Cyclic Regeneration of Carbon Dioxide Acceptor 

 Chl* [0] 



1769 



polytoechoridet 



(ll*IM<) 



(ribulott) 



Fig. 7. — Proposed cycle for carbon reduction in photosynthesis. Heavy lines indicate transformations of carbon com- 

 pounds, light lines the path of conversion of radiant energy to chemical energy and the subsequent use of this energy 

 stored momentarily in some compound (E), to form a reducing agent [H] and oxygen from water. 



.5-phosphate and RDP equal to that between GMP 

 and fructose diphosphate, the free energy change 

 for the reaction below is about —7 kcal.^'-^ 



CH,0© 



c=o 



CHOH 



CHOH + CO, + H,0 



i 



HOH 



CH,0© 



2CHOH + 2H-' 



i 

 CO, 



In the above calculation the concentrations of 

 RDP and PGA measured with Scenedesmus during 

 photosynthesis with 1% COz' are used. The mech- 

 anism of the reaction may consist of the addition of 

 CO2 to the 2,3-enediol sugar formed by enolization 

 of the RDP. The intermediate compound would 

 be 2-carboxypentulose-3. The free energy for the 

 formation of the ion of this acid and H+ {pH 7) 

 from COi and RDP is estimated as zero when the 

 concentration of the intermediate acid is 10~' M. 

 Subsequent hydrolytic splitting of this compound 

 to two molecules of PGA and another hydrogen ion 

 would proceed with a free energy change of —7 

 kcal. 



The energy required to maintain the operation of 

 the proposed carbon reduction cycle might be sup- 

 plied entirely in the reduction of PGA to triose 

 phosphate. If this reduction were accomplished 

 by a reversal of the enzymatic reaction usually writ- 



(27) The intemal energy of the -PCHH" group, exclusive of the 

 energy of bondiog to the remainder of the molecule ia here denoted by 

 © and assumed constant throughout. 



(28) J. A. Bassham, Thesis, submitted as partial fulfillment of re- 

 quirements for the degree of Doctor of Philosophy, University of Cali- 

 fornia, 194B. 



ten, each "turn" of the cycle would be represented 

 by three times the reaction 



2DPN[H,1 -I- 2ATP -|- CO, — *■ |CH,0| -f (A) 

 + 2DPN -t- 2ADP -t- 2© -f H,0 



This is the sum of the reactions 



2[DPN1H,1 + V1O. — »- 



DPN -I- H,0) AF - -101 lead. (B) 



2[ATP — »-ADP-f©I A^ - -21 kcal. (C) 



CO, -f H,0 — *- O, -f- |CH,0| HF " -1-116 kcal. (D) 



The efficiency of the transfer of energy of reactions 

 B and C to reaction D is 116/(21 + 101) = 0.96. 



However, additional energy might be supplied 

 to the operation of the cycle by phosphorylation 

 reactions in which additional molecules of ATP are 

 required. One such reaction may well be the phos- 

 phorylation of ribulose monophosphate to give ribu- 

 lose diphosphate. In this case, one additional 

 molecule of ATP would be required per molecule 

 of CO, reduced. The efficiency of the net reaction 

 (A') would then be 116/132.5 = 0.88. 



2DPN[H,) -I- 3ATP + CO, >- 



|CH,0| -I- 2DPN -f- 3ADP -f 3© -t- H,0 (A') 



The over-all efficiency of photosynthesis would 

 be the product of 0.96 or 0.88 and the efficiency of 

 the process by which water is photolyzed to give 

 oxygen with the production of reducing power, fol- 

 lowed by the conversion of the energy of this re- 

 ducing power to DPN[H,] and ATP. 



If the mechanism for photolysis of water in- 

 volves thioctic acid, as has been proposed,^ the 

 energetics of the photochemical and following steps 

 can be estimated 



[Y + HOH -^ If 



(E) 



S — S 



SH SOH 



(29J J. A. Barltrop, P. M. Hayes and M. Calvin, to be pnblbhed. 



101 



