1770 



Bassham, Benson, Kay, Harris, Wilson and Calvin 



Vol. 76 



(where the symbol / represents the side chain: 

 — (CH2)4COjH). 



+ 1 I + HjO + 'AO, (F) 

 2SH SOH SH SH S — S 



In this process, two quanta are required for each 

 dithiol molecule formed. The stored energy is the 

 sum of the energies of the two half reactions 



2H+ + 2e- + "AOj Af = +37.5 kcal. (G) 



H,0 



C( 



+ 2H 

 S — S 

 which is 



H,0 + 



+ 2e-- 



HS 



E =• -0.3 v.» 

 AF= +13.8 kcal. 



SH (H) 



S — S 



(2A») {\^ 



- — ► I I +'AO, 



i>.F 



HS SH 



51.3 kcal. 



(I) 



Since the energy available from two light quanta at 

 7000 A. is 2 X 40.7 or 81.4 kcal., the efficiency of 

 this process would be 51.3/81.4 = 0.63. 



If Co-I is used in the reduction of PGA, the re- 

 duced coenzyme could be formed with high efficiency 

 from the dithiol 



DPN + SH SH 



DPNlH.l + S — S 



LP = -0.8 kcal. (J) 



The required ATP could be formed in some way 



by oxidation of SH SH or DPN [Hi] by an ener- 

 getic coupling of the reactions 



DPNIH,] + 'AO, — *■ DPN + HjO 



AF = -50.5 kcal. (K) 

 ADP + © > ATP Af - +10.5 kcal. (L) 



Since from one to four molecules of ATP might be 

 formed per DPNfHj] oxidized, a wide range of ef- 

 ficiencies would be possible. A value of three has 

 been suggested*' and if this is used, the resulting 

 coupling reaction could be written 



DPN[H,] + VtO» + 3ADP + 3© *- 



DPN + H,0 + 3ATP (M) 



Multiplying reaction J by 3 and combining with 

 reaction M we have 



(T 



3SH SH + 2DPN + 3ADP + 3© + 'AO, — *■ 



(SO) I. C. GunsaJus, Bymposlnm oa "Mechaaism of Bazyme Ac- 

 tion," McCollum-Pratt Inatitute. Johiu Hopkins Univcnity, 1033, to 

 be published. 



(31) A, L. Lehnioger, "Phosphorus Metabolism," Vol.1, Johns Hop- 

 kins University Press, 1961, page 344. 



+ 2DPN[H,] + 3ATP 







+ H,0 + 3S — S 



(N) 



in which the stored energy is 132.5 kcal. and the en- 

 ergry expended is three times reaction I = 154 kcal. 

 The efficiency of the energy transfers represented 

 by reaction N is then 132.5/154 = 0.86. 



Combining the efficiencies of reactions A', I 

 and N results in a calcidated over-all efficiency for 

 photosynthesis of 0.88 X 0.63 X 0.86 = 0.48. 

 Since Uie mechanism outlined above would require 

 six quanta for each molecule of carbon dioxide re- 

 duced (two quanta for each molecule of dithiol used 

 in reaction N) this efficiency can be obtained di- 

 rectly from the energy of these quata (244 kcal.) and 

 the energy of reaction D: 116/244 = 0.48. 



Higher apparent efficiencies would be obtained 

 at low light intensities where the dark internal con- 

 version of prior storage products (involving no net 

 uptake of oxygen or evolution of COj) would sup- 

 ply appreciable amounts of ATP, DPNH, reduced 

 thioctic acid and possibly intermediates of the Oj 

 evolution chain as well." 



Since reaction I as written stores only 51.3 kcal. 

 of 81.4 kcal. available, it is posable that some 

 mechanism may exist for the storage of some of this 

 energy in the form of either additional reducing 

 power or high energy phosphate. In this case, the 

 over-all efficiency would be higher. 



6. Other Biological Evidence. — The intercon- 

 versions of the five-, six- and seven-carbon sugars 

 are being investigated by several laboratories. The 

 postulated cychc reactions which our data suggest 

 are consistent with the observations of these various 

 groups. Both the work of Axelrod, et al.,'* with 

 spinach preparations and the results reported by 

 Dische and Pollaczek" with hemolysates demon- 

 strate the sequence 



ribose phosphate — >■ heptulose phosphate + 



triose phosphate — *■ hexose phosphate 



Recently studies have been made of the distribu- 

 tion of C" in products resulting from conversion of 

 l-C* labeled pentoses. Neish" has studied the 

 products of bacterial metabolism of several pentoses 

 while Wolin, et al.,** investigated the products of 

 enzymatic conversion of ribose-5-phosphate. In 

 both cases, the distribution of radioactivity in the 

 products coidd be accounted for by a reversal of 

 the reactions herein suggested, although a limited 

 number of other interpretations of their data are 

 possible. 



BCKKBLBV, Cal. 



(32) Z. Dische and B. Pollaczek, paper presented at Second Inter- 

 national Congress of Biochemistry, Paris, France. 1952. 



(33) A. C. Neish. paper presented at American Society of Bacteriolo- 

 guts Meeting. San Francisco, Calif., 1953. 



(34) H. B. Wolin. B. L. Horecker, M. Gibbs and H. Klenow, paper 

 presented at Meeting of American Institute of Biological Sciences, 

 Madison, Wisconsin, 1963. 



102 



