Cloning, characterization and evaluation of potent inhibitors of Shigella sonnei acetohydroxyacid synthase catalytic subunit

Abstract

Acetohydroxyacid synthase (AHAS) is a thiamin diphosphate (ThDP)- and Flavin adenine dinucleotide (FAD)dependent plant and microbial enzyme that catalyzes the first common step in the biosynthesis of essential amino acids such as leucine, isoleucine and valine. To identify strong potent inhibitors against Shigella sonnei (S. sonnei) AHAS, we cloned and characterized the catalytic subunit of S. sonnei AHAS and found two potent chemicals (KHG20612. KHG25240) that inhibit 87-93% S. sonnei AHAS activity at an inhibitor concentration of 100 uM. The purified S. sonnei AHAS had a size of 65 kDa on SOS-PAGE. The enzyme kinetics revealed that the enzyme has a K(m) of 8.01 mM and a specific activity of 0.117 U/mg. The cofactor activation constant (K(s)) for ThDP and (K(c)) for Mg(++) were 0.01 mM and 0.18 mM, respectively. The dissociation constant (K(d)) for ThDP was found to be 0.14 mM by tryptophan fluorescence quenching. The inhibition kinetics of inhibitor KHG20612 revealed an un-competitive inhibition mode with a K(ii) of 2.65 mM and an IC(50) of 9.3 mu M, whereas KHG25240 was a non-competitive inhibitor with a K(ii) of 5.2 mM, K(is) of 1.62 mM and an IC(50) of 12.1 mu M. Based on the S. sonnei AHAS homology model structure, the docking of inhibitor KHG20612 is predicted to occur through hydrogen bonding with Met 257 at a 1.7 angstrom distance with a low negative binding energy of -9.8 kcal/mol. This current study provides an impetus for the development of a novel strong antibacterial agent targeting AHAS based on these potent inhibitor scaffolds. (C) 2011 Elsevier B.V. All rights reserved.