HILL REACTION AND ITS RELATIONSHIP TO PHOTOPHOSPHORYLATION 42 1 



Quinone + COo + HjO + H3PO4 = Hydroquinone + H2PO3-O-COOOH 

 HoPOg-O-COOOH + H.O = H3P04 + 0, + HCOOH 

 Quinone + HCOOH = Hydroquinone + CO2 



Net change : zQuinone + 2H2O = aHydroquinone + O2 



Scheme 7 



Warburg's proposal is based in part on the demonstration that CO2 is an 

 essential requirement for Hill reactions, whether catalyzed by grana or by 

 preparations oi Ch/ore/la cells [31, 52, 53, 54]. In Warburg's mechanism, 

 the reduction of quinone is pictured as occurring in two separate steps. 

 First one mole of quinone is reduced with the simultaneous formation of a 

 phosphorylated peroxide of carbonate. This peroxide is then converted to 

 Oo and "nascent" formate, and the latter substance reduces a second 

 molecule of quinone. Scheme 8 shows how the major flaw in Scheme 6 is 

 corrected by the insertion of the elements of Warburg's reaction sequence. 

 What was needed was a requirement for a cofactor in the oxygen evolving 

 step. This need is provided by the requirement of an oxidant for (CO) (i.e. 

 carbon at the oxidation-reduction stage of formate). The continued 

 operation of the catalytic mechanism requires that [CO] must somehow be 

 reoxidized, and we assume that this oxidation cannot be eftected by O,, 

 even though R can be reoxidized by O.,, with accompanying phosphoryla- 

 tion. It should be noted that the mechanism shown in Scheme 8 does not 



Reduced 

 Hill reagent Hill reagent 



/ \ CO2 < (CO) 



/ Hill reagent ^ V /^ <-'hl 

 / or \ \.^ ^/ ^"' 

 R O, OX (CO;,) ^O, 



2Pi 2 ~ P ~ P ? 



Sc/wnw 8 



provide for any retention of reduced carbon, in keeping with the fact that 

 the grana cause no net fixation of COo. The grana are presumably deficient 

 in the means of causing removal of [CO] in a normal manner. It should also 

 be noted that Scheme 8 accounts nicely for the observed facts that the P/2^ 

 ratio is about one for photophosphorylation coupled to net reduction of a 

 Hill reagent, whereas the P/2f ratio is about two for phosphorylation 

 coupled to photo-oxidation of reduced dye. The reduced dye is assumed 



