Figure 2.

Influence of magnesium ions on the steady-state kinetics of MST enzymes. (A) Plots of percent of maximal velocity vs MgCl₂ concentration for EntC (circles, green line), PchA (squares, red line), and Irp9 (diamonds, blue line). The effect of the magnesium ion on v_max is apparent in this panel. It should be noted that magnesium-dependent inhibition is evident for the isochorismate synthases (isomerases) (EntC and PchA) but not for the salicylate synthase (Irp9). Inset: an enlargement of the upper left-hand corner of the plot. (B) Plots of percent of maximal velocity vs chorismate concentration at 0.5 mM MgCl₂ for EntC (circles, solid green line), PchA (squares, solid red line), and Irp9 (diamonds, solid green line) and at 10 mM MgCl₂ for EntC (circles, dashed green line), PchA (squares, dashed red line), and Irp9 (diamonds, dashed blue line). The decrease in the chorismate K_m with increasing magnesium concentration is easily recognizable in this panel. (C) Table of steady-state kinetic values derived from (B).