Studies on the conformational changes in the bacterial cell wall biosynthetic enzyme UDP-N-acetylglucosamine enolpyruvyltransferase (MurA)

The enzyme UDP-N-acetylglucosamine (UDP-GlcNAc) enolpyruvyltransferase (MurA), the target of the antibiotic fosfomycin, was investigated by small-angle X-ray scattering (SAXS) and fluorescence spectroscopy to detect conformational changes that had been proposed on the basis of the crystal structure of unliganded and liganded MurA. The SAXS data indicate that binding of UDP-GlcNAc to free enzyme results in substantial conformational changes, which can be interpreted as the transition from an open to a closed form. Fosfomycin did not affect the structure of free enzyme or sugar-nucleotide-bound MurA. Phosphoenolpyruvate (pyruvate-P) appeared to induce a structural change upon addition to free enzyme, which differed from that observed upon binding of UDP-GlcNAc. Fluorescence experiments were performed using the hydrophobic fluorescence probe 8-anilino-1-naphthalene sulfonate (ANS). The fluorescence quenching of MurA/ANS solutions upon addition of UDP-GlcNAc or pyruvate-P was concentration dependent in a saturating manner, yielding apparent dissociation constants of K(d(UDP-GlCNAc)) = 59 microM and K(d(pyruvate-P)) = 240 microM. The results suggest that binding of substrates does not exclusively follow an ordered mechanism with UDP-GlcNAc binding first, although binding of UDP-GlcNAc to free enzyme is preferred and possibly influenced by pyruvate-P. The reaction thus appears to follow an induced-fit mechanism, in which the binding site for fosfomycin, and presumably also for pyruvate-P, is created by the interaction of free enzyme with the sugar nucleotide. The methods described here provide a tool for the characterization of site-directed mutants of MurA and the interaction of this enzyme with potential inhibitors.