384 



DANIEL I, ARNON 



13. The photoreductant in plants: non-cyclic photophosphorylation 



Photosynthetic bacteria can reduce pyridine nucleotide either with 

 molecular hydrogen in the dark or with a less reduced electron donor, 

 organic or inorganic, in the light. Green plants do not ordinarily contain 

 hydrogenase, hence they cannot use hydrogen gas during photosynthesis 

 for reducing pyridine nucleotide in the dark. They use water as the 

 electron donor. The reduction of pyridine nucleotides with electrons 

 donated by water requires a considerable input of energy which in photo- 

 synthesis is supplied by light. 



2 3 



//M TPN added 



Fig. 25. Stoicheiometry of oxygen evolution and ATP formation resulting 

 from the photochemical reduction of TPN (Arnon, Whatley, and Allen, [95, 158]). 



As already mentioned, isolated chloroplasts were known to reduce 

 TPN in light with an accompanying evolution of oxygen [42, 43, 5]. This 

 was regarded as a Hill reaction in which TPN served as the hydrogen 

 acceptor. There was no evidence that this photochemical reduction of 

 TPN was in any way linked with photosynthetic phosphorylation. 

 Recently, however, the relation of photosynthetic phosphorylation to the 

 photoreduction of TPN which at first seemed remote, was found to be 

 direct [95, 104]. 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 (4). 



2TPN + 2ADP+2P + 2H0O -> 2TPNH2 + O0 + 2ATP 



(4) 



