ENZYMES 151 



reaction cycle, thus serving to shuttle protons and elcc irons to oxygen 

 and forming water. Such a system ojierates in plants like apples, 

 bananas, mushrooms, and potatoes. When cellular function is seriously 

 impaired, the supply of suitable reducing agents may be interrupted. 

 The quinone accumulates and spontaneously polymerizes to the 

 colored materials characteristic of exposed slices of apple or potato. 

 This same sort of process occurs during the formation of some black 

 and brown pigments in insects, animals, and tumors. 



The principal system for the utilization of oxygen by animal cells 

 and by some microorganisms and plant tissues involves the cyto- 

 chromes. These compounds are proteins possessing prosthetic groups 

 that contain iron. A complex series of reactions transfers electrons 

 from the ferrous ions of cytochromes to oxygen, and hydrogen ions 

 disappear from the solution. 



, ^^ , „,, ^^ .^^^^, cytochrome 



4 Cytochrome-Fe++ + Oo + 4H3O+ > 



oxidase 



4 cytochrome-Fe+++ + 6H2O 



The ferric form is reduced by the last of a sequence of reactions going 

 back to the DPNH formed in reactions like that of page 140. The 

 enzyme cytochrome oxidase catalyzes the transfer of electrons from the 

 last cytochrome of the series to oxygen. This enzyme may contain 

 iron, although some workers believe that copper is present instead. 

 At points along the reaction chain chemical energy is available for 

 use in the physiological processes of the cell. The cytochrome system 

 will be referred to again. 



Some oxidases lead to hydrogen peroxide instead of water, but the 

 biological importance of these systems is not known. The hydrogen 

 peroxide formed is toxic and must be decomposed, probably by 

 catalase or peroxidases. 



Dehydrogenases 



This group of oxidation-reduction enzymes participates in the 

 transfer of hydrogen from one substance to another. Both donor 

 and acceptor can be and usually are quite specific. The number of 

 known dehydrogenases is rather large, and many of them have been 

 intensively studied. The reactions catalyzed by lactic and malic 

 dehydrogenases have already been mentioned. Succinic dehydrogenase 

 is another widespread example and acts upon succinate. 



-OOCCH2CH2COO- '"'''^'"') -OOCCH=CHCOO- + 2H 



succinate fumarate 



