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

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 pK_a 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 (K_i = 1.25 micromolar) and uncompetitive with respect to shikimate-3-P (K_i′ = 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].