ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 173 



The term cytochrome denotes a complex protein with a prosthetic 

 group belonging to the class called porphyrins and in the present 

 case contains iron. The iron undergoes reversible oxidation-reduction 

 in the presence of enzymes called cytochrome reductase and cyto- 

 chrome oxidase. 



The reactions all show negative free-energy changes totaling about 

 50,000 cal. Of this chemical energy, about 31,000 cal. is known to be 

 transferred when three molecules of ATP are formed. The fate of 

 the remaining energy is unknown, and the particular reaction stages 

 yielding the ATP are still not known. Moreover, there may be more 

 than one kind of cytochrome serving as electron carriers in series. But 

 in spite of prolonged investigation little has been learned of the 

 sequence of events in cells. 



Terminal oxidations. The last step of the above series is the only 

 one mentioned so far that involves molecular oxygen as the oxidizing 

 agent. This reaction and others utilizing oxygen are the terminal 

 oxidations of respiration. When the last cytochrome of the sequence 

 is oxidized by oxygen in the presence of hydrogen ions and an enzyme, 

 cytochrome oxidase, water is formed. The free-energy decrease is 

 thought to be about 25,000 cal., an unusually large amount for a 

 single biological reaction. There must certainly be some mechanism 

 for transferring at least part of this energy to useful compounds, and 

 there may be intervening steps allowing the transfer of energy in 

 smaller quantities. 



Other terminal oxidations are known in plants and animals, but 

 the extent to which these reactions function in living cells is still in 

 doubt. They do play roles in various abnormalities and will be 

 discussed later in connection with the metabolism of the two king- 

 doms concerned. Microorganisms appear to possess considerable 

 diversity in their terminal oxidations, but again most of the details 

 have not been worked out. 



The present chapter has summarized part of the information on 

 metabolism, being concerned only with some of the metabolic systems 

 common in higher plants and animals. More details appear in the 

 chapters on plant and animal metabolism. No effort is made to survey 

 the special pathways of the diverse microorganisms. 



