614 A. A. KRASNOVSKIÏ 



Other investigators have shown that the reduced pyridine-nucleotides can be 

 utilized in the system of biosynthetic reactions involved in carbon dioxide 

 reduction [26]. 



A similar path of electron transfer is encountered in photosynthetic bacteria; 

 here, however, an 'enzymic pretrcatmcnt' of the original hydrogen donors 

 probably occurs before their interaction with the pigment molecules. 



Back-reactions of Photoproducts: The Chemosynthetic Path of Coupling 



After illuminating photosynthetic organisms for a certain period, Strehler & 

 Arnold observed a faint afterglow, with a spectrum close to that of the fluor- 

 escence of chlorophyll [27]. This chemilumincscence is probably due to back- 

 reactions of the active photoproducts. On the other hand, multistage reverse 

 reactions of active photoproducts, proceeding with the participation of bio- 

 catalytic systems, can give rise to biochemically important high-energy com- 

 pounds utihzed in the constructive metaboUsm of the organisms. The concept 

 of a 'chemosynthetic path' of the utihzation of active photoproducts is set forth 

 in the latest reports of O. Warburg. 'Chemosynthetic' formation of high-energy 

 phosphate compoimds probably takes place as a result of the biochemical oxi- 

 dation of active reduced compounds generated photochemically. 



Uptake of inorganic phosphates upon illumination and the formation of 

 adenosinetriphosphate has been observed in photosynthetic bacteria [28] and 

 higher plants [29]. 



It has recentiy been estabUshed that isolated chloroplasts are also capable of 

 such 'photosynthetic' phosphorylation [30]. The mechanism of this process 

 evidentiy consists in the following. The initial electron (hydrogen) donors are 

 photochemically generated reduced forms of pyridine-nucleotides or flavins ; the 

 final electron acceptor is not oxygen (as in ordinary oxidative phosphorylation) 

 but rather a photochemically formed oxidizing agent, the nature of which has 

 not yet been established; ascorbic acid, quinones and c>tochromes [30] take 

 part in this process as intermediate systems. 



The anaerobic type of oxidoreductive coupled phosphorylation apparentiy is 

 the older metabohc pattern; with the development of photosynthesis and of the 

 aerobic mode of hfc, molecular oxygen came to be used as the ultimate electron 

 (hydrogen) acceptor. The 'uphill' electron transfer due to the photochemical 

 process thus ensures the generation of high-energy phosphate bonds. 



Calvin and coworkers [31] have pointed out that the reaction cycle of carbon 

 dioxide fixation and reduction is rendered possible by feeding energy into the 

 system in the form of two types of active compounds, 'active hydrogen', prob- 

 ably in the form of reduced pyridine-nucleotides which ensure the reduction 

 stage (phosphoglyceric acid -^ phosphoglyceric aldehyde), and the high-energy 

 phosphate compounds required for the phosphorylation of intermediary products 

 of the cycle. 



The system of heterotrophic metabolism in ancient organisms which were not 

 yet capable of Ught utihzation apparently included the steps of catalytic electron 

 transfer and 'anaerobic' oxidoreductive phosphorylation. The development of 



