Figure 4.

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 Alr_Bax structure is shown as a pale green sphere. Residues labelled in red boxes lack density in the Alr_Sas structure. (b) 2F _o − F _c electron-density map of the active site contoured at 1.0σ. The side chains of the Alr_Sas 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 Alr_Sas 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.