Epoxide hydrolases (EHs), also known as epoxide hydratases, are

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

that metabolize compounds that contain an

epoxide In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...

residue; they convert this residue to two

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

residues through an epoxide hydrolysis reaction to form

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol may also be called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. They are used as protecting gro ...

products. Several enzymes possess EH activity. Microsomal epoxide hydrolase (epoxide hydrolase 1, EH1, or mEH), soluble epoxide hydrolase (sEH, epoxide hydrolase 2, EH2, or cytoplasmic epoxide hydrolase), and the more recently discovered but not as yet well defined functionally, epoxide hydrolase 3 (EH3) and epoxide hydrolase 4 (EH4) are structurally closely related

isozymes In biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. di ...

. Other enzymes with epoxide hydrolase activity include leukotriene A4 hydrolase, Cholesterol-5,6-oxide hydrolase, MEST (gene) (Peg1/MEST), and Hepoxilin-epoxide hydrolase. The hydrolases are distinguished from each other by their substrate preferences and, directly related to this, their functions.

Classification

mEH (EH1), sEH (EH2), EH3, and EH4 isozymes

Humans express four epoxide hydrolase isozymes: mEH, sEH, EH3, and EH4. These isozymes are known (mEH and sEH) or presumed (EH3 and EH4) to share a common structure that includes containing an

Alpha/beta hydrolase fold The alpha/beta hydrolase superfamily is a superfamily of hydrolytic enzymes of widely differing phylogenetic origin and catalytic function that share a common fold. The core of each enzyme is an alpha/beta-sheet (rather than a barrel), containi ...

and a common reaction mechanism wherein they add water to epoxides to form vicinal cis (see ( cis-trans isomerism); see ( epoxide#Olefin (alkene) oxidation using organic peroxides and metal catalysts)) diol products. They differ, however, in subcellular location, substrate preferences, tissue expression, and/or function.

mEH

mEH is widely expressed in virtually all mammalian cells as an

endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

-bound (i.e. microsomal-bound) enzyme with its

C terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When t ...

catalytic domain facing the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

; in some tissues, however, mEH has been found bound to the cell surface

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

with its catalytic domain facing the

extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

space. The primary function of mEH is to convert potentially toxic

xenobiotics A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compo ...

and other compounds that possess epoxide residues (which is often due to their initial metabolism by

cytochrome P450 Cytochromes P450 (P450s or CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that mostly, but not exclusively, function as monooxygenases. However, they are not omnipresent; for examp ...

enzymes to epoxides) to diols. Epoxides are highly reactive electrophilic compounds that form adducts with DNA and proteins and also cause strand breaks in DHA; in consequence, epoxides can cause gene mutations, cancer, and the inactivation of critical proteins. The diols thereby formed are usually not toxic or far less toxic than their epoxide predecessors, are readily further metabolized, and ultimately excreted in the urine. mEH also metabolizes certain epoxides of

polyunsaturated fatty acids In biochemistry and nutrition, a polyunsaturated fat is a fat that contains a polyunsaturated fatty acid (abbreviated PUFA), which is a subclass of fatty acid characterized by a backbone with two or more carbon–carbon double bonds. Some polyunsa ...

such as the

epoxyeicosatrienoic acid The epoxyeicosatrienoic acids or EETs are signaling molecules formed within various types of cells by the metabolism of arachidonic acid by a specific subset of cytochrome P450 enzymes, termed cytochrome P450 epoxygenases. They are nonclassic e ...

s (EETs) but its activity in doing this is far less than that of sEH; mEH therefore may play a minor role, compared to sEH, in limiting the bioactivity of these

cell signaling In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...

compounds (see microsomal epoxide hydrolase).

sEH

sEH is widely expressed in mammalian cells as a

cytosolic The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

enzyme where it primarily serves the function of converting

s (EETs),

epoxyeicosatetraenoic acid Epoxyeicosatetraenoic acids (EEQs or EpETEs) are a set of biologically active epoxides that various cell types make by metabolizing the omega 3 fatty acid, eicosapentaenoic acid (EPA), with certain cytochrome P450 epoxygenases. These epoxygenase ...

s (EQAs), and

epoxydocosapentaenoic acid Epoxide docosapentaenoic acids (epoxydocosapentaenoic acids, EDPs, or EpDPEs) are metabolites of the 22-carbon straight-chain omega-3 fatty acid, docosahexaenoic acid (DHA). Cell types that express certain cytochrome P450 (CYP) epoxygenases meta ...

s (DPAs) to their corresponding diols, thereby limiting or ending their

actions; in this capacity, sEH appears to play a critical

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, an ...

role in limiting the effects of these epoxides in animal models and possibly humans. However, sEH also metabolizes the epoxides of

linoleic acid Linoleic acid (LA) is an organic compound with the formula . Both alkene groups () are ''cis''. It is a fatty acid sometimes denoted 18:2 (n−6) or 18:2 ''cis''-9,12. A linoleate is a salt or ester of this acid. Linoleic acid is a polyunsat ...

viz.,

Vernolic acid Vernolic acid (leukotoxin B or isoleukotoxin) is a long chain fatty acid that is monounsaturated and contains an epoxide. It is a ''cis'' epoxide derived from the C12–C13 alkene of linoleic acid. Vernolic acid was first definitively character ...

(leukotoxins) and Coronaric acids (isoleukotoxins) to their corresponding diols which are highly toxic in animal models and possibly humans (see Vernolic acid#Toxicity, Coronaric acid#toxicity, and soluble epoxide hydrolase). sEH also possesses hepoxilin-epoxide hydrolase activity, converting bioactive hepoxilins to their inactive trioxilin products (see below section "Hepoxilin-epoxide hydrolase").

EH3

Human EH3 is a recently characterized protein with epoxy hydrolase activity for metabolizing

s (EETs) and

vernolic acid Vernolic acid (leukotoxin B or isoleukotoxin) is a long chain fatty acid that is monounsaturated and contains an epoxide. It is a ''cis'' epoxide derived from the C12–C13 alkene of linoleic acid. Vernolic acid was first definitively character ...

s (leukotoxins) to their corresponding diols; in these capacities they may thereby limit the

activity of the EETs and contribute to the toxicity of the leukotoxins.

mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...

for EH3 is most strongly expressed in the lung, skin, and upper gastrointestinal tract tissues of mice. The function of EH3 in humans, mice, or other mammals has not yet been determined although the gene for EH3 has been validated as being hypermethylated on

CpG site The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG isl ...

s in its promoter region in human prostate cancer tissue, particularly in the tissues of more advanced or morphologically-based (i.e. Gleason score) more aggressive cancers; this suggests that the

gene silencing Gene silencing is the regulation of gene expression in a cell to prevent the expression of a certain gene. Gene silencing can occur during either Transcription (genetics), transcription or Translation (biology), translation and is often used in res ...

of EH3 due to this hypermethylation may contribute to the onset and/or progression of prostate cancer. Similar CpG site hypermethylations in the promoter of for the EH3 gene have been validated for other cancers. This promoter methylation pattern, although not yet validated, was also found in human malignant

melanoma Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...

EH4

The gene for EH4, ''EPHX4'', is projected to encode an epoxide hydrolase closely related in amino acid sequence and structure to mEH, sEH, and EH3. The activity and function of EH4 has not yet been defined.

Other epoxy hydrolases

Leukotriene A4 hydrolase

Leukotriene A4 hydrolase (LTA4H) acts primarily, if not exclusively, to hydrolyze

leukotriene A4 Leukotriene A4 (LTA4) is a leukotriene Leukotrienes are a family of eicosanoid inflammation, inflammatory mediators produced in leukocytes by the redox, oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid ...

(LTA4, i.e. 5S,6S-oxido-7''E'',9''E'',11''Z'',14''Z''-eicosatetetraenoic acid;

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

name 4-butanoic acid) to its diol metabolite,

leukotriene B4 Leukotriene B4 (LTB4) is a leukotriene involved in inflammation. It has been shown to promote insulin resistance in obese mice. Biochemistry LTB4 is a leukotriene involved in inflammation. It is produced from leukocytes in response to inflammato ...

(LTB4, i.e. 5''S'',12''R''-dihydroxy-6''Z'',8''E'',10''E'',14''Z''-icosatetraenoic acid; IUPA name 5S,6Z,8E,10E,12R,14Z)-5,12-Dihydroxy-6,8,10,14-icosatetraenoic acid). LTB4 is an important recruiter and activator of

leukocytes White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...

involved in mediation in inflammatory responses and diseases. The enzyme also possess

aminopeptidase Aminopeptidases are enzymes that catalyze the cleavage of amino acids from the N-terminus (beginning), of proteins or peptides. They are found in many organisms; in the cell, they are found in many organelles, in the cytosol (internal cellular f ...

activity, degrading, for example, the leukocyte

chemotactic factor Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...

tripeptide, Pro-Gly-Pro (PGP); the function of the aminopeptidase activity of LTA4AH is unknown but has been proposed to be involved in limiting inflammatory reactions caused by this or other aminopeptidase-susceptible peptides.

Cholesterol-5,6-oxide hydrolase

(Cholesterol epoxide hydrolase or ChEH), is located in the endoplasmic reticulum and to a lesser extent plasma membrane of various cell types but most highly express in liver. The enzyme catalyzes the conversion of certain 3-hydroxyl-5,6-epoxides of cholesterol to their 3,5,6-trihydroxy products (see Cholesterol-5,6-oxide hydrolase). The function of ChEH is unknown.

Peg1/MEST

The substrate(s) and physiological function of Peg1/MEST are not known; however, the protein may play a role in mammalian development and abnormalities in its expression by its gene (PEG1/MEST)by, for example, loss of

Genomic imprinting Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed or not, depending on whether they are inherited from the female or male parent. Genes can also be partially imprinted. Partial imprinting occurs when alleles from b ...

, overexpression, or promoter switching, has been linked to certain types of cancer and tumors in humans such as invasive cervical cancer, uterine leiomyomas, and cancers of the breast, lung, and colon (see MEST (gene)).

Hepoxilin-epoxide hydrolase

Hepoxilin-epoxide hydrolase or hepoxilin hydrolase is currently best defined as an enzyme activity that converts the biologically active monohydroxy-epoxide metabolites of arachidonic acid hepoxilin A3s and hepoxilin B3s to essentially inactive trihydroxy products, the trioxilins. That is, hepoxilin A3s (8-hydroxy-11,12-oxido-5''Z'',9''E'',14''Z''-eicosatrienoic acid) are metabolized to trioxilin A3s (8,11,12-trihydroxy-5''Z'',9''E'',14''Z''-eicosatrienoic acids) and hepoxilins B3s (10-hydroxy-11,12-oxido-5''Z'',8''Z'',14''Z''-eicosatrienoic acids) are metabolized to trioxilin B3s (10,11,12-trihydroxy-5''Z'',8''Z'',14''Z''-eicosatrienoic acids). However, this activity has not been characterized at the purified protein or gene level and recent work indicate that sEH readily metabolizes an hepoxilin A3 to a trioxilin A3 and that hepoxilin-epoxide hydrolase activity is due to sEH, at least as it is detected in mouse liver.

''Mycobacterium tuberculosis''

Mycobacterium tuberculosis ''Mycobacterium tuberculosis'' (M. tb), also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, ''M. tuberculosis'' ha ...

'', the causative agent of tuberculosis, expresses at least six different forms of epoxide hydrolase (forms A-F). The structure of epoxide hydrolase B reveals that the enzyme is a monomer and contains an alpha/beta hydrolase fold. In addition to providing insights into the enzyme mechanism, this hydrolase currently serves as a platform for

rational drug design Drug design, often referred to as rational drug design or simply rational design, is the invention, inventive process of finding new medications based on the knowledge of a biological target. The drug is most commonly an organic compound, organi ...

of potent inhibitors. In particular, urea based inhibitors have been developed. These inhibitors directly target the catalytic cavity. It is hypothesized that the structure of epoxide hydrolase B may allow for drug design to inhibit all other ''Mycobacterium tuberculosis'' hydrolases as long as they contain similar alpha/beta folds. The structure of hydrolase B contains a cap domain, which is hypothesized to regulate the active site of the hydrolase. Furthermore, Asp104, His333, and Asp302 form the catalytic triad of the protein and is critical to function of the protein. At present, other structures of ''Mycobacterium tuberculosis'' hydrolase have not been solved. Model studies on pharmacological susceptibility of these epoxide hydrolases continue.

References

External links

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Epoxide hydrolase characterization and purification
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