201 



Daniel I. Arnon 



reduced ferredoxin are required to reduce one mole of TPN. Thus, 

 the oxidation-reduction of a ferredoxin molecule involves a trans- 

 fer of a single electron. In the absence of evidence to the con- 

 trary, it is attractive to assume that the photor eduction of 

 ferredoxin is the terminal photochemical act of chloroplasts 

 following photon capture--an act that involves a transfer of an 

 electron from excited chlorophyll to the electron acceptor mole- 

 cule in chloroplasts. 



Table h 

 Stoichiometry of Photor eduction of Spinach Ferredoxin 

 and its Subsequent Reoxidation by TPN in the Dark (10) 



umoles 



Ferredoxin (Fd) photoreduced 0.102 



Fd reoxidized by TPN in the dark 0.106 

 TPN reduced 0.0^7 



Ferredoxin and photophosphorvlation . If the photoreduction of 

 ferredoxin by chloroplasts is the terminal photochemical act of 

 chloroplasts, then it follows that photoreduction of ferredoxin 

 should be a common feature of the electron flow pathways of 

 either cyclic or noncyclic photophosphorylation. The two path- 

 ways would differ in the electron acceptors beyond ferredoxin. 

 In the case of noncyclic photophosphorylation, the electrons from 

 ferredoxin would be transferred by the flavoprotein reductase to 

 TPN, whereas in the case of cyclic photophosphorylation they 

 would "cycle" back to "electron-deficient" chlorophyll molecules 

 via a chain of endogenous electron carriers (6). 



A requirement for ferredoxin [then called "TPN-reducing factor 

 (58)] for noncyclic photophosphorylation coupled with TPN reduc- 

 tion was indeed already observed when this process was first dis- 

 covered (3) and this requirement has since been further documen- 

 ted (43-47). Evidence was also obtained several years ago (38) 

 for a requirement of a "TPN-reducing factor" (i.e., ferredoxin) 

 in what is now called pseudocyclic photophosphorylation (8) by 

 chloroplasts. Moreover, Forti and Jagendorf (48) and Black et 

 al. (49) found that under aerobic conditions ferredoxin ("PPNR") 

 stimulates an endogenous photophosphorylation which proceeds in 

 the absence of added cof actors, but is dependent on and consumes, 

 molecular oxygen as the terminal electron acceptor. 



There was no evidence, however, that ferredoxin catalyzes an 

 anaerobic, cyclic photophosphorylation, when oxygen evolution is 

 effectively inhibited by the presence of CMU. Such evidence was 



