Glyphosate enzyme Inhibition

Glyphosate also inhibits the activity of a form of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (13), however, this inhibition is relatively weak. Effects of glyphosate on the enzymes of aromatic amino acid synthesis have been described in more detail in recent reviews (14-17). 

In this chapter, the discussion will concentrate on two inhibitors with a reasonable claim to selective action on enz3ones related to the shikimate pathway glyphosate, which inhibits 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase and L-a-aminooxy-3 phenylpropionic acid (L-AOPP), an inhibitor of phenylalanine ammonia-lyase (PAL) (Fig. 2). In addition to introducing a novel inhibitor of PAL, (R)-(l-amino-2-phenylethyl)phosphonic acid (APEP), previous and current efforts to design inhibitors of other shikimate pathway enzymes will be described. The treatment presented here will show that the deductions and predictions made on the basis of the abstract scheme in Figure 1 can be, and have been, tested on the basis of the real pathway presented in Figure 2. 

Glyphosate kills plants by specifically inhibiting one critical plant enzyme used in the biosynthesis of aromatic amino acids. As such, glyphosate was one of the first commercially successful herbicides to have a primary identified enzyme site of action in plants (4,5). 

A related mechanism is utilized in the biosynthesis of UDP-muramic acid (Eq. 20-6) J There is an enolpyruvoyl adduct analogous to that of EPSP synthase a proposed enolpyruvoyl-enzyme adduct with Cys 115 is not on the major path.k/1 However, this enzyme is not inhibited by glyphosate.1... 

In recent years, agribusiness firms have developed pf empirically several compounds that inhibit essential steps in the biosynthesis of amino acids found in plants but missing in animals. One of these compounds, glyphosate, is a highly specific inhibitor of 5-enol pyruvyl-shikimate-3-phosphate synthase (an enzyme needed for aromatic amino acid biosynthesis). Glyphosate is the active ingredient in the widely used herbicide Roundup. 

The condensation of phospho-shikimic acid with phosphoenol pyruvate is catalyzed by the enzyme 3-enoylpyruvoylhikimate-5-phosphate synthase, or EPSP synthase for short. This reaction is specifically inhibited by the herbicide glyphosate. 

The herbicide glyphosate (Roundup ) kills plants by inhibiting an enzyme needed for synthesis of phenylalanine. Deprived of phenylalanine, the plant cannot make the proteins it needs, and it gradually weakens and dies. Although a small amount of glyphosate is deadly to a plant, its human toxicity is quite low. Suggest why this powerful herbicide has little effect on humans. 

Jaworski (4) reported that growth inhibition of both plant and microbes by glyphosate could be reversed by aromatic amino acids. Further work of Amrhein and his coworkers revealed that glyphosate inhibits the shikimate pathway enzyme, 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase (5). This enzyme catalyzes the reaction shown in Figure 1. Glyphosate-treated plant and bacterial cultures accumulate shikimate and/or shikimate 3-phosphate (S3P), confirming that inhibition of EPSPS is at least a part of the in vivo mechanism of action of this herbicide (6, 7). 

It is clear from the above discussion ensure that glyphosate tolerance may be conferred to plants both by overproduction of wild type EPSPS as well as mutant EPSP synthases. It has been suggested that glyphosate may have multiple sites of action in plant cells (51-56). If this is true, mutant EPSPS enzymes would not confer glyphosate tolerance to plants, which is evidently not the case. It appears, therefore, that reports concerning the effect of glyphosate on other aspects of plant metabolism are due to secondary effects of the herbicide arising as a consequence of the inhibition of aromatic amino acid biosynthesis. 

The toxicant may react with an enzyme or a transport protein and inhibit its normal function. Enzymes may be inhibited by a compound that has a similar, but not identical structure as the true substrate instead of being processed, it blocks the enzyme. Typical toxicants of this kind are the carbamates and the organophosphorus insecticides that inhibit the enzyme acetyl cholinesterase. Some extremely efficient herbicides that inhibit enzymes important for amino acid synthesis in plants, e.g., glyphosate and glufosinate, are other good examples in this category. 

The I50 (the glyphosate concentration necessary to inhibit 50% of the enzyme activity) of EPSP synthase was much higher in leaves of WR plants than in HR plants, in contrast to the situation inside the roots. 

---