Figure 6

Organization of the active site residues in B. anthracis Alr is facilitated by a chloride ion. (A) Electron density map (contoured at 1.5σ in the final refined 2F_o-F_c map) showing details of the active site for Alr _Bax . (B) Structural alignment of residues making the active site of various Alrs (TB structure was not included). For all available Alr structures, Arg138 makes polar contacts to the PLP and, possibly, to the substrate. In Alr _Bax this arginine residue is positioned in the active site by a chloride ion (Cl^-). Polar contacts between the chloride ion and Asn-131 and Arg-138 are shown in Panel A by dashes. For all other alanine racemase structures available to date the equivalent interactions are mediated by a carbamylated lysine (shown in Panel B). Residues in the active site of various Alrs are shown as a stick model. In Panel A, the acetate molecule and the modified lysine residue (LLP) are depicted as ball and stick models; carbons are colored in green, nitrogen in blue, oxygen in red, phosphate in orange, sulfur in yellow and the chloride ion is depicted as a light green sphere. In Panel B residues are shown as stick model and are colored according to the legend on figure 3; the PLP cofactors are shown as ball and stick models. In both panels, primed numbers denote residues from the second monomer.