177 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



redoxpotential fxmol ATP 



o, 1 fxmol cofactor added (E in mV) formed in 



^________ N2 air 



p-benzoquinone 293 



2,3-dimethoxy-p-benzoquinone 198 



2 , 3-diniethyl-p-benzoquinone 177 



2 , 5-benzoquinone-diacetic acid 167 



2 , 3 -dinnethoxy-methylbenzoquinone 151 



1 . 4-naphthoquinone- sulfonic acid 118 



2 , 6-diniethoxy-benzoquinone 53 



phenanthrenquinone 28 



2 -hydroxy-benzoquinone -propionate 27 



vitamin K3 - 1 o 



2 -hydroxy -naphthoquinone -154 



phthiokol -I80 



anthraquinone-2 -sulfonic acid -25o 



Table 3: Quinones as cofactors of photophosphorylation in nitrogen and in 

 air (conditions as in table 1), 



termed cyclic photophosphorylation^ ' , to distinguish it from phosphorylation 

 accompanying non-cyclic electron flow in the Hill reaction, which was discussed 

 above. However, Arnon has surmised that the phosphorylating step is identical 

 in the two systems' ^°'. In the truly cyclic electron flow, the reduced cofactor 

 cannot be reoxidized by oxygen (being absent) , but is presumably reoxidized by 

 a component of the endogenous electron transport chain of chloroplasts^ ' . 

 Since the switching from aerobic to cyclic photophosphorylation occurs with co- 

 factors with a redoxpotential around and below zero'9) (table 3 see also (^6 and 

 '), one might conclude, that the endogenous oxidizing component has a redox- 

 potential of about zero volt. Two endogenous compounds of chloroplasts , as far 

 as discovered, have such a redoxpotential: plastoquinone and cytochrome b^ . 

 It is interesting, that Kamen also found an optimum in bacterial photophosphory- 

 lation at a redoxpotential of zero volt(19) 



The conclusion, that a true cyclic photophosphorylation is possible in 

 broken chloroplasts, has been questioned because stimulation of photophos- 

 phorylation (with suboptimal cofactor concentrations) by oxygen has been ob- 

 served. Also, isotope experiments showed fast O2 -exchange between air and 

 water in cyclic photophosphorylation with Sonne of the original cofactors'^o). 

 However, all this shows is that the reduced cofactor of cyclic photophosphoryla- 

 tion is preferentially reacting with oxygen, if present, rather than with the en- 

 dogenous oxidizing component in the chloroplasts. Whereas oxygen is able to 

 react with very small amounts of a hydroquinone, a certain concentration of the 

 cofactor of true cyclic photophosphorylation has to be used to saturate the re- 

 action with the endogenous oxidizing component of the chloroplasts. Also, a 

 certain specifity in the constitution of a cofactor of cyclic photophosphorylation 



