664 



Martin Gibbs 



of the cycle, and 2 molecules can be utilized for fructose-6-P 

 synthesis. 



Evidence for this carbon reduction cycle from C0„ to fructose- 

 6-P was based on: (a) kinetic tracer studies of in vivo systems 

 (b) presence of enzymes catalyzing the postulated reactions (c) 

 distribution of C in the various intermediates following assim- 

 ilation of C Op, and (d) information obtained with inhibitors. 



Kinetic data 



Evidence for the functional role, of this cycle is derived 

 mainly from kinetic studies with^C Op carried out by Calvin, 

 Benson, Bassham and associates^ . Exposure of photosynthesizing 

 algae to G O2 gave rise to most of the isotope in 3-PGA, labeled 

 mainly in the carboxyl-carbon. The cyclic nature of cycle was 

 revealed by the appearance of isotope in the a and p positions 

 of 3-PGA. The discovery of rapidly labeled D-sedoheptulose-T-P 

 and D-ribulose-l,5-diP led to their inclusion. The reciprocal 

 changes in reservoir sizes of ribulose-l,5-diP and 3-PGA observed 

 when algae were subjected to light and dark periods and to chang- 

 ing partial pressures of COp indicated a close relationship be- 

 tween these 3 compounds^3) . While reinvestigating the kinetics 

 of appearance of C in components of the reductive carbon cycle 

 in Chlorella photosynthesizing under steady- state conditions. 

 Bassham and Kirk^ ' could account for only one molecule of 3-PGA 

 for each COp taken up. This observation contrasts with the for- 

 mation of 2 molecules of 3-PGA per carboxylation when the light 

 is turned off. They postulated that light and the intact cell 

 carry out a reductive carboxylation while in the in vitro system 

 or in the dark carboxylation followed by hydrolysis occurs. Both 

 of these mechanisms propose hydrolysis of the hypothetical 6- 

 carbon ketoacid prior to reduction. An alternative mechanism is 

 outlined in figure 1. Here, reduction of the ketoacid to a keto- 

 aldehyde prior to hydrolysis would result in the formation of one 

 molecule of PGA and one of glyceroldehyde-3-P. 



