AN EVALUATION OF THE CARBON REDUCTION PATHWAYS OF PHOTOSYNTHESIS 



Martin Gibbs 



In general the term photosynthetic carbon reduction pathway, 

 reductive pentose-P pathway or Calvin cycle is conventially used 

 to describe the cyclic reaction sequence involved in the conver- 

 sion of CO2 to carbohydrate in a chlorophyll- containing cell. 

 Carbohydrate is considered here to be D-fructose-6-P. 



The historical development of the cycle has been ably de- 

 scribed by Bassham and Calvin^.-'-) . The cycle as it is usually 

 presented was initially set down in paper XXI of the n classic 

 series of papers deriving from Calvin's laboratory^ . The 

 essential points of the cycle are: 



1. conversion of D-ribulose-5-P to D-ribulose-l,5-diP by ATP 

 in the presence of phosphoribulokinase. 



2. carboxylation of ribulose-l,5-diP to produce a yet- 

 unidentified 6-carbon intermediate which is hydrolytically 

 cleaved to produce to 2 molecules of D-3-P-glycerate catalyzed 

 by ribulose-l,5-diP carboxylase. 



3. phosphorylation of 3-P-glycerate and subsequent reduction 

 by 1,3-diPGA by reduced pyridine nucleotide catalyzed by PGA 

 kinase and glyceraldehyde-3-P dehydrogenase, respectively. 



k. conversion of D-glyceraldehyde-3-P to fructose-6-P 

 catalyzed by triose-P isomerase, D-fructose-l,6-diP aldolase, 

 and fructose-l,6-diphosphatase. 



5. subsequent steps catalyzed by transketolase, aldolase, 

 sedoheptulose-l,T-diphosphatase, and xylulose- 5-P-3-epimerase 

 accomplish the conversion of fructose-6-P to ribulose-5-P. 



The stoichiometry of the cycle may be formulated in terms of 

 conversion of CO2 and fHj, in the form of ATP and reduced pyridine 

 nucleotide, to fructose-6-P in the following way. 6 COo + I8 ATP 



+ 10 H2O + 12 PNH > fructose-6-P + I8 ADP + 12 PN + 1? Pi. 



Thus, for the entrance of 6 molecules of COp into the cycle, 6 

 molecules of ribulose-l,5-diP are required. These give rise to 

 12 molecules of 3-P-glycerate which is converted to glyceralde- 

 hyde-3-P and its equivalent, dihydroxyacetone-P of the 12 mole- 

 cules of triose-P so formed, 10 are needed to complete 2 turns 



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