Potential hydrogen bonds are shown with grey lines, red spheres represent water molecules, and residues are shown as sticks and colored by element. The beginnings of the hot dog helices are shown with faint line ribbons. The images are drawn to scale and superimposable. (A) The active site of T. lanuginosus FAS DH domain (PDB#2UVA), an example of a THD DH, in blue. The crystal structure for this enzyme did not include waters. (B) The active site of A. caviae PhaJ (PDB#1IQ6), an SHD hydratase, in maroon. (C) The active site of C. tropicalis Mfe2p hydratase domain (PDB#1PN2), a DHD DH, in olive.