Fractionation of Stable Carbon Isotopes by Phosphoenolpyruvate Carboxylase from C4 Plants

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Abstract
The active species of CO2 and the amount of fractionation of stable carbon isotopes were determined for a partially purified preparation of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) from corn (Zea mays) leaves. The rates of the enzyme reactions, using substrate amounts of HCO3-, CO2 or CO2 plus carbonic anhydrase, show that HCO3- is the active species of CO2 utilized by PEP carboxylase. the Km values for CO2 and HCO3- are 1.25 mM and 0.11 mM, respectively, which further suggest the preferential utilization of HCO3- by PEP carboxylase. The amount of fractionation of stable C isotopes by PEP carboxylase from an infinite pool of H12CO3- and H13CO3- was -2.03%. This enzyme fractionation (.DELTA.), the fractionation associated with absorption of CO2 into plant cells and the equilibrium fractionation associated with atmospheric CO2 and dissolved HCO3- are discussed in relation to the fractionation of stable C isotopes of atmospheric CO2 during photosynthesis in C4 plants.