178 PROBLEMS OF PHOTOSYNTHESIS 



§ 68 Quinone as a Catalyzer 



In §49 we discussed the necessity of COo in the quinone reaction of grana: 



lighl 



2 quinone + 2H2O > 2 hydroquinone + O2 



The necessary half-value pressure of COo should be 10 mm HoO. At a half- 

 value pressure of 1 mm H2O no O2 is produced. However, the grana do not 

 consume CO2, so that the action of COo must be that of a catalyzer. Inter- 

 mediate reactions must therefore take place and it seems quite logical to as- 

 sume the formation of a peroxide. As a matter of fact, the O2 developed is 

 molecular O2 and therefore has to be formed from a peroxide. From a 

 chemical and energetical point of view evolution of Oo in the atomic state 

 must be excluded. Warburg (21) proposes the following reaction scheme: 



quinone + COo + 2HoO » hydroquinone + HO.COOOH 



HO.COOOH > H.COOH + O2 



quinone + H.COOfI > hydroquinone + CO2 



2 quinone + 2H2O > 2 hydroquinone + O2 



Thus, quinone reduces CO2 to percarbonic acid, a well-known substance 

 with the formula 



O 



HO— C 



\ 



O— OH 



It is reduced to formic acid giving off molecular Oo. The formic acid pro- 

 duced in statu nascendi reacts with a second molecule quinone to produce CO2. 

 The intermediate products do not appear in the over-all reaction. It must 

 be assumed that they are all bound to chlorophyll. These reactions do not 

 take place in solutions and are inhibited by 0.001 N phenylurethane. 



It is of course possible to advance a similar scheme assuming that quinone 

 does not oxidize CO2 but water to produce H0O2 : 



2 quinone + 4HoO > 2 hydroquinone + 2H2O2 



2Hobo > 2H2O + O2 



2 quinone + 2H2O > 2 hydroquinone + Oo 



The over-all reaction is the same as before. However, the percarbonic acid 

 pathway seems a prion to be more probable as the experiments show that COo 

 pressure is absolutely indispensable. On the other hand, a formic acid stage 

 has never been established. An important argument against the H2O0 path- 

 way lies in the fact that catalase is present. This enzyme is inhibited by 

 HCN but the O2 evolution in the quinone reaction is not inhibited by HCN. 

 An important condition for the quinone reaction is a sufficiently acid me- 

 dium so that the hydroquinone produced cannot react back with O2. War- 

 burg (21) pointed out how interesting it is to study conditions for obtaining 



