Characterization of Fosfomycin-Resistant MurA from Borrelia burgdorferi, Fragment-based Inhibitor Design for AroA and DAHP Synthase

Abstract

<p>MurA catalyzes the first committed step of peptidoglycan biosynthesis and it is the target of the antibiotic fosfomycin. Due to a Cys-to-Asp substitution in the active site, MurAs from a number of pathogenic bacteria, including <em>Mycobacterium tuberculosis</em> and <em>Borrelia burgdorferi</em> (Lyme disease), are fosfomycin resistant. His-tagged <em>Borrelia burgdorferi</em> MurA (Bb_MurA) and its D116C mutant have been successfully expressed, purified and characterized. The <em>k</em>_cat value of wild-type Bb_MurA was 0.74 ± 0.01 s^-1. The D116C mutant’s <em>k</em>_cat decreased by 25-fold and was fosfomycin sensitive. The pH profiles of <em>k</em>_cat for both Bb_MurA and its mutant were characterized. There was little difference in p<em>K</em>_a1 values, but the p<em>K</em>_a2 value shifted from 7.4 ± 0.2 in wild-type enzyme to a value >11 in the mutant. This demonstrated that the p<em>K</em>_a2 of 7.4 was due to D116, and that it must be protonated for activity. Fosfomycin inactivation of Bb_MurA_H6(D116C) was time-dependent and only proceeded in the presence of UDP-GlcNAc. The dissociation constant, <em>K</em>_i, was 5.7 ± 0.4 µM and rate of covalent modification, <em>k</em>_inact, was 0.021 ± 0.003 s^-1.</p> <p>DAHP synthase catalyzes the first committed step in the shikimate pathway, and its catalysis has been proposed to proceed through two oxacarbenium ion intermediates. Pyruvate oxime, glyoxylate oxime and 4-imidazolecarboxylic acid have been evaluated as inhibitors of DAHP synthase. In the presence of glycerol 3-phosphate, the fitted <em>K</em>_i values of pyruvate oxime and glyoxylate oxime were 7.6 (± 0.9) × 10^-5 M and 7.4 (± 1.7) × 10^-5 M, respectively. 4-Imidazolecarboxylic acid’s inhibition was cooperative, and its binding was competitive with respect to PEP, and uncompetitive with respect to E4P. Its equilibrium dissociation constant was 3.0 (± 0.2) × 10^-3 M.</p>