Aromatic Amino Acid Production
The first mode of action category we are going to discuss is that of herbicides which inhibit aromatic amino acid synthesis. Glyphosate, one of the most successful herbicides ever discovered, inhibits the production of aromatic amino acids: phenylalanine, tyrosine, and tryptophan. Glyphosate is a potent inhibitor of 5-enolpyruvylshikimate-3-phosphate synthase (often referred to as EPSP synthase or EPSPS). EPSP synthase is an important enzyme in the shikimate pathway which produces many aromatic products such as lignins, alkaloids, flavonoids, benzoic acids, and plant hormones, in addition to amino acids needed for protein synthesis. In fact as much as 20% of the carbon fixed during photosynthesis is utilized by this pathway.
The gene encoding for EPSP synthase is found in the nucleus, but the location of the enzyme and shikimic acid pathway is the chloroplast (Fig 6).
Figure 6: Organelles within a plant cell.
The peptide sequence for EPSP synthase is 444 amino acids long, with an additional 72 amino acid transit peptide (for a total of 516 amino acids). The transit peptide sequence ensures the enzyme will be transported to the chloroplast. Once inside the chloroplast the transit peptide is cleaved and the mature enzyme released. Interestingly, EPSP synthase is one of the few enzymes that has biological activity in the cytoplasm in the immature state, before transit peptide removal.
The function of EPSP synthase is to combine the substrate shikimate-3-phosphate (S3P) with phosphoenolpyruvate (PEP) to form 5-enolpyruvylshikimate-3-phosphate (EPSP). Fig 7-9 illustrates the 3 step process
Figure 7: Step 1, binding S-3-P.
Figure 8: Step 2, binding PEP.
Figure 9: Step 3, new product formed.
This catalyzed reaction is one step in the shikimate pathway. The diagram below illustrates the major steps of the pathway which lead to the development of important aromatic compounds (Fig 10). *This animation has no audio.*
Figure 10:The major steps in the shikimate pathway.