HILL REACTION AND ITS RELATIONSHIP TO PHOTOPHOSPHORYLATION 413 



As a reminder of my ignorance of the intermediates of the Hill reaction, 

 I have liked to draw it occasionally as shown in Scheme 2. Here the initial 

 reaction is formulated as an oxygen transfer from OR to X (equation i) and 

 the regeneration steps are shown in equations 2 and 3. 



hv 



Chi 



/ \ 



/OR, X\ 



/ \ 



R OX > 10, 



OR + X >R + OX (i) 



(H,Oj 



R + Hill reagent > OR + reduced Hill reagent (2) 



OX >X + iO., (3) 



SchetJie 2 



One might just as well formulate the first reaction as an electron trans- 

 fer, but the resulting pictures tend to look more complicated. What should 

 be emphasized is that Schemes i and 2 are intended to represent more or 

 less the same process, and that we should guard against the danger of 

 reading more information into such schemes than is actually justified by 

 experimental evidence. 



Photophosphorylation with a catalytic amount of cofactor 



Let us proceed to the nature of the relationship of the process of 

 photophosphorylation to the reactions of Scheme 2. 



The net process of "cyclic" photophosphorylation is depicted in 

 equation 3. 



ADP=^- + Pf- >ATP3+OH- (3) 



' cofactor ^•^' 



This was the first type of photophosphorylation recognized [6-8]. It is 

 characterized by the fact that only a catalytic amount of cofactor is added 

 to the chloroplast preparation — the cofactor being anv of a large number of 

 oxidation-reduction compounds which must be added (in addition to 

 Mg++), to elicit the photophosphorylation process. It w^as apparent from 

 the first work of Frenkel that the occurrence of a Hill reaction is certainly 

 not necessary for the occurrence of photophosphorylation. Frenkel worked 

 with bacterial chromatophores which have never been shown to cause a 

 photoevolution of Oo, but which give an excellent photophosphorylation 

 reaction [4, 9]. 



There is abundant evidence, however, that chromatophores catalyze 



