PHOTOPHOSPHORYLATION IN FLASHING UGHT 213 



BChf + Y + H^ ^BChl + HY (2) 



Cytochrome c (fc'^^) + HY ^Cytochrome c (fb^^) + Y + H^ (3) 



m^ADP + Pi + hyC')^ >.. m^ATP + H^o) (4) 



In this scheme, Eq, 1 indicates the photochemical oxidation of cyto- 

 chrome c, and Eq, 2 shows the formation of the primary reduced sub- 

 stance HY. Eqs. 3 and 4 indicate electron transfer between the oxidized 

 cytochrome c and the reduced substance in the oxidation- reduction 

 chain and the coupled phosphorylation. 



The maximal yield of delayed photophosphorylation found in this 

 work is 0.90 ATP/cytochrome (Table 1). This figure is calculated on 

 the basis of total cytochrome concentration in the chromatophores. If 

 we assume the light-induced oxidation of c-type cytochrome only, the 

 yield (ATP/photochemically oxidized cytochrome) becomes higher. It 

 is calculated to be around 2 on the basis of relative concentrations of 

 different heme protein species in R. rubruni cells (9), if the saturating 

 yield is obtained when the cytochrome c is fully oxidized by the strong 

 flash. This value (ATP/photochemically oxidized cytochrome = ~2) 

 corresponds to the yield of ATP formation per electron transferred 

 in the oxidation- reduction chain of chromatophores (factor w in Eq. 4). 

 This suggests that the number of phosphorylating sites in the redox 

 chain (Eq. 3) is probably two if the two sites are located in series on 

 the chain. Yet this is a rather tentative value for the number of ATP 

 molecules synthesized per electron transferred by the redox chain, 

 and it remains to be scrutinized further. The increased rate of elec- 

 tron transfer at the rate-determining site resulting from the addition 

 of MPM would lead to an increase in the rate of overall electron trans- 

 port, followed by an increased rate ofphotophosphorylation.lt must be 

 noted, however, that in the presence of MPM (and HOQNO or anti- 

 mycin A) the probable loss of one of the phosphorylating sites is 

 expected. The lowering of the quantum efficiency of photophosphoryla- 

 tion by the addition of these reagents (Baltscheffsky, Baltscheffsky 

 and Olson, 24, and our present data) suggests the loss of a phosphory- 

 lating site. 



The amount of total delayed photophosphorylation per flash was not 

 appreciably affected by temperatures, MPM, or HOQNO when the dark 

 periods were sufficiently long. Other data indicate that these factors 

 affect only the dark steps of photophosphorylation. When the dark 

 period is sufficiently long, the amount of total delayed process is de- 

 termined by the amount of the primary product formed by the photo- 

 chemical process during flash. Therefore, it is understood that the 

 changes of the rate of dark processes (consumption of the primary 



