182 PROBLEMS OF PHOTOSYNTHESIS 



ration as well as CO2 fixation, but under the same conditions upon illumina- 

 don they exhibit neither respiration nor CO2 fixation. They do not respire 

 because the H2O2 produced in the light destroys respiration. Thus, the rule 

 that no CO2 fixation is possible without respiration, has been established. 



§ 69 Quinone Catalysis and Phosphorylation 



In § 51 we discussed Anion's discovery that green grana suspended in phos- 

 phate solution at pH 8 are able to synthesize ATP from ADP upon illumina- 

 tion. Warburg (21) found that substances, such as 0.0001 N phenanthroline 

 and 0.01 A^ HCN (at low O2 pressures), which inhibit quinone catalysis also 

 inhibit light phosphorylation. However, at high O2 pressures 0.01 A^ HCN 

 does not or only very slighdy inhibits both quinone catalysis and light phos- 

 phorylation. This specific behavior with respect to HCN proves that a rela- 

 tionship between both processes must exist. It seems therefore contradictory 

 that nevertheless ATP production has no influence whatever upon quinone 

 catalysis. The speed of O2 evolution elicited by the quinone remains the 

 same whether ADP has been added or not, provided, of course, that phosphate 

 is present in sufficient quantities. The explanation lies in the fact that in 

 quinone reactions phosphate is bound under all circumstances. In the ab- 

 sence of ADP this bound phosphate is freed again during the O2 evolution and 

 the amount of free phosphate remains constant in the over-all reacdon. It 

 acts as a catalyzer. In the presence of ADP, phosphate is trapped and the 

 over-all reaction shows its decrease. 



Warburg found that, in the presence of ADP, one molecule phosphate is 

 bound when one molecule HoOo is produced. In other words, one molecule 

 phosphate is bound when one molecule quinone takes up two H atoms. This 

 relationship of 1 ATP and 2 H is the same as that Warburg and Christian 

 found in the oxidation reaction of fermentation (see §44), the only reaction 

 in which the mechanism of phosphorylation has been chemically defined. 

 Thus, the energetics of light phosphorylation can be identified with the ener- 

 getics of dark phosphorylation (21). 



It is certainly permissible to speculate about the substance in the quinone 

 reactions which may be responsible for the binding of phosphate. Warburg 

 proposes a phosphorylated peroxide of COo according to the following reac- 

 tion scheme: 



OPO3H2 



/ 

 quinone + CO> + H,0 + H:iP04 > hydroquinone + C=0 



O— OH 

 OPOnHo 



C=0 + H..O > H,P04 + H.COOH + O, 



\ 

 O— OH 



