198G EPILOGUE CHAP. 38 



recently discovered RDP-carboxylating and dismuting enzyme ("ribulose 

 carboxydismutase"), only enzymes (PGA hydrogenase, phosphotriose 

 isomerase, aldolase, transketolase, phosphopentokinase, phosphatase' 

 phosphopentoisomerase) which are known to exist. According to this 

 scheme, the reduction of a molecular of carbon dioxide to sucrose could 

 proceed without the help of light, if two molecules of TPNH, and three 

 molecules of ATP could be made available in appropriate concentrations 

 and m proper location within the cell: a conclusion which challenges 

 experimental confirmation. 



The hypothesis that the photochemical reduction of PGA leads to phos- 

 phoglyceraldehyde seems almost unavoidable; the suggestion that this 

 IS the only photochemically induced reduction step in photosynthesis has 

 all the temptation of plausible simplicity. That this reduction occurs by 

 combnied action of a reduced pyridine nucleotide and a high-energy phos- 

 phate, in exact reversal of the corresponding step in respiration, is undoubt- 

 edly suggestive, but remains to be proved. The demonstration that high- 

 energy phosphate (ATP) actually is synthesized in light by whole cells as 

 well as by chloroplasts (c/. p. 1702-1709), lends support to this hypothesis 

 However, the possibility that PGA is reduced in photosynthesis, either 

 directly by excited chloroplasts or by an intermediate other than TPN 

 (and not requiring the assistance of high-energy phosphate), is not yet 

 excluded. 



Despite the demonstration that phosphoglyceric acid alone is tagged in 

 the first second or two of exposure, the large variety of acids, phosphate 

 esters, and aminoacids, which tagging experiments of slightly longer dura- 

 tion characterize as early products of photosynthesis, remains bewildering. 

 The elucidation of their genesis (and fate) remains a large task for the fu- 

 ture. Most puzzling is the role of moMc acid, which appears in large quan- 

 tities immediately upon the beginning of tagging in light. So does alanine 

 and, to a lesser extent, certain other aminoacids. Whether the appearance 

 of the latter is indicative of an early branching-off of protein synthesis 

 from sugar synthesis, remains to be established. 



* * * 



Not only does the reduction of carbon dioxide in photosynthesis, as we 

 see it now, involve a complex interplay of enzymatic transformations, 

 similar m many respects to those that occur in respiration, but running 

 generally in the opposite direction, but it also shows evidence of being 

 connected to respiration, and probably to other metabolic processes as 

 well, by multiple crosslinks, on different levels of reduction and carbon 

 chain synthesis, with common pools of intermediates, to which several 

 reactions can contribute, or into which several of them can dip. 



