Chorismate mutase
Encyclopedia
In enzymology, a chorismate mutase 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.
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
. Its role in maintaining the balance of these aromatic amino acids in the cell is vital. This is the single known example of an enzyme catalyzing a pericyclic reaction
. Chorismate mutase(CM) is only found only in fungi, bacteria, and higher plants.
s, specifically those intramolecular transferase
s 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. For optimal function, they usually have to be accompanied by another enzyme such as prephanate dehydrogenase. These CM are usually bifunctional enzymes meaning they contain two catalytic capacities in the same polypeptide chain. 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. 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.
. 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. 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.
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. 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.
It has also been proposed that the CM could interact with host macrophage
s and might be importance in virulence. 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. There is one gene in particular, Rv1885c, that is the major contributor to CM activity that has become the focus of study. 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.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
that catalyzes
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
the chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
for the conversion of chorismate to prephenate in the pathway to the production of phenylalanine
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
and tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
, also known as the shikimate pathway.
Hence, this enzyme has one substrate
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
, chorismate, and one product
Product (chemistry)
Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...
, 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
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
. Its role in maintaining the balance of these aromatic amino acids in the cell is vital. This is the single known example of an enzyme catalyzing a pericyclic reaction
Pericyclic reaction
In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions...
. Chorismate mutase(CM) is only found only in fungi, bacteria, and higher plants.
Protein family
This enzyme belongs to the family of isomeraseIsomerase
In biochemistry, an isomerase is an enzyme that catalyzes the structural rearrangement of isomers. Isomerases thus catalyze reactions of the formwhere B is an isomer of A.-Nomenclature:...
s, specifically those intramolecular transferase
Transferase
In biochemistry, a transferase is an enzyme that catalyzes the transfer of a functional group from one molecule to another . For example, an enzyme that catalyzed this reaction would be a transferase:In this example, A would be the donor, and B would be the acceptor...
s 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. For optimal function, they usually have to be accompanied by another enzyme such as prephanate dehydrogenase. These CM are usually bifunctional enzymes meaning they contain two catalytic capacities in the same polypeptide chain. 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. 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.
Mechanism of catalysis
The mechanism for the transformation of chorismate to prephenate is formally a Claisen rearrangementClaisen rearrangement
The Claisen rearrangement is a powerful carbon-carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen...
. This transformation is the first committed step
Committed step
In enzymology, the committed step is an effectively irreversible enzymatic reaction that occurs at a branch point during the biosynthesis of some molecules.-What it means:...
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
Exergonic
Exergonic means "releasing energy in the form of work". By thermodynamic standards, work, a form of energy, is defined as moving from the system to the surroundings...
process. 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
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
, 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.
Chorismate mutase and tuberculosis
TuberculosisTuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...
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
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
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. 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.
It has also been proposed that the CM could interact with host macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s and might be importance in virulence. 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. There is one gene in particular, Rv1885c, that is the major contributor to CM activity that has become the focus of study. 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.