The crystal structure of a myristoyl acyl carrier protein specific thioesterase (C14ACP-TE) from a bioluminescent bacterium, Vibrio harveyi, was solved by multiple isomorphous replacement methods and refined to an R factor of 22% at 2.1-A resolution. This is the first elucidation of a three-dimensional structure of a thioesterase. The overall tertiary architecture of the enzyme resembles closely the consensus fold of the rapidly expanding superfamily of alpha/beta hydrolases, although there is no detectable homology with any of its members at the amino acid sequence level. Particularly striking similarity exists between the C14ACP-TE structure and that of haloalkane dehalogenase from Xanthobacter autotrophicus. Contrary to the conclusions of earlier studies [Ferri, S. R., & Meighen, E. A. (1991) J. Biol. Chem. 266, 12852-12857] which implicated Ser77 in catalysis, the crystal structure of C14ACP-TE reveals a lipase-like catalytic triad made up of Ser114, His241, and Asp211. Surprisingly, the gamma-turn with Ser114 in a strained secondary conformation (phi = 53 degrees, psi = -127 degrees), characteristic of the so-called nucleophilic elbow, does not conform to the frequently invoked lipase/esterase consensus sequence (Gly-X-Ser-X-Gly), as the positions of both glycines are occupied by larger amino acids. Site-directed mutagenesis and radioactive labeling support the catalytic function of Ser114. Crystallographic analysis of the Ser77-->Gly mutant at 2.5-A resolution revealed no structural changes; in both cases the loop containing the residue in position 77 is disordered.(ABSTRACT TRUNCATED AT 250 WORDS)