A Global fold of C. immitis RpiB showing iodide ions (violet spheres) and anomalous difference Fourier map shown in violet mesh contoured at 5.0 σ. Protomer A is shown in gray ribbons and protomer B is shown in light green ribbons. The oxidized cysteine residues are shown in sticks representation. B Experimental electron density map from combined SAD/MR is shown contoured at 1.0 σ, and the anomalous difference Fourier map is shown in violet mesh contoured at 5.0 σ. C The active site oxidized cysteine is modeled as Cys-OH (sulfenic acid) in two conformations; the shape of the electron density was not consistent with sulfinic acid or sulfonic acid, the latter of which would have produced major steric clash with the main chain of Gly79 and the side chain of Val82. The 2|Fo|-|Fc| map is shown in blue mesh contoured at 1.0 σ.