Phosphoenolpyruvate Carboxylase from Maize Leaves. Studies Using ß-Methylated Phosphoenolpyruvate Analogues as Inhibitors and Substrates*

Daniel H. Gonzalez and Carlos S. Andreo Centro de Estudios Fotosinteticos y Bioqui'micos, Suipacha 531. 2000 Rosario. Argentina Z. Naturforsch. 41c, 1004—1010 (1986); received June 23/August 15, 1986 Phosphoenolpyruvate Carboxylase, Phosphoenolpyruvate Analogues, Reaction Mechanism, Maize Leaf 1. The phosphoenolpyruvate analogues phosphoenol-a-ketobutyrate and phosphoenol-aketoisovalerate are linear competitive inhibitors of maize leaf phosphoenolpyruvate carboxylase with respect to phosphoenolpyruvate. Phosphoenol-a-ketobutyrate is an excellent inhibitor (K;: 18 |im in the presence of 5 mM MgCl2). The inhibition constant for phosphoenol-a-ketoisovalerate is 0.38 mM under the same conditions. For both compounds, the inhibition is greater in the presence of Mn2+ than with Mg2+. 2. The analogues are dephosphorylated, but apparently not carboxylated, by the enzym e. For the reaction with phosphoenol-a-ketobutyrate, a-ketobutyrate and inorganic phosphate are the reaction products. Bicarbonate and a divalent cation are required for the dephosphorylation reaction. 3. The dephosphorylation reaction is activated by glucose-6phosphate and the Vmax has the same pH dependence as that of the carboxylation of phosphoenol­ pyruvate. The Km for phosphoenol-a-ketobutyrate is reduced in the presence of 5 m M MnCl2 (55 |xm versus 140 |i m with 5 m M MgCl2). The Fmax is essentially the same in the presence of either MgCl2 or MnCl2. These results suggest that the dephosphorylation of the analogues occurs by a mechanism which is similar to that of the carboxylation of phosphoenolpyruvate, and that both reactions have a common rate-determining step.