256 



ELECTRON TRANSPORT 

 TABLE 6. 



Rates of other photoreactions of R. rubrum chromatophores. 



Photoreaction 



Rate and Reference 

 //molesAir/mg Chlorophyll 



Oxidations 



H2 Oxidation (with DPIP) 

 FMNH2 Oxidation (with NAD) 



Reductions 



NAD (with FMNH2) 

 NAD (with Succinate + CN ) 

 NAD (with Succinate) 

 NAD (whole cells) 



Reductions Coupled with Ascorbate 

 and DPIP 



NAD 



Disulfide 



Methyl Red 



Methyl Red + Quinacrine 



Sulfate 



4 (Bose and Gest, 39) 

 9.5 (Frenkel, 7) 



9.1 (Frenkel, 7) 



45 (Vernon and Ash, 8) 



24 (Nozaki et al., 16) 



360 (Amesz, 21) 



37 (Nozaki et al., 16) 



11 (Newton, 18) 



24 (Ash et al., 17) 



145 (Ash et al., 17) 



43 (Ibanez and Lindstrom, 5) 



transfer system in the sense that electrons travel along this electron 

 transport chain in the usual fashion (from the compound with the lowest 

 oxidation potential to that with the highest potential) and are then re- 

 cycled through the light- activated chlorophyll back to the low potential 

 compound once again. Isolated chromatophores have the capacity to 

 carry out the process of photophosphorylation with no added cofactors, 

 and this phosphorylation is very sensitive to the extent of reduction of 

 the various components within the chain, since either overreduction 

 or overoxidation inhibits the cyclic phosphorylation process (42,55, 

 57), Thus, we can imagine the electron transport system of the chroma- 

 tophore as consisting of a closed electron transport system which 

 yields ATP when electrons are cycled around the system. There must 

 be several points of entry onto the system to allow the photochemical 

 reactions to be coupled with the chemical oxidation and reduction re- 

 actions which must take place in the whole cell (NAD reduction and 

 substrate oxidation). 



The fastest rates observed with isolated chromatophores involve 

 the photophosphorylation process and the photooxidation of DPIPH2 in 

 the initial fast reaction. The fast reaction of DPIPH2 must be coupled 

 to the photo reduction of components contained within the cyclic electron 

 transfer system. The most likely candidate for such a photo reduction 

 is ubiquinone. The secondary slow reactions coupled to DPIPH2 oxida- 

 tion appear to involve other enzymatic components on the chromato- 



