J. A. BASSHAM AND M. CALVIN 



if it were derived from carbon 1 of the more recently formed 

 dihydroxyacetone phosphate. 



It is to be noted that the difference in labeling between 

 carbons 3 and 4 of fructose (hence 4 and 5 of sedoheptulose) was 

 explained as arising from the formation of dihydroxyacetone 

 phosphate (from which carbon 3 of fructose is derived) sub- 

 sequent to the formation of glyceraldehyde phosphate (the 

 precursor of carbon 4 of fructose) , 



Of all the enzymes indicated in Figure 1 only two, the 

 enzyme for the carboxylation of RuDP and the enzyme required 

 for the phosphorylation of ribulose monophosphate, have not 

 been found so far in tissues other than green plants. The 

 preparation of the carboxylation enzyme in a cell-free extract 

 has been reported by Quayle and co-workers (45), and Weissbach 

 et al. have isolated an enzyme preparation which causes the phos- 

 phorylation of RuMP to RuDP (63). It appears that the 

 phosphorylation and the carboxylation may be unique steps in 

 the carbon reduction cycle. If these steps are unique to photo- 

 synthesis, they may be so only because they either require 

 enzymes not found outside green plants or because the particular 

 concentrations of metabolites within the chloroplast are required 

 if these reactions are to proceed at a finite rate. Some evidence 

 bearing on this point has been obtained in this laboratory. The 

 purple photosynthetic bacterium, Rhodopseudomonas, is not only 

 capable of photoreducing COo with H2 but it can also reduce 

 CO2 in the dark with Ho or with organic substrates provided O2 

 is present (53,58). This is usually spoken of as an energetic 

 coupling of some of the energy released in the reaction 



2H2 + O2 > H2O 



to help accomplish the reduction reaction 



2H2 + CO2 > (CH2O), + H2O 



Since the free energy change for the latter reaction as written is 

 negative, it appears that the presence of a stronger oxidizing 

 agent than CO2 is required to produce some component or 



38 



