Molecular Basis for Resistance Against Phosphonate Antibiotics and Herbicides

Abstract

Research in recent years have illuminated data on the mechanisms and targets of phosphonic acid antibiotics and herbicides, including fosfomycin, glyphosate, fosmidomycin and FR900098. Here we review the current state of knowledge of the structural and biochemical characterization of resistance mechanisms against these bioactive natural products. Advances in the understanding of these resistance determinants have spurred knowledge-based campaigns aimed towards the design of derivatives that retain biological activity but are less prone to tolerance.

Figure 1

The top row shows the chemical structures of select phosphonic acid antibiotics and herbicides, along with the names of the enzymes that these compounds target. The bottom row lists the actual substrates that the phosphonates mimic.

Figure 2

(A) The chemical transformation catalyzed by the enoylpyruvyl transferase MurA in the first committed step of peptidoglycan biosynthesis. (B) The phosphonate antibiotic fosfomycin irreversibly modified MurA by forming a covalent linkage with Cys115. The structure of MurA indicates that resistance mutations are located near the Cys115 (PDB: 1UAE).

Figure 3

Mechanisms for the enzymatic inactivation of fosfomycin catalyzed by (A) the thiol-S-transferase Mn²⁺-dependent FosA and the Mg²⁺-dependent FosB along with the necessary thiol co-substrates (RS⁻), and (B) the hydrolytic metalloenzyme FosX.

Figure 4

(A) Proposed mechanism of the enolpyruvyl transferase reaction catalyzed by 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which is the target of glyphosate. MurA is thought utilize the same mechanism, using UDP-N-acetylglucosamine instead of S3P. ^,, (B) Crystal structure of E. coli EPSPS in complex with glyphosate and shikimate-3-phosphate (PDB 1G6S) showing that the positively charged nitrogen of glyphosate in stabilized by Glu341 and the co-substrate S3P. (C) Resistance producing mutations are located near the glyphosate-binding pocket.

Figure 5

(A) Reaction mechanism of DOXP reductoisomerase (DXR), which is part of the MEP pathway for synthesis of the essential isoprenoids dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP). Fosmidomycin and FR900098 are competitive inhibitors of DXR. (B) Active site of E. coli DXR (PDB 1ONP) bound to fosmidomycin (FSM, in green) and manganese (purple). Metal-coordinated water is represented as a red sphere.