochromogen, 

 the coordination complex formed by heme with two molecules 

 of nitrogenous base, such as pyridine or piperidine, one on each 

 side of the porphyrin ring (32). It is therefore thought that in 

 cytochrome c there are two links to amino acid residues binding 

 the heme in a crevice in the protein, and that before complex 

 formation with simple ligands can occur, the weaker link has 

 to be broken. Actually ferricytochrome c does react very slowly 



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