The interaction of shikimic acid and protein phosphorylation with PEP carboxylase from the C4 dicot Amaranthus viridis

Shikimic acid has been described as a potent competitive inhibitor of the activity of C-4 phosphoenolpyruvate carboxylase (PEPC) from Amaranthus viridis. In the present study, the effects of shikimic acid were examined further with the dephospho (dark-form) and in vitro phosphorylated forms of homogeneous PEPC from A. viridis. Kinetic analysis showed that the inhibitory effect of shikimic acid was dependent on the phosphorylation state of the enzyme. Thus, the I-50 value of shikimic acid for dark-form PEPC was six times lower than that for the phosphorylated enzyme (12 vs 71 mu M, respectively). When Glc6P, an activator of C-4 PEPC, was present in the assay medium, the I-50 value increased 2- and 3-times with the phospho and dephospho PEPC-forms, respectively. Shikimic acid also markedly decreased P-32 incorporation from Mg[gamma-P-32]ATP into the dark-form of C-4 PEPC, but not casein, catalyzed by protein kinase AI Int his way, shikimic acid mimics the behaviour of L-malate, a well-known inhibitor of PEPC, in that it decreases both the enzyme's activity and phosphorylatability. Based on these data, a possible role for shikimic acid in the regulation of PEPC activity in plants is suggested. (C) 1998 Elsevier Science Ltd. All rights reserved.