for Mn + + in the biosynthetic pathway from glycolaldehyde- 

 ThPP to glycolate. 



Points (1) and (2) are related to the mechanisms sug- 

 gested earlier for the effect of low CO2 pressure on glycolate 

 concentration. 



Acetate 



As shown in Figure 7, acetyl phosphate can be formed 

 from the carbon reduction cycle via the phosphoketolase 

 pathway. This involves dehydration of the ThPP-acetalde- 

 hyde compound derived from carbon atoms 1 and 2 of ketose 

 phosphates. This route is especially attractive as a photo- 

 synthetic pathway, since it conserves chemical energy and re- 

 quires no oxidation or decarboxylation. Known enzyme sys- 

 tems would readily convert the acetyl phosphate to acetyl 

 CoA for fatty acid photosynthesis. 



Another pathway from the carbon reduction cycle to 

 acetyl CoA could be via oxidative decarboxylation of pyruvic 

 acid. This reaction is of the type we have earlier viewed as 

 unlikely in photosynthesizing chloroplasts on grounds of 

 economy. However, this economy takes on a different aspect 

 if one considers the rapid formation of alanine, which we 

 believe might be a reductive amination of phosphoenolpyru- 

 vic acid derived from the carbon cycle (30). Our experiments 

 indicate that about one-third of all NH4+ uptake occurs 

 via this route. The resulting alanine must be used to a 

 considerable extent in transamination reactions, resultine in 

 the production of pyruvic acid. Although pyruvic acid is not 

 labeled soon enough after the introduction of C^^02 to photo- 

 synthesizing plants to permit us to consider it a precursor to 

 alanine, it does become slowly labeled at later times. Thus 

 pyruvic acid could be a product of transamination from ala- 

 nine. The slow labeling of pyruvate may be because alanine 

 has a very large reservoir, which does not saturate with C^* 

 for some minutes. Once formed, the pyruvic acid cannot 



47 



