Common Paihways of Cellular Metabolism - 147 



Organic Substrate 

 e.g. Pyruvic Acid 



TPN and DPN 



Pyridine Acceptors 



FAD and FMN 



Flavin Acceptors 



Primary Acceptor Compounds 



Cytochrome b 



Ubiquinone 



Cytochrome c 



Cytochrome a 



Cytochrome System 

 in Mitochondria 



Cytochrome 

 Oxidase 



Oxygen 



Fig. 8-4. Electron-hydrogen transmission during oxidative phosphorylation. Each time an electron (and hydro- 

 gen) passes from higher to lower level, the oxidation energy is conserved and stored— mainly in the form of 

 high-energy phosphate (ATP). 



cell (Fig. 8-4). At the very end, of course, the 

 electron-hydrogen pairs are passed on to 

 oxygen, forming water as the end product. 

 This final direct oxidation cannot occur, 

 however, in the absence of another enzyme, 2 

 cytochrome oxidase (Fig. 8-4). 



It is difficult to see how such an orderly 

 electron transmission system could operate 

 unless the component enzymes were built 

 into the structure of the cell in a very pre- 

 cisely patterned fashion. Therefore it is not 



2 Cytochrome oxidase is extremely sensitive to in- 

 activation by cyanide (CAT) compounds. This sen- 

 sitivity accounts for the remarkable potency of cyanide 

 poisons in inducing death by asphyxiation in cells 

 and organisms generally. 



surprising to find that the cytochrome system 

 and some of the other enzymes are localized 

 in the mitochondria, presumably at fixed po- 

 sitions in the internal membranes of these 

 complex structures (p. 27). Mitochondria pre- 

 pared from disrupted cells — or even frag- 

 ments of mitochondria — can still carry on 

 oxidative phosphorylation, under the proper 

 conditions. Such a preparation will continue 

 to utilize oxygen and to build up ATP if 

 provided with DPN • H 2 (as an electron-hy- 

 drogen source), plus ADP and inorganic 

 phosphate. 



In essence the nature of the electron trans- 

 fer process can be understood when it is 

 realized that the cytochrome enzymes are 



