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Metabolism and Enzymology

Metal Ion Interactions with Phosphoenolpyruvate Carboxylase from Crassula argentea and Zea mays¹

Department of Chemistry and Biochemistry, California State University, Los Angeles, California 90032

Metal ion interactions with phosphoenolpyruvate carboxylase from the CAM plant Crassula argentea and the C₄ plant Zea mays were kinetically analyzed. Fe²⁺ and Cd²⁺ were found to be active metal cofactors along with the previously known active metals Mg²⁺, Mn²⁺, and Co²⁺. In studies with the Crassula enzyme, Mg²⁺ yielded the highest V_max value but also generated the highest values of K_m(metal) and K_m(PEP). For these five active metals lower K_m(metal) values tended to be associated with lower K_m(PEP) values. PEP saturation curves showed more kinetic cooperativity than the corresponding metal saturation curves. The activating metal ions all have ionic radii in the range of 0.86 to 1.09 Å. Ca²⁺, Sr²⁺, Ba²⁺, and Ni²⁺ inhibited competitively with respect to Mg²⁺, whereas Be²⁺, Cu²⁺, Zn²⁺, and Pd²⁺ showed mixed-type inhibition. V_max trends with the five active metals were similar for the C. argentea and Z. mays enzymes except that Cd²⁺ was less effective with the maize enzyme. K_m(metal) values were 10- to 60-fold higher in the enzyme from Z. mays.

¹ T. N. would like to gratefully acknowledge the receipt of a MARC fellowship from the National Institute of General Medical Sciences (T34 GM07646). This investigation was also supported in part by Public Health Service grant RR-08101 from the MBRS program, Division of Research Resources, National Institutes of Health.