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Articles

Fractionation of Stable Carbon Isotopes by Phosphoenolpyruvate Carboxylase from C₄ Plants ¹

^a Department of Plant Sciences, Texas A&M University, College Station, Texas 77843

The active species of "CO₂" and the amount of fractionation of stable carbon isotopes have been 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 K_m 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 carbon isotopes by PEP carboxylase from an infinite pool of H¹²CO₃^– and H¹³CO₃^– was –2.03. This enzyme fractionation (), together with 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 carbon isotopes of atmospheric CO₂ during photosynthesis in C₄ plants.

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