Model depicting the positioning of the l-serine external aldimine in the active site of ALAS. The model was created by superimposing the l-serine external aldimine of S. paucimobilis SPT (PDB file 2W8J) with the glycine external aldimine of R. capsulatus ALAS (PDB file 2BWP). l-Serine is shown in green and glycine in cyan. Succinyl-CoA was subsequently modeled into the active site by superimposing the generated structure with the structure for the R. capsulatus·succinyl-CoA complex (PDB file 2BWO), using PyMOL. R. capsulatus ALAS amino acids Thr-83, His-142, Lys-248, and Arg-374 correspond to mALAS2 Thr-148, His-207, Lys-313, and Arg-439, respectively.