Nonactive-Site Mutations in S. aureus FabI That Induce Triclosan Resistance

The wide use of the antimicrobial agent/biocide, triclosan, promotes triclosan-resistant bacterial strains, including Staphylococcus aureus, as well as leads to accumulation in the aquatic and terrestrial environments. Knowledge of the molecular actions of triclosan on S. aureus is needed to understand the consequence of triclosan resistance and environmental accumulation of triclosan on S. aureus resistant strains, as well as to develop biphenyl ether analogs as antibiotic candidates. Triclosan inhibits an essential enzyme in the fatty acid biosynthetic pathway, the reduced nicotinamide adenine dinucleotide (NADH)/reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent enoyl-acyl carrier protein (enoyl-ACP) reductase, or FabI. In this study, we used error-prone polymerase chain reaction (epPCR) to generate mutations in the S. aureus FabI enzyme. Instead of using an elaborate FabI enzyme activity assay that involves ACP-linked substrates to determine whether triclosan inhibits the enzyme activities of individual FabI mutants, we used an efficient and economical assay that we developed, based on thermal shift principles, to screen for triclosan binding to FabI mutants in cells. We identified four active-site mutations. More interestingly, we also identified nine triclosan-resistant mutations distant from the active site (G113V, Y123H, S166N, N220I, G227C, A230T, V241I, F252I, and H253P) but located in disparate positions in the monomer-monomer and dimer-dimer interface regions in S. aureus FabI. We suggest that these sites may serve as potential allosteric sites for designing potential therapeutic inhibitors that offer advantages in selectivity since allosteric sites are less evolutionarily conserved.

Structures of SAICAR synthetase (PurC) fromStreptococcus pneumoniaewith ADP, Mg2+, AIR and Asp. Corrigendum

The caption for Figure 3 of the article by Wolfet al.[(2014),Acta Cryst.D70, 841–850] is corrected.

Glutamate racemase dimerization inhibits dynamic conformational flexibility and reduces catalytic rates

Glutamate racemase (RacE) is a bacterial enzyme that converts l-glutamate to d-glutamate, an essential precursor for peptidoglycan synthesis. In prior work, we have shown that both isoforms cocrystallize with d-glutamate as dimers, and the enzyme is in a closed conformation with limited access to the active site [May, M., et al. (2007) J. Mol. Biol. 371, 1219-1237]. The active site of RacE2 is especially restricted. We utilize several computational and experimental approaches to understand the overall conformational dynamics involved during catalysis when the ligand enters and the product exits the active site. Our steered molecular dynamics simulations and normal-mode analysis results indicate that the monomeric form of the enzyme is more flexible than the native dimeric form. These results suggest that the monomeric enzyme might be more active than the dimeric form. We thus generated site-specific mutations that disrupt dimerization and find that the mutants exhibit significantly higher catalytic rates in the d-Glu to l-Glu reaction direction than the native enzyme. Low-resolution models restored from solution X-ray scattering studies correlate well with the first six normal modes of the dimeric form of the enzyme, obtained from NMA. Thus, along with the local active site residues, global domain motions appear to be implicated in the catalytically relevant structural dynamics of this enzyme and suggest that increased flexibility may accelerate catalysis. This is a novel observation that residues distant from the catalytic site restrain catalytic activity through formation of the dimer structure.