FIG. 3.

Effects of divalent metal ions on the catalytic activity of ST-PrpA and ST-PrpB. (A) The activity of 200 ng each of purified ST-PrpA and ST-PrpB was assayed under standard conditions (see Materials and Methods) using [³³P]phosphoseryl MBP as substrate with the indicated concentrations of Mn²⁺ or Ni²⁺. Results are given as the percent activity observed with 1 mM of Mn²⁺ and the standard error of the mean (n = 4). For points with no error bar, the error was smaller than the size of the symbol. For ST-PrpA, 100% activity was equal to 0.0302 pmol of ³³P_i released per min, while the corresponding value for ST-PrpB was 0.0304 pmol/min. (B and C) The activity of purified ST-PrpA (B) and ST-PrpB (C) was assayed under the conditions described for panel A, except that, where indicated, the compounds listed were substituted for the activating divalent metal ion, Mn²⁺. All compounds were present at a final concentration of 1 mM. Iron was maintained in the ferrous state by the addition of ascorbic acid to a final concentration of 1 mM. All results are reported as the percentage of activity relative to that observed with Mn²⁺ and standard error of the mean (n = 4).