1700 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



(Arguments in favor of such a dismutation reaction of a sulfenic acid were 

 presented by Barltrop et al. 1954.) The result of the reaction (36.16) is 

 the utiHzation of two quanta for the oxidation of one molecule of water, and 

 reduction of one molecule of lipoic acid. 

 The normal redox potential of the couple : 



-S / /— SH 

 (36.17) 



was assumed by Calvin et al. (1954) to be close to that of the pyridine 



nucleotides {i. e., about +0.3 volt), so that the standard free energy of 



reaction (36.18): 



/— S /— SH 



(36.18) < I + H2O > < +1^02 



^— S \— SH 



—which is the sum of (36.14), (36.15) and (36.16)— became, at pH 7: 

 (36.19) ^F = 2 X 23.0 (0.8 + 0.3) = 50 . 6 kcal/mole 



(where 0.8 volt is the potential of the oxygen electrode at pW 7). 

 It was suggested that the dithiol : 



,— SH 



-SH 



reduces DPN (or TPN) in the dark, and the reduction of PGA to triose is 

 then achieved by DPNH2 (or TPNH2) with the assistance of ATP, as re- 

 peatedly suggested before. The high energy phosphate, of course, has to 

 be also produced by light, e. g., by an "energy dismutation" of the type 

 repeatedly discussed before (c/. also section B below) . 



According to Lehninger, three high energy phosphates can be produced 

 by autoxidation of one molecule of DPNH2. The supply of the one ATP 

 molecule needed to reduce PGA would thus increase the quantum require- 

 ment from 2 to 2V3 per two H atoms transferred; if this were all the energy 

 needed for the synthesis of a triose, and if no additional energy (e. g., in the 

 form of more ATP) were required to convert triose to hexose, the overall 

 quantum requirement of photosynthesis would be 5V3- Calvin et al. sug- 

 gested, however, that one additional ATP molecule may be needed to 

 phosphorylate ribulose monophosphate to ribulose diphosphate; this would 

 raise the over-all quantum requirement to 6.0. 



We will return to these estimates in chapter 37D (section 4e). Here we 

 must point out that the theory of lipoic acid as the key catalyst in photosyn- 

 thesis is at this writing unsupported by evidence. Calvin et al. (1954) 

 found that under special conditions the rate of the Hill reaction (with 

 quinone as oxidant) can be accelerated by lipoic acid. Barltrop, Hayes 

 and Calvin (1954) made interesting photochemical experiments on sensi- 



