Abstract
1. The phosphoenolpyruvate analogues phosphoenol-α-ketobutyrate and phosphoenol-α-ketoisovalerate are linear competitive inhibitors of maize leaf phosphoenolpyruvate carboxylase with respect to phosphoenolpyruvate. Phosphoenol-α-ketobutyrate is an excellent inhibitor (Ki: 18 μm in the presence of 5 mM MgCl2). The inhibition constant for phosphoenol-α-ketoisovalerate is 0.38 mᴍ 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 enzyme. For the reaction with phosphoenol-α-ketobutyrate, α-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-6-phosphate and the Vmax has the same pH dependence as that of the carboxylation of phosphoenol pyruvate. The Km for phosphoenol-α-ketobutyrate is reduced in the presence of 5 mᴍ MnCl2 (55 μᴍ versus 140 μᴍ with 5 mM MgCl2). The Vmax 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.
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