enzymology Enzymes () are proteins that act as biological catalysts 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 products. ...

, a microsomal epoxide hydrolase (mEH) () is an

enzyme Enzymes () are proteins that act as biological catalysts 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 products ...

that catalyzes the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...

reaction between an

epoxide In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale ...

and

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

to form a

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

This enzyme plays a role in the uptake of bile salts within the

large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before bein ...

. It functions as a Na+ dependent transporter. This enzyme participates in metabolism of

xenobiotic 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 ...

s by

cytochrome p450 Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various co ...

. mEH has been identified as playing a large role in the

detoxification Detoxification or detoxication (detox for short) is the physiological or medicinal removal of toxic substances from a living organism, including the human body, which is mainly carried out by the liver. Additionally, it can refer to the period of ...

and

bioactivation Activation, in chemistry and biology, is the process whereby something is prepared or excited for a subsequent reaction. Chemistry In chemistry, "activation" refers to the reversible transition of a molecule into a nearly identical chemical or ...

of a wide variety of substrates, such as

polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

s (PAHs), which are known for their carcinogenic properties. The human homolog of microsomal epoxide hydrolase is EPHX1 and is located on

chromosome 1 Chromosome 1 is the designation for the largest human chromosome. Humans have two copies of chromosome 1, as they do with all of the autosomes, which are the non- sex chromosomes. Chromosome 1 spans about 249 million nucleotide base pairs, which ...

Nomenclature

This enzyme belongs to the family of

hydrolase Hydrolase is a class of enzyme that commonly perform as biochemical catalysts that use water to break a chemical bond, which typically results in dividing a larger molecule into smaller molecules. Some common examples of hydrolase enzymes are este ...

s, specifically those acting on

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again ...

bonds (ether hydrolases). The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

of this enzyme class is cis-stilbene-oxide hydrolase. Other names in common use include epoxide hydratase (ambiguous), microsomal epoxide hydratase (ambiguous), epoxide hydrase, microsomal epoxide hydrase, arene-oxide hydratase (ambiguous), benzo yrene-4,5-oxide hydratase, benzo(a)pyrene-4,5-epoxide hydratase, aryl epoxide hydrase (ambiguous), cis-epoxide hydrolase, and mEH.

Structure

Microsomal epoxide hydrolase is a single

polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

chain composed of 455

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

s with a molecular weight of 52.96 kilodaltons. It is known that the

N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...

region of the enzyme is responsible for anchoring the protein to the

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

, while the

C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal 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 the protein is ...

region of the enzyme contains catalytic residues.; Microsomal epoxide hydrolase belongs to the superfamily α/β-hydrolase fold enzymes. The center of all α/β-hydrolase fold enzymes is an alpha/beta-sheet consisted of 8

beta strand The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a ge ...

s connected by 6

alpha helices The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...

. The three dimensional structure of mEH has been elucidated from ''Aspergillus niger.'' Although no 3D modeling has been solved for the mammalian mEH enzyme (EPHX1), the overall homology between fungal and mammalian mEH is relatively high. This high homology has allowed for the elucidation overall general structure and subsequent catalytic mechanism of EPHX1 in humans by comparisons to existing structures of fungal mEH.

Mechanism

α/β-hydrolase fold enzymes use a

catalytic triad A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, li ...

in their

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate ( binding site) ...

. The catalytic triad present in microsomal epoxide hydrolase is composed of

glutamine Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral ...

histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the ...

and

aspartic acid Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...

. The substrate is positioned in an orientation poised for nucleophilic attack through hydrogen bonding stabilization from two nearby

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the G ...

residues The proposed mechanism for the mEH-catalyzed reaction first involves a nucleophilic attack on the oxirane ring of the substrate from the aspartic acid residue near the active site, which forms an

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...

intermediate. The second step in this mechanism is

of the ester that occurs by an activated water molecule. The activation of water is facilitated by proton abstraction via the catalytic triad between a water molecule, glutamine, and histidine. After hydrolysis, the substrate is then released from its bond to the aspartic acid residue, liberating the diol product from the enzyme active site. The active site of this enzyme lies within a

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...

pocket in the enzyme, which in turn leads to the enzyme's preferential reactivity with molecules with hydrophobic side-chains. The mEH enzyme typically binds to small organic epoxides, such as styrene epoxide and cis-stillbene-oxide. mEH does not catalyze the hydrolysis of bulkier molecules, as their large side-chains may

sterically Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

disrupt the charge relay system responsible for water activation. MEH Active Site

Function

In humans, mEH has been found in the

ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

lymphocyte A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic a ...

epithelial cells Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...

, and

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...

. Microsomal epoxide hydrolase serves as a protective enzyme against potentially harmful

small molecule Within the fields of molecular biology and pharmacology, a small molecule or micromolecule is a low molecular weight (≤ 1000 daltons) organic compound that may regulate a biological process, with a size on the order of 1 nm. Many drugs ...

s derived from the external environment. This hydrolysis of genotoxic epoxides causes subsequent effects in several signal transduction pathways, rendering this enzyme important to metabolism.

Disease relevance

Microsomal epoxide hydrolase plays a large role in its effects on human health. Studies have shown that mutations EPHX1 in humans may be the cause of hypercholanemia,

preeclampsia Pre-eclampsia is a disorder of pregnancy characterized by the onset of high blood pressure and often a significant amount of protein in the urine. When it arises, the condition begins after 20 weeks of pregnancy. In severe cases of the disease ...

, and may contribute to

fetal hydantoin syndrome Fetal hydantoin syndrome, also called fetal dilantin syndrome, is a group of defects caused to the developing fetus by exposure to teratogenic effects of phenytoin. Dilantin is the brand name of the drug phenytoin sodium in the United States, comm ...

. Research also suggests that

maternal ] A mother is the female parent of a child. A woman may be considered a mother by virtue of having given birth, by raising a child who may or may not be her biological offspring, or by supplying her ovum for fertilisation in the case of gestat ...

Polymorphism (biology), polymorphisms in EPHX1 in pregnant women were related to facial malformations of children born from women taking

phenytoin Phenytoin (PHT), sold under the brand name Dilantin among others, is an anti-seizure medication. It is useful for the prevention of tonic-clonic seizures (also known as grand mal seizures) and focal seizures, but not absence seizures. The in ...

during their

first trimester Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ...

of pregnancy. While mEH participates in the protection of human health via detoxification of various environmental substances, it also has been found to facilitate the activation of

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...

s. mEH detoxifies reactive epoxides that are commonly caused from

cigarette smoke Tobacco smoke is a sooty aerosol produced by the incomplete combustion of tobacco during the smoking of cigarettes and other tobacco products. Temperatures in burning cigarettes range from about 400 °C between puffs to about 900 °C ...

, and as such it is hypothesized that mutations in EPHX1 in humans may have an effect on an individual's susceptibility to

COPD Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by long-term respiratory symptoms and airflow limitation. The main symptoms include shortness of breath and a cough, which may or may not produce ...

emphysema Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alv ...

and

lung cancer Lung cancer, also known as lung carcinoma (since about 98–99% of all lung cancers are carcinomas), is a malignant lung tumor characterized by uncontrolled cell growth in tissues of the lung. Lung carcinomas derive from transformed, mali ...

. Some sources have demonstrated that individuals affected by COPD have a higher rate of containing an under-active variant of the EPHX1 gene, yet also demonstrated that the overactive variant of the gene was also found in higher frequencies in individuals affected by disease as well. Other research has provided evidence supporting the idea that EPHX1 variants do not contribute to susceptibility of disease, but do contribute to disease severity. The role that mEH plays in lung cancer and COPD is still not fully elucidated, as the data on the topic in the literature is not completely unanimous. There is some evidence that mEH variants may contribute to the occurrence of childhood

asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...

in combination with variants on the GSTP1 gene. Compared to soluble epoxide hydrolase, the contribution of mEH to metabolism of beneficial epoxy fatty acids such as

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. These nonclassic eicos ...

is considered minor since they are relatively poor mEH substrates ''in vitro''. Yet, ''in vivo'', it was found that mEH can play a considerable role in regulation of EET levels and hence inhibition of mEH or dual inhibition of mEH and sEH might have therapeutic potential. Amide, amine and urea based mEH inhibitors have been explored. Based on the most potent inhibitors characterized, an amide with a bulky alpha-substituent and a phenyl ring with lipophilic groups at meta-positions appear to be key pharmacophore units. The overall effect that mEH has on human health is still debated, with some sources finding evidence that the overactive EPHX1 gene is the culprit for some diseases, while other evidence supports that the under active variant is the cause of others.

Nomenclature

Structure

Mechanism

Function

Disease relevance

References

Further reading