EFFECTS OF QUINONES AND OXYGEN IN THE ELECTRON TRANSPORT 



SYSTEM OF CHLOROPLASTS 



Achim Trebst, Herbert Eck and Sieglinde Wagner 



Quinones always played a major role in new developments in extracellular 

 photosynthesis with isolated chloroplasts . After Hill had demonstrated oxygen 

 evolution accompanying the photosynthetic reduction of ferric salts by chloro- 

 plasts(l), Warburg introduced p-benzoquinone and naphthoquinone-sulfonate as 

 Hill reagents(^). In the discovery of photosynthetic phosphorylation by Arnon 

 it became soon apparent that vitamin K3 = methylnaphthoquinone is one of the 

 most effective cofactors(-^). When the role of a new class of natural benzoqui- 

 nones in oxidative phosphorylation of mitochondria (the ubiquinones) was investi- 

 gated. Crane also found a similar, but somewhat different endogenous quinone 

 in chloroplasts (plastoquinone)(4) and Bishop showed its importance in the pho- 

 tosynthetic reduction of ferricyanide by chloroplasts^^'. Quinones are there- 

 fore involved in the two principal photosynthetic reactions of chloroplast frag- 

 ments—Hill reaction and photosynthetic phosphorylation— in two ways: as added 

 substrate or cofactor and as an endogenous component of the electron transport 

 chain. 



1 . Quinones as substrates for the Hill reaction 



Numerous compounds have been tested and found suitable as Hill reagents, 

 It does not seem surprising, that organic compounds, functioning as Hill rea- 

 gents, have a quinoid structure, since this is the one most easily reduced. 

 Aronoff and Wessels in 1952 investigated the photosynthetic reduction of a num- 

 ber of substituted benzo- and naphthoquinones by chloroplasts in relation to 

 their redoxpotentials'"" ' ' . Wessels observed no reduction of compounds with 

 a redoxpotential more negative than -loo mV. When Arnon had shown that the 

 Hill reaction with ferricyanide or TPN was coupled to a stoichiometric ATP 

 formation according to the equation^"' 



X + H^O + ADP + P. XH^ + 1/2 O^ + ATP 



2 1 2 ^ 



. (9) 



it seemed desirable to investigate the behavior of benzoqumones again 



Table 1 shows the reduction of a number of substituted p-benzo-, naphtho- 

 and anthraquinones in nitrogen by illumination with broken chloroplasts. Oxy- 

 gen evolution as well as coupled ATP formation was measured. 



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