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Articles

Ribulose 1,5-Bisphosphate and Activation of the Carboxylase in the Chloroplast ¹

Department of Biochemistry, University of Arizona, Tucson, Arizona 85721

Ribulose 1,5-bisphosphate in the chloroplast has been suggested to regulate the activity of the ribulose bisphosphate carboxylase/oxygenase. To generate high levels of ribulose bisphosphate, isolated and intact spinach chloroplasts were illuminated in the absence of CO₂. Under these conditions, chloroplasts generate internally up to 300 nanomoles ribulose 1,5-bisphosphate per milligram chlorophyll if O₂ is also absent. This is equivalent to 12 millimolar ribulose bisphosphate, while the enzyme, ribulose bisphosphate carboxylase, offers up to 3.0 millimolar binding sites for the bisphosphate in the chloroplast stroma. During illumination, the ribulose bisphosphate carboxylase is deactivated, due mostly to the absence of CO₂ required for activation. The rate of deactivation of the ribulose bisphosphate carboxylase was not affected by the chloroplast ribulose bisphosphate levels. Upon addition of CO₂, the carboxylase in the chloroplast was completely reactivated. Of interest, addition of 3-phosphoglycerate stopped deactivation of the carboxylase in the chloroplast while ribulose bisphosphate accumulated. With intact chloroplasts in light, no correlation between deactivation of the carboxylase and ribulose bisphosphate levels could be shown.

In contrast, incubation of purified ribulose bisphosphate carboxylase with ribulose bisphosphate irreversibly inhibited activation, especially in the absence of CO₂. Addition of the same amount of ribulose bisphosphate to lysed chloroplasts did cause some deactivation of the carboxylase in the extract, but full activation returned when the ribulose bisphosphate was consumed. The ribulose bisphosphate carboxylase in the chloroplast is not irreversibly inhibited by high levels of ribulose bisphosphate.

³ To whom reprint requests should be sent.

¹ This work was supported by National Science Foundation grant 77-26284 and by Monsanto Agricultural Products Company to RGJ. This is University of Arizona Agricultural Experiment Station Paper 3399.

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