Enzymatic Properties of Phosphoenolpyruvate Carboxylase Isoforms in the CAM Plant Kalanchoe daigremontiana

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Three isoforms (Types 1, 2 and 3) of phosphoenolpyruvate (PEP) carboxylase in young leaves of the Crassulacean acid metabolism (CAM) plant Kalanchoe daigremontiana were separated by DEAE-cellulose column chromatography and preparative polyacrylamide-agarose gel electrophoresis, and their enzymatic properties were characterized. All three isoforms had similar molecular weights of about 234,000. At pH 8.0 Type 1 showed a high affinity to PEP, (K_m=0.08 mM), whereas Type 3 showed a low affinity (K_m=1.0 mM). K_m values for MgCl_2 were 0.26 mM in Types 1 and 3 and 0.5 mM in Type 2. All three types exhibited the same pH optimum at 8.0, but Type 1 showed relatively low activity below pH 6.0, whereas Type 3 showed high activity. Type 3 was more acid stable than the other forms. In the presence of glucose-6-phosphate, the K_m values of Types 1, 2 and 3 for PEP lowered to 0.027, 0.037 and 0.044 mM at pH 8.0, respectively. Inhibition of activity by organic acids such as malate and pyruvate was pronounced in Type 3. Type 2 exhibited properties intermediate to Types 1 and 3 with regard to pH curve, af Finity to PEP and its effect of various metabolites. The physiological significance of PEP carboxylase isoforms in CAM plants is discussed on the basis of these findings.
日本植物生理学会の論文