Shikimate pathway targets

The Shikimate pathway is responsible for biosynthesis of aromatic amino acids in bacteria, fungi and plants [28], and the absence of this pathway in mammals makes it an interesting target for designing novel antibiotics, fungicides and herbicides. After the production of chorismate the pathway branches and, via specific internal pathways, the chorismate intermediate is converted to the three aromatic amino acids, in addition to a number of other aromatic compounds [29], The enzyme chorismate mutase (CM) is a key enzyme responsible for the Claisen rearrangement of chorismate to prephenate (Scheme 1-1), the first step in the branch that ultimately leads to production of tyrosine and phenylalanine. 

EPSP synthase catalyzes the synthesis of EPSP by an addition-elimination reaction through the tetrahedral intermediate shown in Fig. 2a. This enzyme is on the shikimate pathway for synthesis of aromatic amino acids and is the target for the important herbicide, glyphosate, which is the active ingredient in Roundup (The Scotts Company EEC, Marysville, OH). Transient-state kinetic studies led to proof of this reaction mechanism by the observation and isolation of the tetrahedral intermediate. Moreover, quantification of the rates of formation and decay of the tetrahedral intermediate established that it was tmly an intermediate species on the pathway between the substrates (S3P and PEP) and products (EPSP and Pi) of the reaction. The chemistry of this reaction is interesting in that the enzyme must first catalyze the formation of the intermediate and then catalyze its breakdown, apparently with different requirements for catalysis. Quantification of the rates of each step of this reaction in the forward and reverse directions has afforded a complete description of the free-energy profile for the reaction and allows... 

Chorismate synthase (CS) catalyzes the formation of chorismate, the last step in the shikimate pathway. Chorismate is a branch-point metabolite used for the synthesis of aromatic amino acids, p-aminobenzoic acid, folate, and other cyclic metabolites such as ubiquinone. The shikimate pathway is found only in plants, fungi, and bacteria, making the enzymes of the pathway potential targets for herbicides, antifungals, and antibiotics. 

Fig. 2. The shikimate pathway with the targets of inhibitor action. For abbreviations, see text.

The information obtained from the application of glyphosate to complex systems strongly pointed to one of the following three enzymes as the target of the inhibitor in the shikimate pathway shikimate kinase (EC 2.7.1.71), 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase (EC 2.5.1.19), and chorismate synthase (EC 4.6.1.4). Jointly, these three enzymes convert shikimic acid to chorismic acid in a series of interesting reactions >(Fig. 2). A defined system" had therefore to be found in which the conversion of shikimic acid to chorismic acid could be conveniently studied. 

Ducati RG, Basso LA, Santos DS (2007) Mycobacterial shikimate pathway enzymes as targets for... 

In considering the commercially successful herbicides known to be inhibitors of amino acid biosynthesis, three aspects will be considered the kinetic description of the inhibition, the molecular interactions involved, and the relevance of the proposed target site to the observed physiological effects. Because of the extensive body of research published on the shikimate pathway and the herbicide glyphosate, this example will be taken as a paradigm of the inhibitor-enzyme relationship. The other cases considered will be the branched-chain amino acid family, histidine biosynthesis, and glutamine synthetase. 

Figure 4. Shikimate-derived metabolism in plants. A complicated biosynthetic pathway is a possible genetic engineering target.

It is now well established that the primary metabolic target of glyphosate is an enzyme of the shikimic acid metabolic pathway, enolpyruvyl shikimate-3-phosphate synthase (2.f ). Via this action, glyphosate blocks the synthesis of the end products of this pathway, notably phenylalanine and tryptophan, but also various subsequent products (Figure 1) ( ,i). It has seemed logical to conclude that the herbicidal effect of glyphosate is a direct result of its effect on the shikimic acid pathway. 

The conversion of chorismate into prephenate occurs at a critical point in the shikimic acid pathway the biosynthesis of a variety of aromatics branch off from here. Since CM appears in lower organisms (such as fungi and bacteria) and not in mammals, it is an excellent target for the development of antibacterial and antifungal agents. 

Chorismate mutase (CM) catalyzes the Claisen rearrangement of chorismate to prephenate in the shikimic acid pathway used in the biosynthesis of aromatic amino acids. It represents a reference enzyme to explore the fundamentals of catalysis and has been the subject of extensive experimental and computational research. These have shown both that catalysis proceeds without covalent binding of the substrate to the enzyme, and that the uncatalyzed reaction in water proceeds by the same mechanism. This makes CM a particularly convenient target for QM/MM studies. 

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