PHOTO PHOSPHORYLATION AND ELECTRON TRANSPORT 199 



for this difference in a photosynthetic system is in the region of its 

 unique component, the chlorophyll. The fact that a valinomycin- 

 insensitive phosphorylation appears to be localized in the same part of 

 the cyclic electron transport pathway as the chlorophyll brings into 

 focus a recent suggestion by Calvin (19) that chlorophyll might be a 

 site for an ATP-producing reaction pattern. He considered the pos- 

 sibility that light energy would allow the 9-10 enol in chlorophyll to 

 react with orthophosphate and that a subsequent dehydration reaction 

 would give the necessary energy- rich configuration 



O 



I I II 



C=C-0-P-OH 



6" 



to allow a phosphorylation of ADP to ATP, 



It is tempting to speculate that if such a reaction pattern does exist 

 the rate of phosphate addition to the enol group and thus also the rate 

 of ATP-formation at the chlorophyll level could be determined by the 

 rate of electron transport at another part of the chlorophyll molecule 

 (for example, at the 5 -bridge carbon atom (20)) over the conjugated 

 double-bond system. Phosphorylation would here not be "coupled" to 

 electron transport in the classical sense, but only "linked" to it to give 

 a rate-dependency. The phosphorylation reactions at such a site may 

 well have properties which are quite different from an ordinary elec- 

 tron transport-coupled phosphorylation, as, for example, insensitivity to 

 valinomycin, 



CONCLUDING REMARKS 



Our knowledge today about the electron transport and phosphoryla- 

 tion reactions in bacterial photophosphorylation is far from complete. 

 If indeed, asour evidence indicates, there are two phosphorylation sites 

 in the cyclic electron transport chain oiR.rubrum, and if a difference 

 in response to valinomycin means a basically different reaction pat- 

 tern in an energy- transfer step, then it would seem to be important 

 that the material under investigation should be as active and intact as 

 possible in order to minimize erroneous results due to partial and 

 perhaps selective inactivation. 



REFERENCES 



1. Frenkel, A.W., Light induced phosphorylation by cell-free preparations of 

 photosynthetic bacteria. J. Am. Chem. Soc, 76, 5568 (1954). 



2. Smith, L., and Baltscheffsky, M., Respiration and phosphorylation in ex- 

 tracts of Rhodospirillum rubrum. Fed. Proc, 15, 357 (1956). 



