Ribulose 1,5-bisphosphate and activation of the carboxylase in the chloroplast |
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| Authors: | Sicher R C Hatch A L Stumpf D K Jensen R G |
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| Affiliation: | Department of Biochemistry, University of Arizona, Tucson, Arizona 85721. |
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| Abstract: | 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. |
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