176 METABOLISM AND PHYSIOLOGY 



If the basic photosynthetic mechanisms are the same in green plants 

 and in photosynthetic bacteria, it would be expected that cyclic and 

 noncyclic photophosphorylation of chloroplasts would have appropriate 

 counterparts in photosynthetic bacteria. Such counterparts were indeed 

 found, but only after other investigations revealed fundamental simi- 

 larities that were at first obscured by differences between the chloro- 

 plast and the bacterial systems. Thus, when Frenkel found (6) a light- 

 induced phosphorylation by cell-free preparations of R. rubrum and 

 pointed to its similarity to the photosynthetic phosphorylation dis- 

 covered earlier in spinach chloroplasts (1) he observed that the bac- 

 terial particles became substrate (cy-ketoglutarate) dependent after 

 washing (6). This was at variance with the unique feature of photo- 

 synthetic phosphorylation in chloroplasts, a feature which distinguishes 

 it from oxidative phosphorylation in mitochondria and the anaerobic 

 phosphorylations associated with fermentation: ATP formation in 

 chloroplasts occurs without the contribution of energy by an oxi- 

 dizable substrate and solely at the expense of the energy contributed 

 by absorbed photons (1,2). However, in later experiments, Frenkel (7) 

 and other investigators (8-10) found that the role of o-ketoglutarate 

 and other organic acids in the bacterial system was catalytic and 

 regulatory and not that of a substrate. Once this fundamental point 

 was clarified, the basic similarity of what we now call cyclic photo- 

 phosphorylation in chloroplasts and in bacterial chromatophores was 

 no longer in doubt (4,11). 



Another basic difficulty surrounded attempts to find a noncyclic 

 photophosphorylation in chromatophores. This difficulty seemed at 

 first insurmountable because photosynthetic bacteria never evolve 

 oxygen (12), whereas oxygen evolution was a part of noncyclic photo- 

 phosphorylation in chloroplasts (Eq. 2), However, Losada, Whatley 

 and Arnon (13) separated noncyclic photophosphorylation with chloro- 

 plasts (Eq. 2) into two component photochemical reactions: (a) a photo- 

 oxidation of water3 (0H~) to yield oxygen (Eq. 3), and (b) the noncyclic 

 photophosphorylation reaction proper (Eq. 4) — a reaction not accom- 

 panied by oxygen evolution, but one in which ATP formation is coupled 

 with a light-driven "uphill" electron flow to TPN from an exogenous 

 electron donor other than water (OH~). The two component reactions 

 have been experimentally separated by using an indophenol dye in its 

 reduced and oxidized forms (A~ and A) as an intermediary electron 

 carrier, in accordance with Eqs. 3 and 4: 



3 Water and OH will be used interchangeably. OH represents the hydroxyl ion 

 at neutral pH as in the reaction: 



40H" (lO"'^ n) --O^ + 2H2O + 4e"; E' = 0.815 V (pH 7) 



