542 LIGHT AND LIFE 



16. Oxygen-catalyzed Cyclic Photophosphorylation 



The mechanisms of photosynthetic phosphorylation in chloroplasts 

 discussed thus far include anaerobic cyclic photophosphorylation 

 (Figs. 4 and 5) and non-cyclic photophosphorylation (Fig. 19) . Re- 

 cent work by Tsujimoto, Hall, and Arnon (156) suggests the opera- 

 tion in chloroplasts of a third mechanism, an oxygen-catalyzed cyclic 

 photophosphorylation. 



As already discussed in Section 6, a catalytic role for oxygen was 

 envisaged in explaining the first experiments on photosynthetic phos- 

 phorylation, in which the presence of oxygen was required but no 

 oxygen consumption was observed (11). Interest in the role of oxy- 

 gen was heightened when several laboratories reported that at low, 

 "microcatalytic" concentrations of FMN or vitamin K (Fig. 3) , photo- 

 phosphorylation remained dependent on oxygen (169, 77, 111). 



From the standpoint of cellular physiology, it was interesting to 

 contrast the role of oxygen in ATP formation in photosynthesis with 

 that in respiration. The participation of oxygen as the terminal elec- 

 tron acceptor in oxidative phosphorylation has conferred a marked 

 superiority on respiration over fermentation, in the efficiency of con- 

 verting the free energy of the substrate into the energy of the pyro- 

 phosphate bonds of ATP. Was the efficiency of conversion of light 

 energy into ATP also increased by the presence of oxygen? 



To answer this question, photophosphorylation by chloroplasts was 

 investigated in air and in nitrogen, at different concentrations of 

 FMN or vitamin K, and particularly, at a limiting light inteyisity, 

 when the efficiency of the energy conversion process could be best 

 observed (compare Section 10) . The results are shown in Fig. 20. 



In limiting light, the highest rate of photophosphorylation was 

 obtained in nitrogen at a concentration of approximately 10-* M 

 of either FMN or vitamin K. No photophosphorylation occurred in 

 nitrogen without added cofactors but when these were added at an 

 optimal concentration, the anaerobic system was about twice as effi- 

 cient in converting light energy into ATP as the aerobic system. 



The experiments represented by Fig. 20 were carried out with 

 relatively high concentrations of chloroplast material. Under these 

 conditions the aerobic system showed little increase in photophos- 

 phorylation from adding FMN or vitamin K. However, high con- 

 centrations of chloroplast material were found to be necessary to in- 

 sure the effective operation of the anaerobic FMN system. The 

 anaerobic vitamin K system functioned optimally at lower concentra- 

 tions of chloroplast material, suggesting that it required less or fewer 



