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

Metal ion interactions with phosphoenolpyruvate carboxylase from the CAM plant Crassula argentea and the C(4) plant Zea mays were kinetically analyzed. Fe(2+) and Cd(2+) were found to be active metal cofactors along with the previously known active metals Mg(2+), Mn(2+), and Co(2+). In studies with the Crassula enzyme, Mg(2+) 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 A. Ca(2+), Sr(2+), Ba(2+), and Ni(2+) inhibited competitively with respect to Mg(2+), whereas Be(2+), Cu(2+), Zn(2+), and Pd(2+) showed mixed-type inhibition. V(max) trends with the five active metals were similar for the C. argentea and Z. mays enzymes except that Cd(2+) was less effective with the maize enzyme. K(m) ((metal)) values were 10- to 60-fold higher in the enzyme from Z. mays.