Characterization of a Cys115 to Asp Substitution in the Escherichia coli Cell Wall Biosynthetic Enzyme UDP-GlcNAc Enolpyruvyl Transferase (MurA) That Confers Resistance to Inactivation by the Antibiotic Fosfomycin

Abstract

The antibiotic fosfomycin inhibits bacterial cell wall biosynthesis by inactivation of UDP-GlcNAc enolpyruvyl tranferase (MurA). Prior work has established that Cys115 of Escherichia coli and Enterobacter cloacae MurA is the active site nucleophile alkylated by fosfomycin and implicated this residue in the formation of a covalent phospholactyl-enzyme adduct derived from substrate, phosphoenolpyruvate (PEP). On the basis of sequencing information from putative MurA homolog from Mycobacterium tuberculosis, we generated a C115D mutant of E. coli MurA that was highly active but fully resistant to time-dependent inhibition by fosfomycin. Fosfomycin still bound to the active site of C115D MurA, as established by the observed reversible competitive inhibition by fosfomycin. Fosfomycin still bound to the active site of C115D MurA, as established by the observed reversible competitive inhibition vs PEP. In contrast to the broad pH-independent behavior of wild-type (WT) MurA, C115D mutant activity titrated across the pH range examined (pH 5.5-9) with an apparent pKa approximately 6, with kcatC115D ranging from approximately 10kcatWT at pH 5.5 to <0.1kcatWT at pH9.0. Km(PEP)115D was relatively constant in the pH range examined and increased approximately 100-fold relative to Km(PEP)WT. A fosfomycin-resistant C115E mutant with -1% activity of the C115D mutant was found to follow a pH dependence similar to that observed for C115D MurA. The contrasting pH dependences of WT and C115D MurA was also observed in the reaction with the pseudosubstrate, (Z)-3-fluorophosphoenolpyruvate, strongly suggesting a role for Cys/Asp115 as the general acid in the protonation of C-3 of PEP during MurA-catalyzed enol ether transfer. The difference in nucleophilicity between the carboxylate side chains of Asp115 and Glu115 and the thiolate group of Cys115 suggests that covalent enzyme adduct formation is not required for catalytic turnover and, furthermore, provides a chemical rationale for the resistance of the C115D and C115E mutants to fosfomycin inactivation.

Recruitment of genes and enzymes conferring resistance to the nonnatural toxin bromoacetate

“…To distinguish between these possibilities, we took advantage of the fact that fosfomycin producing Mycobacteria possess a MurA that contains an activesite aspartate in place of cysteine. This natural substitution renders the protein resistant to modification by fosfomycin (27), and we also found that it was resistant to alkylation with bromoa- cetate (Fig. S3).…”

Phosphate Analogues as Probes of the Catalytic Mechanisms of MurA and AroA, Two Carboxyvinyl Transferases

“…The oxacarbenium ion was also supported by site-directed mutagenesis experiments (11). When Cys115 was mutated to Asp, the enzyme lost the ability to be inhibited by fosfomycin.…”

“…The reaction mechanisms of both MurA and AroA have been studied intensively with pre-steady state experiments (11,14,15), site directed mutagenesis (11), substrate analogues (16)(17)(18), x-ray crystallography (7,19,20), fluorescence titration (21) and NMR experiments (6,22). Their mechanisms are similar in several aspects.…”

“…Based on the lack of formation of a fluoro-EP-UDP-GicNAc like product from fluorine substituted THI, Walsh and coworkers believed it went through an oxacarbenium ion intermediate (11). They assumed an E2-type mechanism for the elimination step would not be affected by the substitution of fluorine at C3.…”

“…Cys115 is located in a loop which undergoes a large conformational change as the enzyme binds to the substrates (26). Site-directed mutagenesis experiments suggest it acts as a general acid in protonating the C3 position in the addition step, and a general base for decomposition of THI in both forward and backward directions (11). Its pKa inside the enzyme was determined to be 8.3, suggesting that Cys115 is appreciably protonated at physiological pH (29).…”