Active site of alanine racemase from S. pneumoniae. (A) Electron density 2Fo-Fc map of the active site contoured at 1.5σ, excluding solvent. Residues from the first monomer are colored pink, residues from the second monomer are blue and are denoted with primed numbers. The PLP-bound Lys residue (LLP) is grey. (B) Superposition of the active site residues from Gram-positive alanine racemase structures with AlrSP; only S. pneumoniae residues are labeled. Residues pictured are from G. stearothermophilus (yellow) , E. faecalis (green) , B. anthracis (blue) , S. lavendulae (red) , and S. pneumoniae (pink). The chloride ion from the B. anthracis structure is depicted as a blue sphere. (C) Unmodeled electron density (green) found in the active site. 2Fo-Fc (light blue) and Fo-Fc (green and red) maps are contoured at 1.5 and 3.0 σ, respectively. Residues are colored and labeled as described for Figure 4A.