In Figure 6 are shown hypothetical pathways leading 

 from PGA to alanine, serine, aspartic acid, and malic acid. 

 These pathways differ somewhat from known enzymatic 

 pathways, in that, in each step leading to the amino acid, 

 ammonia reacts with a phosphoric acid ester. 



The rapid incorporation of inorganic nitrogen into 

 organic compounds would be brought about by the large 

 negative free-energy change associated with each of these re- 

 actions. Thus these reactions, and not the reductive amina- 

 tion of ketoglutaric acid alone, would account for a major 

 portion of ammonia incorporation during photosynthesis. 



COg 



Reduction Cycle 



H2C-0(P) 



HC-OH 

 I 

 CO2" 



PGA 



H2C-OH H,C-OH 



I I 



HC-0® '^"^ HC-NH2 + HO (?) 



CO2" CO2" 



2-PGA 



SERINE 



-HOH 



H2C 



H2CH 



" r^ NH4+ I ^ 



C-0(P) ». HC-NH2 + HO (P) 



TPNH 



C02' 



CO2 



HO(P)i 



CO2" 



HCH 



I 

 HC-OH 



I 

 CO2" 



MALIC ACID 



HOH 



TPNH 



C02~ 



I 

 HC 



c-o(p) 



COf 



HHi 



CO2- 



ALANINE 



I 

 HCH 



TPNH HC-NH2+H0(£) 



I 

 CO2- 



ASPARTIC ACID 



Figure 6. Hypothetical pathways of photosynthesis of alanine, 

 aspartic acid, serine, and malic acid. 



33 



