666 



i 



Martin Gibbs 



Another explanation to account for the unexpected finding of 

 Bas sham and Kirk was recently proposed by Bassham^-'' . Using 

 Lynen's^ '' system for the biosynthesis of fatty acids as a model, 

 he proposed that the enzymes of the carbon cycle are highly or- 

 ganized as a multifunctional enzyme system. He has also specu- 

 lated that sulfhydryl groups and thiamine pyrophosphate would 

 form the bridge between the enzymes and substrates. The impor- 

 tance of an enzyme disulfide functioning in photosynthesis was 

 suggested by Gibbs and Calo^ '^ who observed that arsenite did 

 not inhibit the photochemical act (formation of ATP, TPNH and 

 0_) or any of the enzymes usually associated with the Calvin 

 cycle. This ineffectiveness toward the broken system contrasts 

 sharply with a 90^ inhibition of COp fixation by the intact 

 chloroplast in the presence of 1 x 10 ^M arsenite. 



The observation of Bassham that the ribulose-l,5-diP carboxy- 

 lase system may act differently in the dark or in vitro as 

 contrasted to in vivo in the light indicates that the primary 

 carboxylation reaction needs reinvestigation. 



Enzyme data 



Enzymes of the reductive pentose-P cycle are widespread in 

 nature. None of them with the possible exception of the TPN- 

 linked glyceraldehyde-3-P dehydrogenase^ ^ appear to be the 

 exclusive property of the photo synthetic cell. In contrast to 

 studies with glycolysis, the urea cycle and pyrimidine synthesis 

 where the least active enzyme had a capacity several- fold higher 

 than the overall system, a comparison of the activities of the 

 individual enzymes of reductive pentose-P cycle with the rate 

 of photosynthetic COg fixation of intact cells revealed certain 

 deficiencies. Peterkofsky and Racker^^j reported low activities 

 for transaldolase, ribulose-l,5-diP carboxylase, sedoheptulose- 

 1,7-diphpsphatase as well as forsfructose-l,6-diphosphatase. 

 Richter '^ and Fewson et al. reported the absence of 

 fructose-l,6-diP aldolase from extracts of blue-green algae. 

 Szymona and Doudoroff^ -' and later Richter reported only small 

 amounts of this aldolase in extracts of Rhodop seudomona s 

 spheroides . While caution must be exercised in evaluation of 

 these observations, it does suggest that a search should be made 

 for alternative enzymes catalyzing individual steps of the cycle, 

 particularly where apparent enzyme deficiencies are observed. 



Isotope distribution data /, \ 



The earliest experiments of Calvin and coworkers^ showed 

 that brief exposure of photosynthe sizing higher plants and algae 

 to C^^Op produced 3- PGA labeled mainly in the carboxyl carbon and 



