176 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



2 . Quinones as cofactors of cyclic photophosphorylation 



If the added components of the system are cycling, no substrate amounts 

 of a quinone should be required. Indeed, all tested substituted o- and p-benzo- 

 quinones are cofactors of a cyclic photophosphorylation^'': by illuminating 

 chloroplasts with catalytic amounts of a quinone (or hydroquinone) ATP is formed 

 (table 2 and 3). Since ATP formation with benzoquinones in catalytic amounts 

 occurs only in the presence of oxygen'^', this has to be called an aerobic or 

 pseudocyclic photophosphorylation. 



o, 1 /imol cofactor added /imoles ATP formed 



chlorogenic acid 8,2 



caffeic acid 8,1 



dihydroxyphenylalanin (DOPA) 7,8 



dihydroxyphenylethylamin (Dopamin) 8,6 



catechol 6,3 



vitamin K_ 6,4 



Table 2: o-Hydroquinones as cofactors of aerobic photophosphorylation 

 (conditions as in table 1, 15 min light in air). 



The term pseudocyclic was introduced by Arnon' '-'' to distinguish an oxygen 

 dependent photophosphorylation from true oxygen independent cyclic photo- 

 phosphorylation. In aerobic (pseudocyclic) photophosphorylation the cofactor 

 is first reduced in a coupled Hill reaction. The reduced cofactor is then re- 

 oxidized by oxygen. 



Q + H^O + ADP + P. -QH_, + ATP + 1/2 0_ 



2 1 2 i. 



QH^ + 1/2 O^ ►Q + H^O 



sum: ADP + P. ►ATP 



1 



The higher efficiency of certain cofactors of photophosphorylation in an 

 atmosphere of air or oxygen has been observed^ ~ ' . All oxidizable com- 

 pounds, whose oxidized form can act as Hill reagent have to be included in the 

 list of cofactors of aerobic photophosphorylation, among them numerous o- and 

 p- hydroquinones , sonne of which are mentioned in tables 2 and 3. 



Substituted p- and o-benzoquinones , in catalytic amounts are not active 

 as cofactors of true cyclic photophosphorylation in an atmosphere of nitrogen'"'. 

 But a number of naphtho- and anthraquinones are effective in nitrogen^^ ' "' . 

 Such ATP formation independent of oxygen was discovered by Arnon and later 



