FIGURE 8.

Reduced PcrAB structures with substrate analog bound and open and closed conformation of PcrA Phe¹⁶⁴. A, active site of PcrA (blue) with selenite (SeO₃²⁻) coordinated to the molybdenum atom. The occupancy of selenite in the structure is 0.6. The surfaces in A and C are colored in a hydrophobicity scale (26) where red is the most hydrophobic and blue is the most hydrophilic. B, electron density of 2|F_o| − |F_c| simulated annealing omit maps at 1.5 σ contour level of PcrA cofactors and active site for reduced PcrAB with selenite bound. The anomalous map is shown as a green surface at 3 σ contour level. C, overlay of the active sites of oxidized (magenta) and reduced (tan) PcrA along with the tunnel to the active site. The reduced PcrA structure reveals dual conformations of the Phe¹⁶⁴ side chain in open and closed states. The tunnel is shown in the open conformation of Phe¹⁶⁴. The water molecule in reduced PcrA (tan) is located inside the open tunnel and hydrogen-bonded with residues Asp¹⁷⁰ and Asn³⁵. D, electron density of 2|F_o| − |F_c| simulated annealing omit maps at 1.5 σ contour level of PcrA cofactors and active site for reduced PcrAB with the side chain of Phe¹⁶⁴ in the open and closed conformations. E, structure of the open tunnel in PcrA with Phe¹⁶⁴ in the open tunnel conformation shown as a surface (light purple).