The essential role of cobalt in the inhibition of the cytosolic isozyme of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Nicotiana silvestris by glyphosate.

The prime molecular target of glyphosate (N-[phosphonomethyl]glycine), a potent herbicidal and antimicrobial agent, is known to be the shikimate-pathway enzyme, 5-enol-pyruvylshikimate-3-phosphate synthase. Inhibition by glyphosate of an earlier pathway enzyme that is located in the cytosol of higher plants, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DS-Co), has raised the possibility of dual enzyme targets in vivo. With the recent appreciation that magnesium (and manganese) can replace cobalt as the divalent-metal activator of DS-Co, it has now been possible to show that sensitivity of DS-Co to inhibition by glyphosate is obligately dependent upon the presence of cobalt. Evidence for a cobalt(II):glyphosate complex with octahedral coordination was obtained through examination of the effect of glyphosate upon the visible electronic spectrum of aqueous solutions of cobalt(II) chloride. The presence of glyphosate increased the concentration of cobalt(II) chloride required for enzyme activity, and the concentration of cobalt(II) chloride markedly affected the concentration of glyphosate required for inhibition of DS-Co activity. The extent to which DS-Co is vulnerable to inhibition by glyphosate in vivo depends, therefore, upon the unknown extent to which DS-Co molecules in the cytosol might be associated with cobalt.