200 



Daniel I. Arnon 



ported by Horio and Yamashita (39), Katoh and Takamiya (^0), Fry 

 and San Pietro (4l) and Gewitz and Voelker (3^)- Fry and San 

 Pietro (^1) found that the iron in this protein is associated 

 with "labile" sulfide groups. The presence of "labile" sulfide 

 groups in the protein was also independently found by Gewitz and 

 Voelker (3^+) . 



Table 3 



Iron Analysis of Spinach Ferredoxin 



(Tagawa, Chain and Arnon, 1963) 



The chemical similarities and the functional interchangeabil- 

 ity of bacterial and chloroplast ferredoxin in the photoreduction 

 of TPN (9) suggest that these two substances, although not iden- 

 tical, belong to a family of ferredoxins. Ferredoxins appear to 

 function as electron carriers that transfer to appropriate en- 

 zyme systems the roost "reducing" electrons in cellular metabolism, 

 that is, electrons at a potential of about -420 mV. These come 

 from two sources: hydrogen gas (or substrates producing H^) or 

 illuminated chloroplasts . 



The role of ferredoxins in photosynthetic electron transport 

 and in utilization and production of hydrogen gas is diagramma- 

 tically represented in Fig. 5' In this scheme, crystalline 

 spinach ferredoxin (Fig. 6) was found to be replaceable by one 

 of several crystalline ferredoxins: that from Clostridium pas - 

 teurianum (9) and those from the photosynthetic bacterium 

 Chroma tium (Fig. 7) and the blue-green alga, Nostoc muscorum 

 (Fig. 8). 



Separation of the light and dark reactions in photoreduction 

 of TPN . The recognition of the role of ferredoxin in the TPN- 

 reducing system was followed by the physical separation of the 

 photoreduction of TPN by illuminated chloroplasts into two steps: 

 (1) a photochemical reduction of ferredoxin by chloroplasts in 

 the absence of TPN, followed by (2) the dark reoxidation of re- 

 duced ferredoxin by chloroplasts to which TPN was added (42). 



The results, summarized in Table 4, show that two moles of 



