Plant Physiol.
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Plant Physiology 98:780-783 (1992)
© 1992 American Society of Plant Biologists

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Metabolism and Enzymology

Role of Cysteine in Activation and Allosteric Regulation of Maize Phosphoenolpyruvate Carboxylase 1

Thierry P. Chardot and Randolph T. Wedding

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

The effect of 5-5'-dithiobis-2-nitrobenzoate (DTNB) on the kinetic parameters and structure of phosphoenolpyruvate carboxylase purified from maize (Zea mays L.) has been studied. The Vmax is found to be independent of the presence of this thiol reagent. The Km is increased upon oxidation of cysteines by DTNB. At a substrate concentration higher than Km (3.1 millimolar Mgphosphoenolpyruvate), a significant reversible decrease of the activity is observed. Malate has little effect in preventing the modification of these cysteines. The V type inhibition by malate was also studied at a saturating phosphoenolpyruvate level (9.3 millimolar Mgphosphoenolpyruvate). In the presence of 50 micromolar DTNB, up to 60% inhibition is caused by 15 millimolar malate; however, in the presence of both 50 micromolar DTNB and 50 millimolar dithiothreitol (DTT) this inhibition is reduced to 20%. The presence of DTT alone increases the size of the phosphoenolpyruvate carboxylase molecule as determined by light scattering. The activity at nonsaturating substrate concentration is increased by 36% in the presence of DTT. The oligomerization equilibrium between the dimer and the tetrameric form of the enzyme is affected by cysteine. The Km for the substrate, the sensitivity toward malate, and the size of the enzyme are found to be modified upon incubation in the presence of DTT.

1 Supported in part by grant DCB 88-12484 from the National Science Foundation.






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Copyright © 1992 by the American Society of Plant Biologists