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J. A. Bassham 



that ferredoxin reduces an enzyme disulfide to disulfhydryl and that the enzyme 

 disulfhydryl in turn functions directly in the reductive carboxylation reaction 

 (24). 



It is also possible that ferredoxin could function directly in the carbon 

 reducing reaction. Valentine et_ al. (25) have shown that in the oxidation of 

 pyruvate in Chlostridlum acidi-urici , ferredoxin takes the place occupied by 

 lipoic acid in other systems and mediates the electron transfer between pyru- 

 vate and NAD. Since ferredoxin contains free sulfide groups we may suppose 

 that it is functioning here as a disulfide cofactor similar to lipoic acid, and 

 that it is accepting an acetaldehyde moiety from the carboxylation of pyruvate. 

 By analogy with the lipoic acid system, we may suppose that the intermediate 

 is acetyl ferredoxin sulfhydryl which then reacts with inorganic phosphate to 

 produce acetyl phosphate and ferredoxin disulfhydryl. 



Possibly ferredoxin contains chains of PeS-FeS-FeS . . arranged so that two of 

 the terminal sulfides have the same orientation as the disulfide of a lipoic 

 acid molecule. At the other ends of these FeS chains, the Fe+3 atoms could 

 accept electrons which would then be transported along the chain to the di- 

 sulfide grouping which would then become disulfhydryl. In this way, the ferre- 

 doxin could function as a mediator between one electron and two electron oxida- 

 tion reduction reactions. 



Accepting electrois singly from the photochemical apparatus of the green 

 cell, ferredoxin might transfer two electrons at the potential of hydrogen gas 

 to the enzyme system responsible for the reductive carboxylation reaction. With- 

 out attanpting to guess the detailed mechanism of this carboxylation reaction, 

 we may nonetheless note that it bears a formal similarity to a reversal of the 

 pyruvate oxidation discussed above. 



With these thoughts in mind, we have attempted to investigate the kinetics 

 of the carbon cycle of photosynthesis in the presence of added chanical agents 

 which might interact with disulfide disulfhydryl systems. In one such study (26) 

 we allowed Chlorella to photosyntheslze in the presence of 1^002 for about 10 

 minutes under steady state conditions at pH 5.0. Without disrupting these 

 conditions we introduced an amount of 8-methyl lipoic acid which gave an approx- 

 imately 0.5 milliraolar solution of this lipoic acid analog. Preliminary studies 

 had shown that such an addition caused an immediate complete inhibition of 

 oxygen evolution and CO2 uptake. 



The effect of the addition of this inhibitor upon the levels of various in- 

 termediates of the carbon reduction cycle and other photosynthetic products are 

 shown in Figs. 3 and 4. The most dramatic effect is the immediate drop in the 

 level of PGA which falls during the first 15 seconds after addition of the 

 inhibitor to about lA of its steady state value. At the same time, the levels 

 of fructose diphosphate and of sedoheptulose diphosphate rise quite rapidly. 

 Surprisingly, in view of the PGA effect, ribulose diphosphate undergoes only a 

 small positive transient and then a slight decrease to a constant level. It was 

 noted that lipoic acid itself also caused inhibition of photosynthesis, and in 

 more recent but unpublished experiments we find that the effects of 8-methyl 

 lipoic acid are reproduced by the same concentration of lipoic acid. 



If the disulfide compound which v;e have added is accepting electrons from 

 the light reaction and thereby keeping them frcm being used in carbon reduction, 

 one might expect the resulting transient changes in the intermediates of the 

 carbon cycle to resemble those seen upon turning off the light. It is clear 



