Identification and characterization of inhibitors of Haemophilus influenzae acetohydroxyacid synthase.

Acetohydroxyacid synthase (AHAS), a potential target for antimicrobial agents, catalyzes the first common step in the biosynthesis of branched-chain amino acids. The gene coding for the AHAS catalytic subunit from Haemophilus influenzae (Hi) was cloned, overexpressed in Escherichia coli, and purified. To identify new inhibitory scaffolds, we used a high-throughput screen to test 221 small diverse chemical compounds against Hi-AHAS. Compounds were selected for their ability to inhibit AHAS in vitro. The screen identified 3 compounds, each representing a structural class, as Hi-AHAS inhibitors with an IC(50) in the low micromolar range (4.4-14.6 μM). The chemical scaffolds of the three compounds were oxa-1-thia-4-aza-cyclopenta[b]naphthalene (KHG25229), phenyl-2,3-dihydro-isothiazole (KHG25386), and phenyl-pyrrolidine-3-carboxylic acid phenylamide (KHG25056). Further, molecular docking of the two most potent chemicals, KHG25229 and KHG25386, in Hi-AHAS yielded binding energies of -10.41 and -9.21 kcal/mol, respectively. The binding modes were consistent with inhibition mechanisms, as both chemicals oriented outside the active site. As the need for novel antibiotic classes to combat drug resistant bacteria increases, screening compounds that act against Hi-AHAS may assist in the identification of potential new anti-Hi drugs.