Structural and functional characterization of fosfomycin resistance conferred by FosB from Enterococcus faecium

Structural Studies of Klebsiella pneumoniae Fosfomycin-Resistance Protein and Its Application for the Development of an Optical Biosensor for Fosfomycin Determination

Fosfomycin-resistance proteins (FosAs) are dimeric metal-dependent glutathione transferases that conjugate the antibiotic fosfomycin (Fos) to the tripeptide glutathione (γ-Glu-Cys-Gly, GSH), rendering it inactive. In the present study, we reported a comparative analysis of the functional features of two FosAs from Pseudomonas aeruginosa (FosAPA) and Klebsiella pneumoniae (FosAKP). The coding sequences of the enzymes were cloned into a T7 expression vector, and soluble active enzymes were expressed in E. coli. FosAKP displayed higher activity and was selected for further studies. The crystal structure of the dimeric FosAKP was determined via X-ray crystallography at 1.48 Å resolution. Fos and tartrate (Tar) were found bound in the active site of the first and second molecules of the dimer, respectively. The binding of Tar to the active site caused slight rearrangements in the structure and dynamics of the enzyme, acting as a weak inhibitor of Fos binding. Differential scanning fluorimetry (DSF) was used to measure the thermal stability of FosAKP under different conditions, allowing for the selection of a suitable buffer to maximize enzyme operational stability. FosAKP displays absolute specificity towards Fos; therefore, this enzyme was exploited for the development of an enzyme-based colorimetric biosensor. FosAKP was tethered at the bottom of a plastic cuvette using glutaraldehyde chemistry to develop a simple colorimetric method for the determination of Fos in drinking water and animal plasma.

Small molecule inhibitors of the fosfomycin resistance enzyme FosM from Mycobacterium abscessus

Fosfomycin is a safe broad-spectrum antibiotic that has not achieved widespread use because of the emergence of fosfomycin-modifying enzymes. Inhibition of fosfomycin-modifying enzymes could be used to help combat pathogens like Mycobacterium abscessus. Our previous work identified several inhibitors for the enzyme FosB from Staphylococcus aureus. We have tested those same compounds for inhibition of FosM, the fosfomycin-modifying enzyme from M. abscessus. The work described here will be used as the basis for more detailed studies into the inhibition of FosM.

Identification and analysis of small molecule inhibitors of FosB from Staphylococcus aureus

Antimicrobial resistance (AMR) poses a significant threat to human health around the world. Though bacterial pathogens can develop resistance through a variety of mechanisms, one of the most prevalent is the production of antibiotic-modifying enzymes like FosB, a Mn2+-dependent l-cysteine or bacillithiol (BSH) transferase that inactivates the antibiotic fosfomycin. FosB enzymes are found in pathogens such as Staphylococcus aureus, one of the leading pathogens in deaths associated with AMR. fosB gene knockout experiments establish FosB as an attractive drug target, showing that the minimum inhibitory concentration (MIC) of fosfomycin is greatly reduced upon removal of the enzyme. Herein, we have identified eight potential inhibitors of the FosB enzyme from S. aureus by applying high-throughput in silico screening of the ZINC15 database with structural similarity to phosphonoformate, a known FosB inhibitor. In addition, we have obtained crystal structures of FosB complexes to each compound. Furthermore, we have kinetically characterized the compounds with respect to inhibition of FosB. Finally, we have performed synergy assays to determine if any of the new compounds lower the MIC of fosfomycin in S. aureus. Our results will inform future studies on inhibitor design for the FosB enzymes.

Inhibition of Fosfomycin Resistance Protein FosB from Gram-Positive Pathogens by Phosphonoformate

The Gram-positive pathogen Staphylococcus aureus is a leading cause of antimicrobial resistance related deaths worldwide. Like many pathogens with multidrug-resistant strains, S. aureus contains enzymes that confer resistance through antibiotic modification(s). One such enzyme present in S. aureus is FosB, a Mn²⁺-dependent l-cysteine or bacillithiol (BSH) transferase that inactivates the antibiotic fosfomycin. fosB gene knockout experiments show that the minimum inhibitory concentration (MIC) of fosfomycin is significantly reduced when the FosB enzyme is not present. This suggests that inhibition of FosB could be an effective method to restore fosfomycin activity. We used high-throughput in silico-based screening to identify small-molecule analogues of fosfomycin that inhibited thiol transferase activity. Phosphonoformate (PPF) was a top hit from our approach. Herein, we have characterized PPF as a competitive inhibitor of FosB from S. aureus (FosB^Sa) and Bacillus cereus (FosB^Bc). In addition, we have determined a crystal structure of FosB^Bc with PPF bound in the active site. Our results will be useful for future structure-based development of FosB inhibitors that can be delivered in combination with fosfomycin in order to increase the efficacy of this antibiotic.