Glyphosate: mechanism of action
Glyphosate is a herbicide used in agriculture and non-crop situations for the control of a wide range of weeds.
Chemically, the active ingredient glyphosate (N-phosphonomethyl-glycine) is a derivative of glycine, the smallest amino acid found in proteins. In the glyphosate molecule, one of the amino hydrogen atoms of glycine is replaced with a phosphonomethyl group.
Compared to other active ingredients in herbicides, glyphosate is a small molecule with a molecular weight of 169 g. Glyphosate is a derivative of the amino acid glycine, where one of the amino hydrogen atoms has been replaced with a phosphonomethyl group. (Phosphorus atoms in orange, hydrogen atoms in white, oxygen atoms in red, nitrogen atom in blue)
Once absorbed by the plant, glyphosate binds to and blocks the activity of the enzyme enolpyruvylshikimate-3-phosphate synthase (EPSPS). The EPSPS enzyme comes at the start of the shikimic acid pathway that converts simple carbohydrate precursors derived from glycolysis and the pentose phosphate pathway to aromatic amino acids and many other important plant metabolites. The enzyme is normally located within the chloroplasts where it catalyses the reaction of shikimate-3-phosphate (S3P) and phosphoenol pyruvate to form 5-enolpyruvyl-shikimate-3-phosphate (ESP). ESP is a precursor for aromatic amino acids and, ultimately, hormones, vitamins and other essential plant metabolites. Structural similarities to phosphoenol pyruvate enable glyphosate to bind to the substrate binding site of the EPSPS, inhibiting its activity and blocking its import into the chloroplast.
Since the active site of the EPSPS enzyme is highly consistent in higher plants, glyphosate affects a broad spectrum of weeds indiscriminately. Inhibiting the function of the shikimic acid pathway causes a deficiency in aromatic amino acids, eventually leading to the plant’s death by starvation.
Despite the fact that glyphosate is a small and simple molecule, its water solubility is too low for it to be easily sprayed in the field. The most common glyphosate formulations for commercial purposes therefore mix it with other substances to improve its efficiency. In many plant protection products glyphosate acid is formulated as a salt to enhance its water solubility.
A wide range of different glyphosate herbicide formulations have been registered in Europe. These include granular (SG) and liquid formulations (SL), various salts of glyphosate including isopropylamine (IPA), potassium (K), ammonium (NH4) and dimethyl ammonium (DMA).
Fast uptake of glyphosate is also crucial to prevent the herbicide being washed off by rain after spraying. Many glyphosate plant protection products also contain surfactants of various types and concentrations that improve leaf absorption, retention and coverage.
Last update: 19 June 2013