Plant Physiol.
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Plant Physiology 84:1080-1083 (1987)
© 1987 American Society of Plant Biologists

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Metabolism and Enzymology

Regulation of Phosphoenolpyruvate Carboxylase from Crassula argentea1

Further Evidence on the Dimer-Tetramer Interconversion

Min-Xian Wu2 and Randolph T. Wedding

Department of Biochemistry, University of California, Riverside, California 92521

Phosphoenolpyruvate carboxylase in Crassulacean acid metabolism plants during the day exists in dimeric form the activity of which is strongly inhibited by malate. Enzyme purified from Crassula leaves collected during the day and stored at –70°C for 49 days shows a steady progression of change from dimer to tetramer, and this change in oligomeric state is accompanied by a decrease in the sensitivity of the enzyme to inhibition by malate. At 10 minutes preincubation of enzyme after 11 days storage—which is composed of an equilibrium mixture of dimer and tetramer—with malate causes most of the enzyme to be converted to dimer and increases the sensitivity of the enzyme to malate inhibition during assay. Preincubation with phosphoenolpyruvate shifts the equilibrium toward the tetrameric form and reduces the maximal inhibition produced by 5 millimolar malate to less than 20%. However, none of the treatments used resulted in shifting the oligomerization equilibrium completely in either direction. Thus the question of whether some covalent modification of the enzyme, such as phosphorylation, is required to permit complete changes in equilibrium remains open.

2 Present address, Laboratory of Photosynthesis, Institute of Plant Physiology, Academia Sinica, 300 Fenglin Road, Shanghai, China 200032.

1 Supported in part by National Science Foundation grant DMB 85-15181.






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Copyright © 1987 by the American Society of Plant Biologists