2.8.3.18 crystal structures of a C-terminally His6-tagged form of several wild-type and mutant complexes, including freeze-trapped acetylglutamyl anhydride and glutamyl-CoA thioester adducts. The latter shows the acetate product bound to an auxiliary site that is required for efficient carboxylate substrate recognition. Mutant E294A crystallizes in a closed complex containing dethiaacetyl-CoA, which adopts an unusual curled conformation. A model of the acetyl-CoA Michaelis complex reveals that the nucleophilic glutamate is held at a near-ideal angle for attack as the thioester oxygen is forced into an oxyanion hole composed of Gly388 NH and CoA N2'' CoA is nearly immobile along its entire length during all stages of the enzyme reaction 721675 2.8.3.18 enzyme bound to dethiaacetyl-CoA and acetate, hanging drop vapor diffusion method, using 0.9 M sodium citrate, 0.1 M imidazole-HCl, pH 8.2, and 25 mM 2-mercaptoethanol 738287 2.8.3.18 native and C-terminally His6-tagged wild-type enzymes in complexes including freeze-trapped acetylglutamyl anhydride and glutamyl-CoA thioester adducts, hanging drop vapor diffusion method, mixing of 0.002 ml of protein solution containing 5.6 mg/ml AarC in 45 mM potassium phosphate, pH 8.0, 90 mM potassium chloride, and 2 mM CoA or 6.0 mg/ml His6-tagged AarC in 45 mM Tris-HCl, pH 8.0, 90 mM potassium chloride, and 2 mM CoA, with 0.002 ml of reservoir solution containing 0.8-1.0 M sodium citrate, 0.1 M imidazole, pH 8.2, and 25 mM 2-mercaptoethanol for orthorhombic crystals or 1.7-2.0 M ammonium sulfate, 0.2 M sodium chloride, 0.1 M sodium cacodylate, pH 6.5, and 25 mM 2-mercaptoethanol for hexagonal crystals, room temperature of about 22°C, X-ray diffraction structure determination and analysis 721675