Fig. 1.

A top-down view from the N terminus of the coiled coil and a side view of the trigonal thiolate As(III)-binding site illustrate its pyramidal coordination in the interior of the three-stranded coiled coil. (A) Top-down view of the As(III) ion coordinated to the three cysteine residues in position 9 of the heptad repeat with 2F_o − F_c electron density contoured at 1.5 σ overlaid. (B) Side view of the metal-binding site with an F_o− F_c omit map contoured at 5 σ demonstrating the endo coordination of the As(III) ion 1.3 Å below the plane of the Cys Sγ-atoms and on an equal level with the β-methylene protons.