Photo-oxidation of 5-enolpyruvoylshikimate-3-phosphate synthase from Escherichia coli: evidence for a reactive imidazole group (His385) at the herbicide glyphosate-binding site

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Abstract
Photo-oxidation of Escherichia coli 5-enolpyruvoylshikimate-3-phosphate synthase, a target for the non-selective herbicide glyphosate (N-phosphonomethylglycine), in the presence of pyridoxal 5′-phosphate resulted in irreversible inactivation of the enzyme. The inactivation followed pseudo-first-order and saturation kinetics with a Kinact. of 50 microM. The inactivation is specifically prevented by preincubation of the enzyme with the combination of shikimate 3-phosphate and glyphosate. Increasing glyphosate concentration during preincubation resulted in a decreasing rate of inactivation. On 95% inactivation, approximately one histidine per molecule of enzyme was oxidized. Tryptic mapping of the enzyme modified in the absence and presence of shikimate 3-phosphate and glyphosate as well as analyses of the histidine content in the isolated peptides indicated that His385, in the peptide Asn383-Asp-His-Arg386, was the site of oxidation. These results suggest that His385 is the most accessible reactive imidazole group under these conditions and is located close to the glyphosate-binding site.