Active site of the anthrax toxin lethal factor (1YQY.pdb55), with MOLCAD electrostatic potential mapping (red = positive, purple = negative); catalytic Zn2+ (magenta sphere); zinc-binding residues His686, His690, and Glu735; and illustrating three binding subsites: S1’, S1–S2, and S2’,56 visualized in SYBYL 8.0., Tripos, Inc.