Metal dependence of LpxK activity. (A) Purified LpxK, which was first incubated with EDTA, was then diluted into assay buffer containing 2 mM ATP and the indicated cation, and reaction was quenched by spotting on a TLC plate after 8 minutes. (B) The standard assay was performed in the presence of 5 mM ATP, and Mg2+ ranging from 0.015 to 128 mM. The cation is necessary for activity, but becomes inhibitory at high concentrations with respect to ATP.