Plant and Cell Physiology, 2000, Vol. 41, No. 8 905-910
© 2000 Oxford University Press
Illumination Increases the Affinity of Phosphoenolpyruvate Carboxylase to Bicarbonate in Leaves of a C4 Plant, Amaranthus hypochondriacus
1 Department of Plant Sciences, School of Life Sciences, University of Hyderabad, Hyderabad 500 046, India 2 Molecular Biology and Agriculture Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India 3 Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto, 606-01 Japan
Illumination increased markedly the affinity to bicarbonate of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) in leaves of Amaranthus hypochondriacus L., a C4 plant. When leaves were illuminated, the apparent Km for (HCO3–) of PEPC decreased by about 50% concurrent with a 2- to 5-fold increase in Vmax and 3- to 4-fold increase in Ki for malate. The inclusion of ethoxyzolamide, an inhibitor of carbonic anhydrase, during the assay had no effect on kinetic and regulatory properties of PEPC indicating that carbonic anhydrase was not involved during light-induced sensitization of PEPC to HCO3–. Pretreatment of leaf discs with cycloheximide (CHX), a cytosolic protein synthesis inhibitor, suppressed significantly the light-enhanced decrease in apparent Km (HCO3–). Further, in vitro phosphorylation of purified dark-form PEPC by protein kinase A (PKA) decreased the apparent Km (HCO3–) of the enzyme, in addition increasing Ki (malate) as expected. Such changes, due to in vitro phosphorylation of purified PEPC by PKA, occurred only with wild-type PEPC, but not in the mutant form of maize (S15D) which is already a mimic of the phosphorylated enzyme. These results suggest that phosphorylation of the enzyme is important during the sensitization of PEPC to HCO3– by illumination in C4 leaves. Since illumination is expected to increase the cytosolic pH and the availability of dissolved HCO3– in mesophyll cells, the sensitization by light of PEPC to HCO3– could be physiologically quite significant.
4 Corresponding author: E-mail, asrsl@uohyd.ernet.in; Fax, +91-40-3010120 (or) 3010145.
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