Dynamics of the photosynthesis of carbon compounds I. Carboxylation reactions

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Kinetic studies have been made of the rates of appearance of 14C in individual compounds formed by Chlorella pyrenoidosa during steady state photosynthesis with 14CO 2. These rates have been compared with rates of CO 2 and 14C disappearance from the gas phase during the same experiments. 1. The following results were obtained: 2. 1. After the first few seconds, the rate of appearance of 14C in compounds stable to drying on planchets at room temperature is 95 to 100% of the rate of uptake of carbon from the gas phase. 3. 2. After the first few seconds, the rate of appearance of carbon in compounds isolable by usual methods of paper chromatography constitutes at least 73 to 88% of the rate of uptake of carbon from the gas phase. Compounds formed from the carbon reduction cycle via the carboxylation of ribulose diphosphate account for a least 70 to 85% of the uptake, while carboxylation of phosphoenolpyruvic acid appears to account for at least another 3%. 4. 3. The induction period in the appearance of 14C in stable compounds may be due to a reservoir of intracellular CO 2 and HCO 3 − or to some other volatile or unstable compound. If so, this reservoir contains no more than 1.5 μmoles of carbon, corresponding to about 7 sec carbon fixation in the experiment in which it was measured. 5. 4. No other carboxylation reactions, such as the carboxylation of γ-aminobutyric acid, could be observed. The rate of labeling of glutamic acid after 5 min of exposure of the algae to 14CO 2 reached a maximum rate of about 5% of the total uptake rate, but this labeling appears to be due to conversion of labeled intermediates formed from the carbon reduction cycle or phosphoenolpyruvic acid carboxylation. 6. 5. The in vivo carboxylation of ribulose diphosphate in the light appears to be followed by conversion of the product to one molecule of phosphoglyceric acid, containing the newly incorporated 14CO 2 and one molecule of some other (kinetically distinguishable) three carbon compound. This reaction would be different from the one reported for the isolated enzyme system and the in vivo reaction in the dark, which produces two molecules of 3-phosphoglyceric acid.