Glyphosate Inhibition of 5-Enolpyruvylshikimate 3-Phosphate Synthase from Suspension-Cultured Cells of Nicotiana silvestris

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Abstract
Treatment of isogenic suspension-cultured cells of Nicotiana silvestris Speg. et Comes with glyphosate (N-[phosphonomethyl]glycine) led to elevated levels of intracellular shikimate (364-fold increase by 1.0 millimolar glyphosate). In the presence of glyphosate, it is likely that most molecules of shikimate originate from the action of 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthase-Mn since this isozyme, in contrast to the DAHP synthase-Co isozyme, is insensitive to inhibition by glyphosate. 5-Enolpyruvylshikimate 3-phosphate (EPSP) synthase (EC 2.5.1.19) from N. silvestris was sensitive to micromolar concentrations of glyphosate and possessed a single inhibitor binding site. Rigorous kinetic studies of EPSP synthase required resolution from the multiple phosphatase activities present in crude extracts, a result achieved by ion-exchange column chromatography. Although EPSP synthase exhibited a broad pH profile (50% of maximal activity between pH 6.2 and 8.5), sensitivity to glyphosate increased dramatically with increasing pH within this range. In accordance with these data and the pKa values of glyphosate, it is likely that the ionic form of glyphosate inhibiting EPSP synthase is COO−CH2NH2+CH2PO32−, and that a completely ionized phosphono group is essential for inhibition. At pH 7.0, inhibition was competitive with respect to phosphoenolpyruvate (Ki = 1.25 micromolar) and uncompetitive with respect to shikimate-3-P (Ki′ = 18.3 micromolar). All data were consistent with a mechanism of inhibition in which glyphosate competes with PEP for binding to an [enzyme:shikimate-3-P] complex and ultimately forms the dead-end complex of [enzyme:shikimate-3-P:glyphosate].