16.6 The EPSP Synthase Enzyme (Aromatic Amino Acid Synthesis) – Group 9

Now we turn our attention to the Site of Action, EPSP Synthase Inhibitors. 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.

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 have biological activity in the cytoplasm in the immature state, before transit peptide removal.

EPSP synthase catalyzes one step in the shikimate pathway, leading to the development of important aromatic amino acids. The diagram below illustrates the major steps of the pathway.

In the next video clip, Dr. Namuth-Covert highlights the key concepts of the shikimate path diagram and points out where herbicides interfere with it.