PIIOTOSYNTHKTIC PHOSPHORYLATION IO5 



Chroincitium have been puhlishcd which suggest strongly that this analogy is not 

 far-fetched (30). 



These considerations lead to the expectation that a varied pattern ol enzymatic 

 composition should he found in all of these subcellular photo-active particles, de- 

 pending on their degree of fragmentation. 'Whole' chloroplasts such as those 

 isolated from Spirogyra (29) or spinach (4) appear to contain a great number of 

 enzymes, sufficient in fact to enable the chloroplast to t unci ion as a complete 

 photosynthetic unit(i). Fragmented chloroplasts, grana and chromatophores 

 show fewer enzymatic activities, a number of soluble enzymes being recovered in 

 the supernatant fluid from broken particles (11, 12, 2, 20). However, all of these 

 particles, fragmented or not, exhibit a general reaction initiated by light absorp- 

 tion and presumably intimately connected with the photochemical act. This re- 

 action is the light-activated esterification of adenosine diphosphate (ADP) by 

 inorganic phosphate (Pi) to form adenosine triphosphate (ATP); e.g., ADP + 

 Pi + light— >ATP. The characteristics of this reaction, called 'photophosphoryla- 

 tion,' are c|uite similar in all the difTerent particles encountered, regardless of 

 structural complexity, and can be described adequately by considering any one 

 of the many systems which have been studied. This report will be based on re- 

 sults obtained using bacterial chromatophores. 



HISTORY 



The hypothesis which is favored at present for the general mechanism of photo- 

 phosphorylation pictures the process as a phosphorylation coupled to electron 

 transport in an oxidative system, analogous to the coupled oxidative phosphoryla- 

 tion seen in mitochondria of plant and animal cells. The notion of an oxidation 

 coupled with the photochemical act is quite old. R. Hill was probably the first to 

 formulate such a process specifically in biochemical terms (14). Many others have 

 considered it since, particularly in connection with phosphorylations. An example 

 is the mechanism proposed by Ruben, who formulated a scheme in 1943 whereby 

 the eventual carbon dioxide acceptor — an aldehyde — was activated for carboxyla- 

 tion by an enzymic phosphorylation reaction (25). The energy for this reaction 

 was supplied in his scheme by coupled oxido-reduction reactions utilizing a por- 

 tion of the reductant and oxidant generated photochemically. As models he 

 proposed the well-known phosphorylation mechanisms operative in the oxidation 

 of triosephosphate to i, 3-diphosphoglycerate (33) and in the generation of acetyl 

 phosphate during pyruvic acid oxidation ( 17). Lipmann and Tuttle (18) formu- 

 lated a similar scheme based on stepwise reduction of acylphosphate bonds. 

 Davenport and Hill (9) postulated dismutation reactions between heme protein 

 components to be involved in the energy storage reactions. 



In 1953, Vernon and Kamen noted an enzymic cytochrome c photo-oxidase 



