538 LIGHT AND LIFE 



dine nvicleotides with electrons donated by Avater requires a considera- 

 ble input of energy which in photosynthesis is supplied by light. 



We have seen in Section 5 that isolated chloroplasts can reduce 

 TPN in light with an accompanying stoichiometric evolution of oxy- 

 gen, in a reaction that is catalyzed by a specific TPN reductase, an 

 enzyme that is localized in chloroplasts (Equation 1) . Recently, the 

 relation of photosynthetic phosphorylation to the photoreduction of 

 TPN, and indeed to the photoreduction of non-physiological Hill 

 reagents, was found to be very direct (18) . In the presence of ADP 

 and orthophosphate (P) , the photoreduction of TPN and oxygen 

 evolution was coupled with the formation of ATP in accordance 

 with Equation 5. 



2TPN -f 2ADP -f 2P 4- 2HoO -^ 2TPNH2 



+ Oo + 2ATP (5) 



Under appropriate experimental conditions (18a) , the evolution 

 of one mole of oxygen was accompanied by the reduction of two 

 moles of TPN, and the esterification of 2 moles of orthophosphate 

 (Fig. 18). The stoichiometry of this reaction was the same when 

 TPN was replaced by ferricyanide. With either TPN (41) or ferri- 

 cyanide (18, 74) , the rate of oxygen evolution is greatly increased 

 when it is couj^led with phosphorylation. The conventional Hill re- 

 action could thus be viewed as an uncoupled photophosphorylation, 

 i.e., a photochemical electron transport that is proceeding without its 

 normally associated phosphorylation reaction'^. 



The novel features of reaction 5 were, first, that its ATP forma- 

 tion, luilike any other known phosphorylation, was coupled with an 

 evolution of oxygen. Second, contrary to an analogy with oxidative 

 phosphorylation, ATP formation resulted not in the ultimate oxida- 

 tion but in the reduction of pyridine nucleotide. 



The mechanism of reaction 5 was also different, notably in its 



dependence on chloride (30) , from that of the anaerobic cyclic photo- 



j)hosphorylation that is conmion to chloroplasts and chromatophores 



(Section 9) . In the presence of chloride, ferricyanide was not an 



'It may be of interest to record in passing tliat the fiiuiine: of Reaction 5 in 

 chloroplasts was as nnexpcctcd to its discoverers as it was to other investigators in 

 tliis field. The extent to which it was unexpected was expressed by Jagendorf in 

 these words: "When it was announced at the AlBS meetings in August lO.")? that 

 llie 'Hill reaction' is really due to uncoupled phosphorylation, i.e., that ferricyanide 

 reduction was more rapid if phosphorylation occurred simultaneously, we were as 

 surprised and skeptical as anyone else. However, by now we have been able to 

 confimi this very amply" (74. p. 215). 



