546 



LIGHT AND LIFE 





cof„ 



\ 



cof 



red. 



02^,.H20 



\ 



cytox ,^ Cytred. 



Chi [ T u^^m^ ^ 



LIGHT -^P-ADP- 



^T^ 



02-catalyzecl cylic photophosphorylation 



Fig. 21. Sclieme for oxygen-catalyzed cyclic photophosphorylation in 

 chloroplasts. Details in text. 



in agreement with the O^^ exchange data recently reported by Naka- 

 moto and Vennesland (112) and Jagendorf (75). 



In sinnmary, then, FMN and vitamin K seem to catalyze two path- 

 ways of cyclic jjhotophosphorylation, one anaerobic and one catalyzed 

 by molecular oxygen (cf. 168) . The anaerobic pathway, when in- 

 vestigated in an atmosphere of nitrogen, requires appreciable, al- 

 though still catalytic, concentrations of cofactors and, particularly in 

 the case of FiMN, high concentrations of chloroplast material that 

 evidently supply the additional factor (s) needed for the efficient 

 conversion of Hght energy into ATP under anaerobic conditions. The 

 oxygen-catalyzed pathway for FMN or vitamin K is catalyzed by very 

 low, "micro(atalytic" concentrations of these cofactors and is much 

 less dependent on additional (hloroplast material than the anaerobic 

 pathway. 



These findings are intcrjjrctcd to mean that oxygen, when present 

 in a system catalyzed by either FMN or vitamin K, is able to com- 

 pete effectively for the electrons of cyclic j^hotophosphorylation. Once 

 the electrons are accepted by oxygen and form water, the cyclic path- 

 way can be maintained only by a release of electrons in the oxygen- 

 forming reaction ol non-cyclic j)hotopliosphorylation in chloroplasts 

 (Section 15). Hy contrast, phenazine methosidfate catalyzes the trans- 

 fer of electrons to cytochrome so effectively (105) that it is able to 

 prevent their "escape" to oxygen, and hence the phenazine metho- 



