50 



OXIDATION-REDUCTION POTENTIALS 



chromes are protein compounds of an iron porphyrin. The general skeleton of iron 

 porphyrin is shown below. By varying the substituents in the four pyrole rings the 

 different porphyrins are derived. 



Chlorophyll is a magnesium porphyrin and catalase and peroxidase are iron- 

 porphyrin compounds. In haemoglobin the iron is divalent and the protein com- 

 ponent, globin is, therefore, combined with hsem ; the trivalent iron porphyrin is 

 bsematin, which is present in peroxidase. In the cytochromes, on the other hand, the 

 enzymic properties are attributable to the alternate oxidation and reduction of the 

 iron. The pigment turacin in feathers of the touraco is a copper -porphyrin compound, 

 presumably without respiratory significance. 



CYTOCHROME 



MacMunn's observation in 1886 of a pigment probably of respiratory significance 

 in tissues had to remain for nearly forty years before its fruitful development by 

 Keilin (1925). The presence of the cytochromes can be detected by virtue of the 

 characteristic absorption spectra possessed by the reduced pigments. The cyto- 

 chrome absorption bands in a yeast suspension, for example, disappear when oxygen 

 is bubbled through and they reappear during reduction on standing. There are 

 systems in cells which can oxidise cytochromes aerobically and reduce them when 

 air is excluded. The oxidised and reduced forms are now generally known as ferri- 

 and ferro-cytochrome respectively. 

 The reaction 



Fe®® ^ Fe®®® + electron 

 in the case of cytochrome does not occur spontaneously and appropriate enzyme 

 systems have to be present for the reactions to proceed. Dehydrogenases effect 

 reduction and cytochrome oxidase (or indophenol oxidase) effects oxidation. 



succinate 



Succinate + Ferricytochrome > Fumarate + Ferrocytochrome 



dehydrogenase 

 Cytochrome 



Ferrocytochrome + Og > Ferricytochrome + H2O 



Oxidase 



Study of this system, the details of which are still the subject of controversy, has 

 been aided by the use of chemical compounds which act as differential inhibitors of 



