The 1.8 angstrom crystal structure and active-site architecture of beta-ketoacyl-acyl carrier protein synthase III (FabH) from Escherichia coli

Background: beta-Ketoacyl-acyl carrier protein synthase III (FabH) initiate s elongation in type II fatty acid synthase systems found in bacteria and p lants. FabH is a ubiquitous component of the type II system and is position ed ideally in the pathway to control the production of fatty acids. The elu cidation of the structure of FabH is important for the understanding of its regulation by feedback inhibition and its interaction with drugs. Although the structures of two related condensing enzymes are known, the roles of t he active-site residues have not been experimentally tested. Results: The 1.8 Angstrom crystal structure of FabH was determined using a 12-site selenium multiwavelength anomalous dispersion experiment. The activ e site (Cys112, His244 and Asn274) is formed by the convergence of two a he lices and is accessed via a narrow hydrophobic tunnel. Hydrogen-bonding net works that include two tightly bound water molecules fix the positions of H is244 and Asn274, which are critical for the decarboxylation and condensati on reactions, Surprisingly, the His244-->Ala mutation does not affect the t ransacylation reaction suggesting that His244 has only a minor influence on the nucleophilicity of Cys112. Conclusions: The histidine and asparagine active-site residues are both req uired for the decarboxylation step in the condensation reaction. The nucleo philicity of the active-site cysteine is enhanced by the alpha-helix dipole effect, and an oxyanion hole promotes the formation of the tetrahedral tra nsition state.