PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 385 



Under appropriate experimental conditions [158] 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. 25). The 

 stoicheiometry of this reaction was the same when TPN was replaced by 

 ferricyanide. With either TPN [159] or ferricyanide [95, 104] the rate of 

 oxygen evolution is greatly increased when it is coupled with phosphoryla- 

 tion. The conventional Hill reaction could thus be viewed as an uncoupled 

 photophosphorylation, i.e. a photochemical electron transport that is 

 proceeding without its normally associated phosphorylation reaction. 



It was proposed elsewhere [94, i] that the reduction of TPN by 

 chloroplasts in Reaction 4 involves a non-cyclic electron flow mechanism. 

 Reaction 4 may thus be viewed as being analogous to the non-cyclic 

 electron flow in bacteria (Fig. 24) and diff"ering from it only in those 



/iA oxygen formed 



fiM TPNH2 formed 



3-7 



0-3 



-CI 



+ CI 



•CI 



+ CI 



Fig. 26. Effect of chloride on reduction of TPX and evolution of oxygen. 

 The reaction mixture contained in a volume of 3 ml. chloroplasts (Pi,) containing 

 o • 25 mg. chlorophyll ; and the following, in micromoles : tris acetate buffer, pH 

 8-2, 80; TPN, 4; and a partly purified preparation of photosynthetic phosphopyri- 

 dine nucleotide reductase. The plus chloride treatment received 10 /xmoles KCl. 

 Oxygen evolution was measured manometrically, and the TPNHj formed was 

 measured by its absorption at 340 m/n (Bove, Bove, Whatley, and Arnon [103]). 



aspects that reflect the special enzymic composition of chloroplasts. 

 Unlike photosynthetic bacteria, chloroplasts contain neither X.j-fixing 

 enzymes nor hydrogenase. As a consequence, the electron acceptor end 

 of the non-cyclic electron flow mechanism in chloroplasts can be coupled 

 neither to photofixation of nitrogen nor to photoproduction of hydrogen 

 gas, but only to COg reduction (by way of TPNH.,). 



The most characteristic difference between the non-cyclic electron 

 flow mechanism of chloroplasts (equation (4)) and bacteria (Fig. 24) is in 

 the electron donor system. In chloroplasts the eieciron donor is water 

 (i.e. OH ~) whereas bacteria cannot use water but use inorganic or organic 

 electron donors such as thiosulphate or succinate [94, 136]. 



VOL. U. 2C 



