Evolution of Photosynthetic Mechanisms 21 



it seems that there is a molecule very similar to this, but involving 

 another phosphate group on one of the ribose molecules, and so I 

 shall actually use the triphosphopyridine nucleotide in its reduced 

 form as the structural formula for the reducing agent which is re- 

 quired to run the carbon reduction cycle. 



The possibility exists that still another, and perhaps more specific, 

 reducing agent might be used by photosynthetic organisms in the 

 reductive splitting of the initially produced carboxylation product 



NH C 



H 



c 



hct ^c 



I! I 



+ N 



/ C -NH 



2 N 

 I 

 HC 



**, 



HC 



I 

 HC-OH 



I 



HC-OH I 



I 

 HC- 



I 

 HC- 



H 



— ' 0" 

 I 



-0-P- 

 II 

 



0" 



I 

 ■o— P- 



II 







"C-N^ 

 ,C-N' 



.CH 



HC 



I 

 HC-OPO3H" 



HC-OH 



I 

 HC 



CH 

 H 



NHo 



I <L 



N C-N v 



I II "CH 



HC^. „C-N' 

 N I 



HC 



I 

 HC-OH 



I 



HC-OH 



HC- 



I 

 HC- 



H 



0" 



1 I I 

 -0-P-0-P-0-P-0H 



TRIPH05PH0P YRIDINE NUCL EO TIDE 

 (OXIDIZED FORM) (TPN+) 



HC 

 II 



c: 



HC X CH 

 N 



I 

 R 



NH 2 



NICOTINAMIDE PORTION OF 

 TPNH (REDUCED TPN +) 



A DENOSINE TRIPHOSPHA TE (A TP) 



IN ADENOSINE DIPHOSPHATE (ADP), 

 TERMINAL PHOSPHATE IS 

 REPLACED BY -OH 



FIG. 2. Structural formulas of triphosphopyridine nucleotide and adenosine tri- 

 phosphate, two of the agents required to run the photosynthetic carbon cycle. 



(Step 1, Fig. I). 10 If so, it is almost certainly as good a reducing 

 agent as TPNH and may or may not be structurally and kinetically 

 related to it. If such a specific photosynthetic reducing agent functions 

 in green plants, it will, in all probability, have been a late addition 

 in the evolutionary development of a higher efficiency, since we al- 

 ready know that the cycle can operate through TPNH. 



The other molecule which is essential for running the cycle and 

 which clearly must come somewhere from the p/jofochemical reaction 

 is the adenosine triphosphate (ATP). Here, there are two pyrophos- 

 phate linkages, and the important one for our purposes is the ter- 



