Chorismate synthase

Biological and practical function

The shikimate pathway synthesises precursors to aromatic amino acids, as well as other aromatic compounds that have various involvement with processes such as "UV protection, electron transport, signaling, communication, plant defense, and the wound response". Because humans lack the shikimate pathway, but it is required for the survival of many microorganisms, the pathway and chorismate synthase in particular are considered to be potential targets for new antimicrobial treatments. For example, chorismate synthase is known to be essential to the survival of Mycobacterium tuberculosis, making the enzyme an attractive antibiotic target for control of this pathogen.

Structural studies

As of late 2007, 9 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , , and .

The crystal structure of chorismate synthase is a homotetramer with one FMN molecule non-covalently bound to each of the four monomers. Each monomer is made up of 9 alpha helices and 18 beta strands and the core is assembled in a unique beta-alpha-beta sandwich fold. The active sites for FMN-binding are made up of clusters of flexible loops and the area around these regions have highly positive electromagnetic potential. There are two histidine residues located at the active site which are thought to protonate the reduced flavin molecule and the leaving phosphate group of the substrate.

Mechanism

The formation of chorismate from EPSP involves two eliminations, of phosphate and a proton (H+), from the substrate. In the first step of catalysis, phosphate is eliminated, assisted by proton transfer from a conserved histidine residue. At the same time, an electron is transferred from the FMN to the substrate, forming an FMN radical and a substrate radical. Next, the FMN radical rearranges, and then a hydrogen atom is transferred to FMN from the substrate, eliminating both radicals and generating the product. The reduced FMN then re-tautomerizes to its active form by donating a proton to a second conserved histidine. Although the chorismate synthase reaction is FMN-dependent, there is no net redox change between substrate and product; the FMN merely acts as a catalyst.

Two classes of chorismate synthase exist, differing in how the reduced state of the FMN cofactor is maintained. Bifunctional chorismate synthase is present in fungi and contains an NAD(P)H-dependent flavin reductase domain. Monofunctional chorismate synthase is found in plants and E.coli and lacks a flavin reductase domain. It depends on a separate reductase enzyme to reduce the FMN.

References

References

1. Dias. (2006). "Structure of chorismate synthase from Mycobacterium tuberculosis". Journal of Structural Biology.
2. (1991). "Molecular cloning and analysis of a cDNA coding for chorismate synthase from the higher plant Corydalis sempervirens Pers". J. Biol. Chem..
3. (1991). "Molecular cloning, characterization and analysis of the regulation of the ARO2 gene, encoding chorismate synthase, of Saccharomyces cerevisiae". Mol. Microbiol..
4. (1999). "A unique reaction in a common pathway: Mechanism and function of chorismate synthase in the shikimate pathway". Planta.
5. (2008). "The Mycobacterium tuberculosis Rv2540c DNA sequence encodes a bifunctional chorismate synthase". BMC Biochemistry.
6. (2004). "Crystal Structure of Chorismate Synthase: A Novel FMN-binding Protein Fold and Functional Insights". Journal of Molecular Biology.
7. (March 2004). "Mechanism of chorismate synthase: Role of the two invariant histidine residues in the active site". Journal of Biological Chemistry.