646 



J. A. Bassham 



4.8 

 4.4 



4.0 



ijj 3.6 ~ 

 < 



3 3.21- 



< 



f^ 2.8- 

 ^2.4h 

 CO 2.0 

 I 1.6 

 12- 



B- 

 .4- 



CO2-AIR 



I ■ I 



/l^ 



(;■<%> -/^//? 



-OOP- I 



14 16 18 20 22 24 26 28 30 32 

 TIME (MIN.) 



Pig. 6. Effects of sudden removal and readditlon of CO2 on levels of PSCR cycle . 

 Sugar diphosphates and PGA. • Ribulose-l,5-diphosphate, D 3-phosphoglyceric 

 acid , B f r uctose-1 , 6-diphosphate , O sedoheptulose-1 , 7-diphosphate . 



reduction of newly incorporated carbon to the level of bound triose phosphate 

 would be required if we are to invoke the bound glyceraldehyde phosphate moiety 

 as an explanation for the asymmetry of the hexose phosphate labeling and the 

 slow labeling of carbon atan '4 of sedoheptulose phosphate. 



The mediation of the reactions of the cycle may be accomplished by a multi- 

 functional, organized enzyme systan. This systan should be closely linked to 

 the photochemical apparatus, perhaps by such a compound as PPIE (chloroplast 

 ferredoxin). The systan should include enzyme "handles" for holding some of the 

 intermediate compounds in a bound form. Bound forms might include glycolalde- 

 hyde, bound to thiazolium groups in transketolase reactions, and aldehyde 

 moieties bound to disulfide/disulfhydryl groups in carboxylation, condensation 

 and epimerisation reactions. Attenpts to isolate active enzymes from the cell may 

 result in the loss of the organization and primary enzymic activities. Residual 

 enzymic activities found in the soluble isolated protein could mimic the reactions 

 of the intact system. Such activities might well be variable and in many cases 

 inadequate to accomplish the reactions of the PSCR cycle at anything like the 

 in vivo rate. 



I 

 I 



I 



The work described in this paper was sponsored by the United States Atomic 

 Energy Commission. 



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