652 XIV. PYRROLE PIGMENTS IN EVOLUTION 



and evolution of oxygen, need may have arisen for the protection of 

 the oxygen-sensitive anaerobic enzyme systems. As we have dis- 

 cussed in Chapter IX, the role of catalase as protective enzyme is not 

 beyond doubt. It is also necessary to keep in mind that catalase 

 could only protect anaerobic enzA'me systems against the action of 

 hydrogen peroxide, produced indirectly from oxygen, but not against 

 the action of oxygen itself. Another possible early evolutionary pre- 

 cursor of the hematin enzymes, in particular of the cytochrome sys- 

 tem, is an enzyme which according to Vogler and co-workers {2899) 

 is probably present in Thiobacillus thiooxidans. This enzyme oxidizes 

 sulfur to sulfate; it is inhibited by carbon monoxide, the inhibition 

 being reversed by light. This enzyme deserves further study. 



The evolution of the hematin enzymes is largely a matter of the 

 development of suitable proteins. With the exception of the respira- 

 tory ferment and cytochrome a, the other more important hematin 

 enzymes all contain the same prosthetic group, protohematin; for 

 our present purpose we may also include cytochrome c in this group, 

 considering it a protohematin compound having the prosthetic 

 group in a peculiar kind of combination with the protein. We know 

 also, from the synthesis of hybrid enzymes with nonbiological hema- 

 tins and globin or the peroxidase protein, that it is the protein not the 

 prosthetic group which determines the nature of the enzyme. The 

 unique role of the proteins for biological processes holds in this field 

 as in many others, and the problem of adaptation of the hematin 

 enzymes to their function becomes largely one of protein adaptation. 



It has been shown in Chapter VIII that the specific proteins greatly 

 increase, for one specific catalytic action, the rudimentary catalytic 

 potentialities residing in the hematin, and that they are also of great 

 importance for the protection of the enzyme against destruction in 

 the catalytic process itself. This decrease in lability may have been 

 of importance in the evolution of the present-day enzymes, which 

 unite a very high turnover number with a remarkable stability. They 

 may have evolved from much less stable hemoproteins by adaptation 

 of the protein, the prosthetic group having already reached the maxi- 

 mum degree of resonance and thus the limit of its adaptation. 



This assumption cannot be supported by evidence demonstrating differ- 

 ences of stability of enzymes in forms now living. We can only summarize 

 once more the various mechanisms by which such a protection is achieved. 

 Ferrous heme compounds are particularly likely to undergo autodestruction 

 (c/. Chapter X). Hence enzymes which are reactive in the ferric form, such 

 as catalase and peroxidase, are stabilized in this form and do not undergo 



