RECENT KINETIC STUDIES ON THE CARBON REDUCTION CYCLE 

 J. A. Bassham 



The photosynthetic carbon reduction cycle (PSCR cycle) was proposed in 

 essentially its present form about ten years ago (1), following seme seven 

 years of study by Calvin and coworkers, who used 1^*0 as a tracer to follow the 

 path of carbon in photosynthesis. Since that time, it has been rather widely, 

 though not universally, accepted as being the correct primary pathway of carbon 

 dioxide reduction during photosynthesis. Studies from this laboratory (2) 

 showed the PSCR cycle, or a cycle utilizing the same intermediate compounds, to 

 be responsible for by far the greater part of the carbon dioxide reduction. 

 Other later studies showed that carbohydrates are not unique as secondary prod- 

 ucts formed from the intermediates of the catrbon cycle. For example, it was 

 shown that the synthesis of carbon skeletons of amino acids could account for 

 305Z or more of the rate of carbon uptake by the carbon reduction cycle during 

 photosynthesis in Chlorella pyrenoidosa (3;. The pursuit of such quantitative 

 studies led to the development of more sophisticated steady-state, steady 

 tracer level techniques (2, 3). Development of these techniques in tur'n per- 

 mitted us to restudy sane kinetic properties of the PSCR cycle, and to investi- 

 gate seme puzzling facets of the -'-^C labeling patterns which had been noted 

 earlier in this and other laboratories. 



Before presenting these questions and our efforts to find answers to them, 

 I will discuss briefly the PSCR cycle, shown in Pig. 1. In this cycle, the 

 first stable carboxylation product is 3-phosphoglyceric acid (PGA) (^) . 

 All the remaining stable intermediates are sugar phosphates and diphosphates, 

 with carbon skeletons from 3 to 7 carbon atans in length. 



There are four stages in the cycle: 1) Rlbulose-1 , 5-diphosphate (RuDP) is 

 carboxylated and gives two molecules of PGA; 2) PGA is reduced to triose phos- 

 phate; 3) A series of reactions convert five triose phosphate molecules to 

 three ribulose monophosphate molecules; H) The ribulose monophosphate molecules 

 are then phosphorylated with ATP to give RuDP. 



Carbon which enters this cycle as CO2 is later "drained off" in the form of 

 reduced carbon ccmpounds such as PGA or sugar phosphates, by secondary photosyn- 

 thetic pathways. Such pathways lead to the synthesis of fats, proteins, carbo- 

 iTydrates and other products. Part of these products are then used in the 

 synthesis of new chloroplast structure while the remainder is "exported" to 

 non-green parts of the plant cell, or to other parts of the plant in multicellu- 

 lar organisms. 



The asterisks are intended to show the degree of labeling of various carbon 

 atcms in the cycle following a short period (a few seconds) of photosynthesis 

 with 14C02. Newly incorporated I'^COo becomes the carboxyl group of PGA and the 

 unphosphorylated terminal carbon atom of phosphoglyceraldehyde and of phospho- 

 dihydroxyacetone . Condensation of these two triose phosphates with each other 

 results in fruct03e-l,6-diphosphate labeled in carbon atoms 3 and H. However, 



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