420 BIRGIT VENNESLAND 



A in Scheme 3. Scheme 6 is drawn to indicate that the dye reacts rather 

 directly with OX as well as with R. 



In addition to the first oxidation-reduction dismutation to form R and OX, 

 the Hill reaction with oxidized dye would include reactions A and B. 

 To explain photo-oxidative photophosphorylation we assume also that 

 molecular oxygen can substitute for the other Hill reagents to reoxidize R 

 by way of a phosphorylating electron transport chain. The regeneration of 

 OR and of X should be understood to proceed according to reactions C 

 and D when a sufficient amount of reduced dye is present. The electron 

 transport chain may contain plastoquinone and cytochrome b. The 

 cyanide-insensitive auto-oxidizability of cytochrome b would be compatible 

 with a position at this point. 



It should be understood in connection with Scheme 6, that though the 

 occurrence of reaction A excludes the occurrence of reaction C for a given 

 molecule of R, and B similarly excludes D, nevertheless, all the reactions 

 could be occurring simultaneously in a given chloroplast suspension, with 

 the relative rates determined by the concentrations of reduced and oxidized 

 dye and by the oxygen tension. "Oxidative" photophosphorylation is 

 rather slow relative to other types of photophosphorylation, although the 

 Hill reaction with the dye is quite rapid. Thus reactions A and B represent 

 the preferred reaction sequence. 



There is one very serious difficulty, however, with the otherwise rather 

 plausible picture in Scheme 6. If reactions C and B can both occur, then 

 why should one need a cofactor at all to elicit photophosphorylation or 

 oxygen exchange ? If the system is really constituted as shown in Scheme 6, 

 it should form ATP in the light while it evolves and reconsumes Og, 

 whether a cofactor is added or not. It is established, however, that grana 

 unsupplemented by cofactor do not cause photophosphorylation or 

 oxygen exchange at an appreciable rate. 



In order to get around this difficulty one must either postulate different 

 or additional phosphorylation sites, or one must establish a necessity for 

 added cofactor in the oxygen evolving step whereby O2 and X are formed 

 from OX. Reactions 3a and 3a' of Scheme i show in principle how the 

 oxygen evolving step might be dependent on cofactor. Here the evolution 

 of oxygen involves the formation of a second reductant, which is reoxidized 

 by a Hill reagent. 



The COo requirement of the Hill reaction 



A rather specific mechanism for the Hill reaction recently proposed by 

 Warburg [31] is depicted in Scheme 7. 



