FIG. 3.

Activation of FosB with Ni²⁺ and Mg²⁺ using l-cysteine as the thiol substrate. The experimental data for Ni²⁺ (●) and Mg²⁺ (▪) were fit to the quadratic form of a simple binding isotherm to obtain the activation constant for the metal (K_act^M) and the maximum rate constant (k_max^M) for activation under the conditions of the assay. The solid lines are fits of the experimental data for the two metals, with K_act^Ni = 9 ± 3 μM, k_max^Ni = 1.0 ± 0.1 s⁻¹, K_act^Mg = 180 ± 30 μM, and k_max^Mg = 0.75 ± 0.04 s⁻¹. For comparison, the results for Mn²⁺ under the same conditions were K_act^Mn = 3.5 ± 0.9 μM and k_max^Mn = 0.11 ± 0.01 s⁻¹.