MECHANISTIC STUDIES OF PHOSPHOENOLPYRUVATE CARBOXYLASE FROM ZEA-MAYS WITH (Z)-3-FLUOROPHOSPHENOLPYRUVATE AND (E)-3-FLUOROPHOSPHOENOLPYRUVATE AS SUBSTRATES

The catalytic mechanism of phosphoenolpyruvate (PEP) carboxylase from Zea mays has been studied using (Z)- and (E)-3-fluorophosphoenolpyruvate (F-PEP) as substrates. Both (Z)- and (E)-F-PEP partition between carboxylation to produce 3-fluorooxalacetate and hydrolysis to produce 3-fluoropyruvate. Carboxylation accounts for 3% of the reaction observed with (Z)-F-PEP, resulting in the formation of (R)-3-fluorooxalacetate, and for 86% of the reaction of (E)-F-PEP forming (S)-3-fluorooxalacetate. Carboxylation of F-PEP occurs on the 2-re face, which corresponds to the 2-si face of PEP. The partitioning of F-PEP between carboxylation and hydrolysis is insensitive to pH but varies with metal ion. Use of O-18-labeled bicarbonate produces phosphate that is multiply labeled with O-18; in addition, O-18 is also incorporated into residual (Z)- and (E)-F-PEP. The 13(V/K) isotope effect on the carboxylation of F-PEP catalyzed by PEP carboxylase at pH 8.0, 25-degrees-C, is 1.049 +/- 0.003 for (Z)-F-PEP and 1.009 +/- 0.006 for (E)-F-PEP. These results are consistent with a mechanism in which carboxylation of PEP occurs via attack of the enolate of pyruvate on CO2 rather than carboxy phosphate. In this mechanism phosphorylation of bicarbonate to give carboxy phosphate and decarboxylation of the latter are reversible steps. An irreversible step, however, precedes partitioning between carboxylation to give oxalacetate and release of CO2, which results in hydrolysis of PEP.