J. A. BASSHAM AND M. CALVIN 



H2O2 complexes under aerobic conditions. It appears that this 

 complex can form with concentrations of H2O2 as low as 10~^ M, 



Consideration of the above mechanism of oxygen evolution 

 suggests that the place where energy might possibly be available 

 for use in forming ATP, or perhaps additional reducing agent, is 

 in the liberation of oxygen from peroxide. Some 17 kcal. are 

 available from each molecule of peroxide. However, it is 

 difficult, with only our present knowledge, to visualize the 

 possible mechanism of this energetic coupling. On the other 

 hand, it is attractive to suppose that the peroxide may be used 

 in part to oxidize some ferrocytochrome (19) in a reaction 

 catalyzed perhaps by peroxidase (19). Thus the requirement 

 for an oxidizing agent to react ultimately with the primary 

 reducing agent could be met without requiring oxygen. This 

 is rather useful because of the evidence that molecular oxygen 

 is not required for photosynthesis. Allen (1) has reduced the 

 concentration of molecular oxygen in contact with photosyn- 

 thesizing organisms to a value of 0.004 mm. Hg and finds, at 

 this level, no diminution of the rate of photosynthesis. The 

 absence of a back reaction involving molecular oxygen was also 

 shown by the studies of Brown (10) with the mass spectrometer 

 and iso topically labeled oxygen. 



It is interesting to suppose that the ferrocytochrome 

 oxidized by H2O2 is reduced cytochrome /, discovered by Hill 

 and Scarisbrick (35). It was found that this compound was 

 present in considerable amounts in green chloroplast material 

 and that it had an oxidation-reduction potential of +0.365 v., 

 about 0.1 V. better as an oxidizing agent than cytochrome c. 

 Since the oxidizing potential of H2O2 for the physiological 

 conditions as given above is +1.2 v., there is a potential dif- 

 ference between these two half reactions of 0.8 v., or a negative 

 free energy change of 37 kcal. /mole for the oxidation of two 

 reduced cytochrome / molecules with one H2O2 molecule. 



The oxidized cytochrome could then react with other 

 electron carriers, perhaps other cytochromes, and eventually, 

 with the primary reducing agent, or with TPNH. Since the 



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