338 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



compounds play as biological catalysts in respiratory processes is 

 bound up with their ability to react with oxygen and hydrogen per- 

 oxide, or to undergo changes of iron valency. It has been shown in 

 Chapter V that simple hematin compounds possess these abilities in 

 a rudimentary way. In the chapters on hemoglobin it has been 

 demonstrated how profoundly the properties of hematin are altered 

 by combination with the protein globin. In this instance the altera- 

 tion is of such a nature that the resulting compound, hemoglobin, has 

 not the properties of an oxidative catalyst, nor is it greatly superior 

 to hematin or hemochromes as 'a peroxidative or catalatic enzyme. 

 The oxygen in oxyhemoglobin is less, rather than more, reactive than 

 molecular oxygen; it is, as Warburg put it, transport oxygen, not 

 activated oxygen. We shall see in Section 6.3.6 that certain altera- 

 tions of the protein activate the chemically inert oxygen of oxy- 

 hemoglobin but such alterations have not yet been shown to occur 

 in vivo. If an enzyme is defined as a catalyst of biological origin, 

 hemoglobin is not an enzyme. 



In the next chapter we shall find several instances of the same 

 protoheme found as the prosthetic group of hemoglobin acquiring, 

 when combined with other proteins, extremely powerful catalytic 

 properties (horse-radish peroxidase, catalase). Similarly it has not 

 been demonstrated, and is unlikely, that the related hematins which 

 constitute the prosthetic groups of the oxidative enzymes (cyto- 

 chromes, respiratory catalyst) are powerful catalysts in themselves; 

 again they are certainly combined with specific proteins which modify 

 and increase the catalytic activity of their prosthetic groups or active 

 centers. 



We must distinguish between several ways in which the protein 

 exerts an influence on the catalytic efficiency of a hematin. First, 

 it may alter the electronic configuration of the hematin iron atom, 

 causing, for example, peroxidase to combine with hydrogen peroxide 

 to a relatively stable compound, or catalase to form an explosively 

 unstable compound; oxygen with hemoglobin results in a relatively 

 inert oxygen compound, but with the respiratory enzyme presumably 

 a very reactive one. Secondly, the protein " alters the oxidation- 

 reduction potential of the prosthetic groups. Thirdly, it will provide 

 the point of combination with the specific substrate, for example 

 between the hydrogen peroxide heme group and pyrogallol in perox- 

 idase, or between cytochrome c and a suitable hydrogen donor. In 

 order to make this distinction clear we shall restrict the use of the 



