Active site of S. aureus alanine racemase. (a) Superposition of the active-site residues of alanine racemases from S. aureus (green), G. stearothermophilus (blue), B. anthracis (red), M. tuberculosis (orange) and P. aeruginosa (purple). The chloride present in the AlrBax structure is shown as a pale green sphere. Residues labelled in red boxes lack density in the AlrSas structure. (b) 2Fo − Fc electron-density map of the active site contoured at 1.0σ. The side chains of the AlrSas active site are depicted as sticks; C atoms are coloured green, O atoms red, N atoms blue, S atoms yellow and phosphate orange. The PLP cofactors from each structure are depicted as ball-and-stick models. Important water molecules that are alluded to in the text are shown as grey spheres. In both panels the acetate and sulfate from the AlrSas structure are represented as ball-and-stick models; C atoms are coloured black, O atoms red and S atoms yellow. Primed numbers denote residues that are contributed by the second monomer.