PHOTOSYNTHESIS 



conceivable that there is still another type of process for the 

 generation of ATP which differs fundamentally from the one 

 outlined above, involving the intermediate formation by 

 oxidation of a "high-energy phosphate group" in the form of an 

 enol ester or anhydride. This is suggested by the increasing 

 knowledge of "oxidative phosphorylation" and the participation 

 of metalloproteins in this process, particularly metalloflavins 

 and/or porphyrin, together with the fact that tripositive (higher 

 valence state) ions form stronger and more compact complexes 

 than do dipositive ions (lower valence state). An example of 

 the manner in which such a process might operate is shown in 

 Figure 3. The metal (in this case Fe) in its reduced form 

 (Fe+2) could bind into a complex the terminal phosphate of 

 ADP and one orthophosphate ion. This complex, upon oxida- 

 tion to the higher valence state (Fe+^), would contract and 

 induce the displacement of an OH~ from orthophosphate by 

 the — 0~ atom of the terminal phosphate of ADP, thus produc- 

 ing the stable ATP chelate of the Fe+3, in order that the ATP 

 be liberated for other uses, the Fe+^ must be reduced again to 

 Fc+\ for which the chelation constant is much smaller. The 

 cycle is thus complete, the net result being the transfer of an 

 electron from the reducing agent to some oxidizing agent of 

 higher potential via the Fe atom with the trapping of some (if 

 not all) of the energy of this transfer in the form of ATP. It is 

 interesting to note that Chance (20) has observed an apparent 

 requirement of the reduction step for the liberation of ATP in 

 the course of oxidative phosphorylation. 



The process postulated for the formation of primary 

 reductant and H2O2 has a quantum requirement of 4 for each 

 H2O2 formed or 2 for each RH2 formed. The over-all quantum 

 requirement will depend on the number of RH2 molecules 

 which must be used to form ATP. If all required ATP can be 

 supplied from respiration reactions outside the chloroplast, as 

 may be the case at very low light intensities, the over-all quantum 

 requirement will be 4. At high light intensities the over-all 

 quantum requirement will be 10, 7, 6, or 5, depending on 



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