Crystal structure of R. capsulatus ALAS. The distribution of color indicates: in pink, amino acids with ionizable side groups whose theoretical pKa is lower than 5 (i.e., Asp and Glu); in cyan, amino acids with ionizable side groups whose theoretical pKa is greater than 5 (i.e., His, Cys, Tyr, Lys, and Arg); in yellow, amino acids with hydrophobic side chains (i.e., Ala, Val, Leu, Ile, Trp, Phe, and Met) and in white, all other amino acids (i.e., Ser, Thr, Asn, Gln, Pro, and Gly) [PDB 2BWO].