PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVTIRSION PROCESS 367 



Two such cases of photosynthesis in Chromatium have recently been 

 described by Losada et al. [121]. In one case the sole source of carbon was 

 acetate and in the other, COo. The photoassimilation of acetate occurred 

 in the absence of an external hydrogen donor whereas in the photo- 

 assimilation of COo the reductant was exogenous hydrogen gas. The sole 

 contribution of light in both cases was the formation of ATP. 



In the photoassimilation of CO.,, ATP was required for the formation 



ATP 



V 



ribulose-S-P 



ribulose di-P 

 CLUTAMATE 



triose-P 



itaconate P-glycerate 



II 



P-enolpyruvate 



DPN + H; 

 ATP ASPARTATE 



citramalate 



acetate/ yhc-Cok-^ — | ACETATE 



ATP 



_ . [acetaldehyde] + CO2 



• pyruvate -"^ citrate 



^~^ --ALANINE 



Ac-CoA 



CO. 

 ATP— ^^ TPNH 



ate-<—'socitrate-»-rt-ketogluta rate-*- GLUT A MATE 



glyoxylate 



Fig. 13. Reactions of carbon assimilation in CJimmatinm. Further details are 

 given by Losada, Trebst, Ogata, and Arnon [121]. 



of an activated intermediate (ribulose diphosphate, phosphoenolpyruvate, 

 or 1,3-diphosphoglycerate) for a subsequent carboxylation or reduction, 

 whereas in the photoassimilation of acetate, ATP was required for the 

 activation of the carbon source itself, by forming acetyl-CoA from acetate 

 and coenzyme A. The activated compounds then become ready for 

 participation in the synthetic reactions that are catalyzed by specific 

 enzyme systems, all of which function in the dark. A summary of the 

 reactions of ATP, that have now been experimentally documented [121] 

 in the carbon metabolism oi Chromatium, is given in Fig. 13. 



Evidence that the sole contribution of light in these reactions is the 



