Identification, Substrate Specificity, and Inhibition of theStreptococcus pneumoniaeβ-Ketoacyl-Acyl Carrier Protein Synthase III (FabH)*

In the bacterial type II fatty acid synthase system, β-ketoacyl-acyl carrier protein (ACP) synthase III (FabH) catalyzes the condensation of acetyl-CoA with malonyl-ACP. We have identified, expressed, and characterized theStreptococcus pneumoniaehomologue of Escherichia coliFabH. S. pneumoniaeFabH is ∼41, 39, and 38% identical in amino acid sequence to Bacillus subtilis,E. coli,and Hemophilus influenzaeFabH, respectively. The His-Asn-Cys catalytic triad present in other FabH molecules is conserved in S. pneumoniaeFabH. The apparentKmvalues for acetyl-CoA and malonyl-ACP were determined to be 40.3 and 18.6 µm, respectively. PurifiedS. pneumoniaeFabH preferentially utilized straight short-chain CoA primers. Similar to E. coliFabH, S. pneumoniaeFabH was weakly inhibited by thiolactomycin. In contrast, inhibition of S. pneumoniaeFabH by the newly developed compound SB418011 was very potent, with an IC50value of 0.016 µm. SB418011 also inhibited E. coliand H. influenzaeFabH with IC50values of 1.2 and 0.59 µm, respectively. The availability of purified and characterized S. pneumoniaeFabH will greatly aid in structural studies of this class of essential bacterial enzymes and facilitate the identification of small molecule inhibitors of type II fatty acid synthase with the potential to be novel and potent antibacterial agents active against pathogenic bacteria.