Myristoyl-ACP thioesterase (LuxD) from Vibrio harveyi causes the slow release of fatty acids for reduction into the aldehyde substrate required for the bacterial bioluminescence reaction. The active site Ser nucleophile (S-114) of the LuxD thioesterase is in a gamma-turn with a sequence (AXS(114)XS) quite different from the standard motif of GXSXG found in almost all (thio) esterases and lipases. The presence of an Arg residue (R-118) in the first turn of the helix after the gamma-turn also distinguishes Euro from other enzymes. Mutation of R-118 to Leu inactivated the enzyme and prevented acylation of the Ser(114) nucleophile, while even a conservative replacement with Lys resulted in over 75% loss of the same functions, suggesting that R-118 helps maintain the configuration of the active site. In contrast, replacement of S-116 With Gly but not Ala stimulated the esterase and deacylation rates by over threefold. Purification of the S116G mutant to homogeneity and analyses of its intrinsic fluorescence on acylation with myristoyl-CoA clearly demonstrated that this mutant was much more active than wild-type LuxD. The presence of S-116 rather than the expected Gly residue in the gamma-turn containing the Ser nucleophile may function so that release of fatty acids from LuxD is restricted allowing a more efficient delivery of fatty acids to the luminescent system.