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The monomeric enzyme '''5-Enolpyruvlyshikimate-3-phosphate synthase (EPSP synthase)''' is an enzyme involved in the shikimate pathway found only in plants and microorganisms. The shikimate pathway is essential for the biosynthesis of chorismate, a molecule that is a precursor to majority of the aromatic compounds produced in the cell, including the aromatic amino acids (L-tyrosine, L-phenylalanine, L-tryptophan) <ref>PMID:10353849</ref>. These aromatic compounds are essential for the synthesis of proteins thereby making chorismate and the shikimate pathway essential for plants and microorganisms. EPSP synthase is a transferase or an enzyme that catalyzes the transfer of a specific functional group from one molecule to another. EPSP synthase catalyzes the transfer of the enolpyruvyl group from phosphonenolpyruvate (PEP) to shikimate-3-phosphate (S3P) to form 5-enolpyruvylshikimate-3-phosphate (EPSP) and inorganic phosphate. EPSP is converted into chorismate, which is then introduced into synthesis pathways of aromatic compounds <ref>PMID:10353849</ref>. EPSP synthase has two distinct globular domains, composed of beta sheets and alpha helices <ref>PMID:11607190</ref>. Each of the domains are composed of three protein folding subunits. The subunits contain two alpha helices and four beta sheets <ref>PMID:11607190</ref>. As shown in Figure 3 below, each domain has six alpha helices and twelve beta sheets. The two domains, which are linked together by two crossover chain segments, exist in an open conformation until the binding of shikimate-3-phosphate, which is the enzyme's binding substrate <ref>PMID:17348837</ref>. Upon the binding of shikimate-3-phosphate, the domains close together to form the active site in an interdomain cleft. Phosphoenolpyruvate then enters the active site enabling EPSP synthase to catalyze the transfer of the enolpyruvyl group through an addition-elimination reaction. EPSP synthase is attractive for drug research because of its potential to serve as a selective target for antimicrobial drugs <ref>PMID:17348837</ref>. Because EPSP synthase is found only in plants and microorganisms, its use in antimicrobial drugs will limit the harmful side effects for humans.   

The three-dimensional structure of EPSP synthase in ''Escherichia coli'' was determined using X-ray crystallography. The enzyme has two distinct domains, each of which have a radius of approximately 25 Angstroms <ref>PMID:11607190</ref>. The two domains are joined together by two crossover chain segments with both the amino and carboxyl terminus of the polypeptide chain located in the C-terminal domain. In its entirety, EPSP synthase is composed of the following <scene name='56/564040/Epsp_synthase/2'>secondary structures</scene>: twelve alpha helices and twenty-four beta sheets. The domains are formed by protein folding subunits consisting of two parallel helices and  four beta sheets <ref>PMID:11607190</ref>. Each domain is formed from three of these subunits, forming the threefold axis of symmetry shown below in Figure 3. Three alpha helices are positioned around the axis of each domain to form its core. The domain surfaces consist of three beta-sheets and three parallel helices that are all solvent accessible <ref>PMID:11607190</ref>. The beta sheets can be found in both the parallel and anti-parallel positions. The alpha helices are positioned in a parallel fashion that establishes equivalent chain polarities throughout the enzyme. It is believed that the helix dipole establishes optimal interaction between the enzyme and charged phosphate groups of the substrates <ref> W. G. J. Hol, P. T. van Duijnen & H. J. C. The [alpha]-helix dipole and the properties of proteins: Nature 273, 443-446 (8 June 1978).[http://dx.doi.org/10.1038/273443a0 DOI:10.1038/273443a0</ref>. Studies suggest that these helical dipole efffects create a cationic well that draws the anionic substrates into the active site located in the interdomain cleft.