HILL REACTION AND ITS RELATIONSHIP TO PHOTOPHOSPHORYLATION 415 



occur in association with the photon-requiring oxidation-reduction 

 reaction (i.e. equation i in Scheme 2). This is a possibihty that has to be 

 considered. The mechanism in Scheme 3 has the advantage of providing 

 an analogy to mitochondrial oxidative phosphorylation. The grana contain 

 cytochrome b [16, 18] and the quinone derivative Q255 [19, 20], and these 

 components are very similar to some of the mitochondrial constituents 

 thought to be rather intimately involved in the oxidative phosphorylation 

 process [21-25]. In mitochondrial oxidative phosphorylation, the phos- 

 phate " pick-up " occurs during a dark reaction with a negative free-energy 

 change. This would presumably also be the case in Scheme 3. If the 

 phosphorylation were associated with a reaction occurring against a 

 thermochemical gradient and consequently requiring light energy, one 

 might expect it to be quite different in nature. 



Stoicheiometric photophosphorylation 



If the mechanism shown in Scheme 3 is accepted for the purpose of 

 the argument, a question may be posed with the aid of the diagram in 

 Scheme 4. Here a catalytic amount of cofactor is visualized as shuttling in 

 the manner indicated by the equations. Does the phosphorylation occur 

 during reaction A or B ? This question requires an admission that there 

 may actually be several sites for photophosphorylation. Let us assume at 

 this stage that there is only one site. 



R OX 



cofactor 



A. Oxidized cofactor +R > reduced cofactor + OR 



B. Reduced cofactor + OX > oxidized cofactor + X 



Scheme 4 



There is convincing evidence that if the phosphorylation occurs as 

 depicted in Scheme 3 and at only one site, then this site must be A and 

 not B. It has been shown that photophosphorylation can be coupled to the 

 net photoreduction of Hill reagents such as TPN, ferricyanide, and 

 naphthoquinone sulphate [8, 26-31]. Arnon has termed the process 

 "stoicheiometric" photophosphorylation, to distinguish it from "cyclic" 

 photophosphorylation. The stoicheiometry of the photophosphorylation 



