Chorismate mutase

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In enzymology, a chorismate mutase (EC 5.4.99.5) is an enzyme that catalyzes the chemical reaction for the conversion of chorismate to prephenate in the pathway to the production of phenylalanine and tyrosine, also known as the shikimate pathway.

Chorismate

Prephenate

Hence, this enzyme has one substrate, chorismate, and one product, prephenate. Chorismate mutase is found at a branch point in the pathway. The enzyme channels the substrate, chorismate to the biosynthesis of tyrosine and phenylalanine and away from tryptophan (Qamra, R. et al. 2006)[8]. Its role in maintaining the balance of these aromatic amino acids in the cell is vital (Kast, P. et al. 2000)[1]. This is the single known example of an enzyme catalyzing a pericyclic reaction (Kast, P. et al. 2000)[1]. Chorismate mutase(CM) is only found only in fungi, bacteria, and higher plants.

1 Protein Family
2 Mechanism of Catalysis
3 Chorismate Mutase and Tuberculosis
4 Structural studies
5 References
6 External links
- 6.1 Gene Ontology (GO) codes

[edit] Protein Family

Yeast chorismate mutase active site containing the transition state analogue, endo-oxabicyclic (yellow). Polar residues of the active site surround the substrate and interact through noncovalent Hydrogen bonding. Important active site residues include: Arg16, Arg157, Thr242, Glu246, Glu198, and Lys168, as well as the Nitrogen on the back bone of Asn 194 (Ma, J. et al. 1998)[6].

This enzyme belongs to the family of isomerases, specifically those intramolecular transferases transferring other groups. The systematic name of this enzyme class is chorismate pyruvatemutase. This enzyme is also called hydroxyphenylpyruvate synthase. This enzyme participates in phenylalanine, tyrosine and tryptophan biosynthesis.

The structures of chorismate mutase vary in different organisms, but the majority belongs to the AroQ family. The chorismate mutase of this family look most like that of Escherichia coli. That is, they are characterized by an intertwined homodimer of 3-helical subunits. For example, the secondary structure of the CM of yeast is very similar to that of E. coli. Chorimate mutase in the AroQ family are more common in nature and are widely distributed among the prokaryotes. (Qamra, R. et al. 2006)[8]. For optimal function, they usually have to be accompanied by another enzyme such as prephanate dehydrogenase (Qamra, R. et al. 2006)[8]. These CM are usually bifunctional enzymes meaning they contain two catalytic capacities in the same polypeptide chain (Qamra, R. et al. 2006)[8]. The CM of eukaryotic organisms are usually monofunctional and are controlled by the aromatic amino acids. There are organisms such as Bacillus subtilis whose chorismate mutase have a completely different structure. These enzymes belong to the AroH family and are characterized by a trimeric α/β barrel topology (Babu, M. 1999)[2]. CM of this class are monofunctional, containing a single catalytic capacity. E. coli and Yeast chorismate mutase have a limited sequence homology, but their active sites contain similar residues. The active site of the Yeast chorismate mutase contains Arg16, Arg157, Thr242, Glu246, Glu198, Asn194, and Lys168. The E. coli active site contains the same residues with the exception of these noted exchanges: Asp48 for Asn194, Gln88 for Glu248, and Ser84 for Thr242 (Guo, H. et al. 2001)[5].

[edit] Mechanism of Catalysis

Yeast chorismate mutase active site. Indicates the proximity of hydrogen bonding between the polar amino acid residues of chorismate mutase active site and the transition state inhibitor (endo-oxabicyclic in cyan). Arginine (orange), Threonine (green), Glutamic acid (blue), Asparagine (pink), and Lysine (yellow) (Ma, J. et al. 1998)[6].

The mechanism for the transformation of chorismate to prephenate is formally a Claisen rearrangement.

Mechanism for the enzyme's subtrate chorismate claisen rearrangement reaction forming the product prephenate (Schnappauf G. et al. 1997)[7].

This transformation is the first committed step in the pathway to production of the aromatic amino acids: tyrosine and phenylalanine. In the absence of enzyme catalysis this mechanism proceeds as a concerted, but asynchronous step and is an exergonic process (Kast, P. et al. 2000)[1]. As a result there is no formal intermediate, but rather a chair-like transition state. The addition of chorismate mutase, increases the rate of the reaction a million fold. There have been extensive studies on the exact mechanism of this reaction, but the rate-determining step has yet to be uncovered. Some questions that remain surrounding the mechanism are how conformational constraint of the flexible substrate, specific hydrogen bonding to the transition state, and electrostatic interactions actually contribute to catalysis. One study using CM from B. subtilis showed evidence that the when a cation was aptly placed in the active site, the electrostatic interactions between it and the negatively-charged transition state promoted catalysis (Kast, P. et al. 2000)[1].

[edit] Chorismate Mutase and Tuberculosis

Tuberculosis is a terrible disease that accounts for more deaths than any other single infection around the world. The fact that chorismate mutase is found only in fungi, bacteria and higher plants makes it an easy target for the creation of anti-bacterials as well as herbicides. In addition, the low sequence homology between known chorismate mutase provides the opportunity to develop unique inhibitors. This has been the case in the fight against antibiotic-resistant Tuberculosis (TB). The CM found in Myobacterium tuberculosis is vital to its overall survival and subsequent pathogenicity. Humans do not contain CM, so it is being used as a target to develop an antibiotic to fight the disease (Agrawal, H. et al. 2007)[3]. M. tuberculosis contains two genes, Rv1885c and Rv0948c, that code for its chorismate mutase. These CM are secreted out of the cell to provide support for M. tuberculosis when it is in an aromatic amino acid deficient medium (Kim, S-K et al. 2007)[4]. It has also been proposed that the CM could interact with host macrophages and might be importance in virulence (Kim, S-K et al. 2007)[4]. This idea is supported by evidence that the AroQ CM of other pathogenic bacteria, such as Salmonella typhi have been shown to be involved in virulence. (Qamra, R. et al. 2006)[8]. There is one gene in particular, Rv1885c, that is the major contributor to CM activity that has become the focus of study (Kim, S-K et al. 2007)[4]. Rohini Qamra et al. have solved a crystal structure of the CM of Myobacterium tuberculosis (MtbCM) and propose that a proline-rich portion of the protein is responsible for binding to the host cell macrophage's cell surface receptors. (Qamra, R. et al. 2006)[8].

[edit] Structural studies

As of late 2007, 24 structures have been solved for this class of enzymes, with PDB accession codes 1COM, 1DBF, 1FNJ, 1FNK, 1ODE, 1UFY, 1UI9, 1YBZ, 2AO2, 2CHS, 2CHT, 2CSM, 2D8D, 2D8E, 2F6L, 2FP1, 2FP2, 2GBB, 2GTV, 2PV7, 2QBV, 3CSM, 4CSM, and 5CSM.

[edit] References

[1] *Kast, P.; Grisostomi, C.; Chen, I.A.; Li, S.; Krengel, U.; Xue, Y.; Hilvert, D. A strategically positioned cation is crucial for efficient catalysis by chorismate mutase. The Journal of Biological Chemistry. 2000 Nov 24; 275(47):36832-8.

[2] *Babu,M. “Annotation of Chorismate Mutase from the Myobacterium tuberculosis and the Myobacterium leprae genome” Undergraduate Thesis for the Center of Biotechnology. 1999.

[3] *Agrawal, H.; Kumar, A.; Bal, N.; Siddiqi, M.; Arora, A. Ligand based virtual screening and biological evaluation of inhibitors of chorismate mutase (Rv1885c) from Mycobacterium tuberculosis H37Rv. Bioorganic & Medicinal Chemistry Letters. 17. (2007) 3053-3058.

[4] *Kim, S-K.; Ladner, J.; Reddy, P. Health and Medical Technologies Forensics and Homeland Security. (2007) Measuring Enzyme Activity to Counter the Threat of Drug-Resistant Tuberculosis.

[5] *Guo, H., Cui, Q., Lipscomb, W.N., Karplus, M. (2001). “Substrate conformational transition in the active site of chorismate mutase: Their role in the catalytic mechanism”. PNAS. 98, 9032-9037.

[6] *Ma, J., Zheng, X., Schnappauf, G., Braus, G., Karplus, M. "Yeast chorismate mutase in the R state: Simulation of the active site". Proc. Natl. Acad. Sci. USA. 95, (1998) 14640-14645.

[7] *Schnappauf, G., Strater, N., Lipscomb, W.N., Braus, G.H. "A glutamate residue in the catalytic center of the yeast chorismate mutase restricts enzyme activity to acidic conditions". Proc. Natl. Acad. Sci. USA. 94, (1997) 8491-8496.

[8] *Qamra, R., Prakash, P., Hasnain, S.E., Mande, S.C. (2006) “The 2.15 Å Crystal Structure of Mycobacterium tuberculosis Chorismate Mutase Reveals an Unexpected Gene Duplication and Suggests a Role in Host-Pathogen Interactions”. Biochemistry. 45 (23), 6997 -7005, 2006. 10.1021/bi0606445 S0006-2960(06)00644-1.