Identification, substrate specificity, and inhibition of the Streptococcuspneumoniae beta-ketoacyl-acyl carrier protein synthase III (FabH)

In the bacterial type II fatty acid synthase system, beta -ketoacyl-acyl ca rrier protein (ACP) synthase IH (FabH) catalyzes the condensation of acetyl -CoA with malonyl-ACP. We have identified, expressed, and characterized the Streptococcus pneumoniae homologue of Escherichia coli FabH. S. pneumoniae FabH is similar to 41, 39, and 38% identical in amino acid sequence to Bac illus subtilis, E. coli and Hemophilus influenzae FabH, respectively. The H is-Asn-Cys catalytic triad present in other FabH molecules is conserved in S. pneumoniae FabH. The apparent K-m values for acetyl-CoA and malonyl-ACP were determined to be 40.3 and 18.6 muM, respectively. Purified S. pneumoni ae FabH preferentially utilized straight short-chain CoA primers. Similar t o E. coli FabH, S. pneumoniae FabH was weakly inhibited by thiolactomycin. In contrast, inhibition of S. pneumoniae FabH by the newly developed compou nd SB418011 was very potent, with an IC50 value of 0.016 muM. SB418011 also inhibited E. coli and H. influenzae FabH with IC50 values of 1.2 and. 0.59 muM, respectively. The availability of purified and characterized S. pneum oniae FabH will greatly aid in structural studies of this class of essentia l bacterial enzymes and. facilitate the identification of small molecule in hibitors of type II fatty acid synthase with the potential to be novel and potent antibacterial agents active against pathogenic bacteria.