OXIDASES, PEROXIDASES, AND CATALASE 99 



of thioether bindings from the side chains of the porphyrin to the 

 protein, and by means of two histidine-imidazole groups strongly 

 bound to Fe on each side of the flat heme disc. Thus the heme group 

 appears to be built into a crevice in the protein molecule. This ex- 

 plains why cytochrome c is not autoxidizable, since oxygen can never 

 approach the iron atom, and why no CO-compounds or cyanide 

 compounds are formed at physiological pH values." 



Oxygen Transfer in Living Cells 



In every aerobically living cell we find a number of hemin pro- 

 teins, e.g., a respiratory ferment, three diflFerent cytochromes, cata- 

 lase and/or peroxidase. There is also frequently present what Keilin 

 calls the "unspecific cell hematin." More recently another functional 

 type of hemin enzymes has been found in such cells, which cata- 



CHAIN OF RESPIRATORY CATALYSTS 



Op- 



RESPIRATORY CYTOCHROMES ^CYTOCHROME ^PYRIDINE. 



■ ENZYME * A-»C^B REDUCTASE ENZYMES 



SEQUENCE DURING PASTEUR REACTION 



PASTEUR 

 ENZYME 



FERROUS 



IRON 

 CATALYST 



FERI^ENTATION 

 ENZYME 

 SYSTEM 



SUBSTRATES 



STARCH 



GLYCOGEN 



GLUCOSE 



FRUCTOSE 



Figure 2. — Function of molecular oxygen in respiration and Pasteur reaction 



lyzes the inhibiting eflFect of oxygen on fermentation and glycolysis 

 (Pasteur reaction). Some data pertaining to these various iron por- 

 phyrin proteins are given in Tables 4 and 5. 



Two groups of these intracellular hemin proteins are endowed 

 with the power to react directly with molecular oxygen: the oxygen- 

 transferring enzymes of respiration (for short, "Warburg enzymes") 

 and the aerobic fermentation-inhibiting catalysts (for short, "Pasteur 

 enzymes"). Inasmuch as the Pasteur reaction is the subject of an- 

 other paper, it must suffice here to present only the particular 

 working hypothesis which the writer is advancing on the basis 

 of recent work in this laboratory. See Figure 2. 



Photochemical work (44, 61) has revealed the pheohemin nature 

 of the prosthetic group of the Pasteur enzymes in rat retina and in 



