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Myeloperoxidase (MPO) is a
peroxidase Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides, and should not be confused with other ...
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
that in humans is encoded by the ''MPO''
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
on
chromosome 17 Chromosome 17 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 17 spans more than 84 million base pairs (the building material of DNA) and represents between 2.5 and 3% of the total DN ...
. MPO is most abundantly expressed in neutrophils (a subtype of
white blood cell 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 ...
s), and produces hypohalous acids to carry out their
antimicrobial An antimicrobial is an agent that kills microorganisms (microbicide) or stops their growth (bacteriostatic agent). Antimicrobial medicines can be grouped according to the microorganisms they are used to treat. For example, antibiotics are used aga ...
activity, including hypochlorous acid, the sodium salt of which is the chemical in bleach. It is a lysosomal protein stored in azurophilic granules of the neutrophil and released into the extracellular space during degranulation. Neutrophil myeloperoxidase has a
heme Heme (American English), or haem (Commonwealth English, both pronounced /Help:IPA/English, hi:m/ ), is a ring-shaped iron-containing molecule that commonly serves as a Ligand (biochemistry), ligand of various proteins, more notably as a Prostheti ...
pigment, which causes its green color in secretions rich in neutrophils, such as
mucus Mucus (, ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both Serous fluid, serous and muc ...
and
sputum Sputum is mucus that is coughed up from the lower airways (the trachea and bronchi). In medicine, sputum samples are usually used for a naked-eye examination, microbiological investigation of respiratory infections, and Cytopathology, cytological ...
. The green color contributed to its outdated name verdoperoxidase. Myeloperoxidase is found in many different organisms including mammals, birds, fish, reptiles, and amphibians. Myeloperoxidase deficiency is a well-documented disease among humans resulting in impaired immune function.


Function

MPO is a member of the XPO subfamily of peroxidases and produces
hypochlorous acid Hypochlorous acid is an inorganic compound with the chemical formula , also written as HClO, HOCl, or ClHO. Its structure is . It is an acid that forms when chlorine dissolves in water, and itself partially dissociates, forming a hypochlorite an ...
(HOCl) from
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
(H2O2) and
chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
(Cl) (or hypobromous acid if Br- is present) during the neutrophil's respiratory burst. It requires heme as a cofactor. Furthermore, it oxidizes
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 conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
to tyrosyl radical using hydrogen peroxide as an
oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ''electron donor''). In ot ...
. However, this hypochlorous acid may also cause oxidative damage in host tissue. Moreover, MPO oxidation of apoA-I reduces HDL-mediated inhibition of
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
and inflammation. In addition, MPO mediates protein nitrosylation and the formation of 3-chlorotyrosine and dityrosine crosslinks. Myeloperoxidase is the first and so far only human enzyme known to break down
carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range ( nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''S ...
s, allaying a concern among clinicians that using nanotubes for targeted delivery of medicines would lead to an unhealthy buildup of nanotubes in tissues. *


Role in innate immunity

Neutrophils use myeloperoxidase to produce the substances needed for their respiratory burst. Hypochlorous acid and tyrosyl radical are
cytotoxic Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are toxic metals, toxic chemicals, microbe neurotoxins, radiation particles and even specific neurotransmitters when the system is out of balance. Also some types of dr ...
, so they are used by the neutrophil to kill
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
and certain types of fungi.


Structure

The 150- kDa MPO protein is a cationic heterotetramer consisting of two 15-kDa light chains and two variable-weight glycosylated heavy chains bound to a prosthetic
heme Heme (American English), or haem (Commonwealth English, both pronounced /Help:IPA/English, hi:m/ ), is a ring-shaped iron-containing molecule that commonly serves as a Ligand (biochemistry), ligand of various proteins, more notably as a Prostheti ...
group complex with calcium ions, arranged as a
homodimer In biochemistry, a protein dimer is a macromolecular complex or protein multimer, multimer formed by two protein monomers, or single proteins, which are usually Non-covalent interaction, non-covalently bound. Many macromolecules, such as proteins ...
of
heterodimer In biochemistry, a protein dimer is a macromolecular complex or multimer formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ...
s. Both are proteolytically generated from the precursor peptide encoded by the ''MPO'' gene. The light chains are glycosylated and contain the modified iron protoporphyrin IX
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, the ''binding s ...
. Together, the light and heavy chains form two identical 73-kDa monomers connected by a
cystine Cystine is the oxidized derivative of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of redox reactions and a mec ...
bridge at Cys153. The protein forms a deep crevice which holds the heme group at the bottom, as well as a
hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...
pocket at the entrance to the distal heme cavity which carries out its catalytic activity. Variation in glycosylation and the identity of the heavy chain lead to variations in molecular weight within the 135-200 kDa range. In mice, three
isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...
exist, differing only by the heavy chain. One of the ligands is the
carbonyl In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...
group of Asp 96. Calcium-binding is important for structure of the active site because of Asp 96's close proximity to the catalytic His95
side chain In organic chemistry and biochemistry, a side chain is a substituent, chemical group that is attached to a core part of the molecule called the "main chain" or backbone chain, backbone. The side chain is a hydrocarbon branching element of a mo ...
.


Reaction mechanism

The central heme group acts as the
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, the ''binding s ...
. The reaction starts when hydrogen peroxide donates an oxygen to the heme group, converting it to an activated form called "Compound I". This compound then oxidizes the chloride ions to form the hypochlorous acid and Compound II, which can be reduced back down to its original heme state. This cycle continues for as long as the immune system requires.


Clinical significance


Myeloperoxidase deficiency

Myeloperoxidase deficiency is a hereditary deficiency of the enzyme, which causes a mild
immune deficiency Immunodeficiency, also known as immunocompromise, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that affec ...
against certain
pathogens In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term ...
. People with myeloperoxidase deficiency are most at risk of infection by '' Candida'' species, which are pathogenic fungi. The most common species found in humans is '' Candida albicans''. There may be an increased risk of certain other infections, such as with '' Klebsiella pneumoniae'', but recurrent
candidiasis Candidiasis is a fungal infection due to any species of the genus '' Candida'' (a yeast). When it affects the mouth, in some countries it is commonly called thrush. Signs and symptoms include white patches on the tongue or other areas of the ...
is the only common clinical consequence, if the patient is noticeably affected at all.


Vasculitis

Antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...
against MPO have been implicated in various types of
vasculitis Vasculitis is a group of disorders that destroy blood vessels by inflammation. Both artery, arteries and veins are affected. Lymphangitis (inflammation of lymphatic vessels) is sometimes considered a type of vasculitis. Vasculitis is primarily c ...
, most prominently three clinically and pathologically recognized forms:
granulomatosis with polyangiitis Granulomatosis with polyangiitis (GPA), formerly known as Wegener's granulomatosis (WG), after Nazi German physician Friedrich Wegener, is a rare, long-term, systemic disorder that involves the formation of granulomas and vasculitis, inflammatio ...
(GPA), microscopic polyangiitis (MPA); and eosinophilic granulomatosis with polyangiitis (EGPA). Antibodies are also known as anti-neutrophil cytoplasmic antibodies (ANCAs), though ANCAs have also been detected in
staining Staining is a technique used to enhance contrast in samples, generally at the Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...
of the perinuclear region.


Atherosclerosis and heart disease

Myeloperoxidase is known to contribute to
atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis, characterized by development of abnormalities called lesions in walls of arteries. This is a chronic inflammatory disease involving many different cell types and is driven by eleva ...
and diseases related to it, including
coronary artery disease Coronary artery disease (CAD), also called coronary heart disease (CHD), or ischemic heart disease (IHD), is a type of cardiovascular disease, heart disease involving Ischemia, the reduction of blood flow to the cardiac muscle due to a build-up ...
. MPO oxidizes
LDL cholesterol Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall dens ...
, and as a result, the
LDL receptor The low-density lipoprotein receptor (LDL-R) is a mosaic protein of 839 amino acids (after removal of 21-amino acid signal peptide) that mediates the endocytosis of cholesterol-rich low-density lipoprotein (LDL). It is a cell-surface receptor ...
in liver cells becomes unable to bind to LDL and remove it from the blood stream. However, in its oxidized state, LDL can still contribute to foam cell formation and other atherosclerotic processes. Thus, elevated levels of MPO are a risk factor for atherosclerosis.


Medical tests

An initial 2003 study suggested that MPO could serve as a sensitive predictor for
myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when Ischemia, blood flow decreases or stops in one of the coronary arteries of the heart, causing infarction (tissue death) to the heart muscle. The most common symptom ...
in patients presenting with
chest pain Chest pain is pain or discomfort in the chest, typically the front of the chest. It may be described as sharp, dull, pressure, heaviness or squeezing. Associated symptoms may include pain in the shoulder, arm, upper abdomen, or jaw, along with n ...
. Since then, there have been over 100 published studies documenting the utility of MPO testing. The 2010 Heslop et al. study reported that measuring both MPO and CRP (C-reactive protein; a general and cardiac-related marker of inflammation) provided added benefit for risk prediction than just measuring CRP alone.
Immunohistochemical Immunohistochemistry is a form of immunostaining. It involves the process of selectively identifying antigens in cells and tissue, by exploiting the principle of antibodies binding specifically to antigens in biological tissues. Albert Hewett ...
staining for myeloperoxidase used to be administered in the diagnosis of
acute myeloid leukemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with haematopoiesis, normal blood cell production. Sympt ...
to demonstrate that the leukemic cells were derived from the
myeloid Myeloid tissue, in the bone marrow sense of the word '' myeloid'' ('' myelo-'' + '' -oid''), is tissue of bone marrow, of bone marrow cell lineage, or resembling bone marrow, and myelogenous tissue (''myelo-'' + '' -genous'') is any tissue ...
lineage. Myeloperoxidase staining is still important in the diagnosis of myeloid sarcoma, contrasting with the negative staining of
lymphoma Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). The name typically refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include enlarged lymph node ...
s, which can otherwise have a similar appearance. In the case of screening patients for vasculitis, flow cytometric assays have demonstrated comparable sensitivity to
immunofluorescence Immunofluorescence (IF) is a light microscopy-based technique that allows detection and localization of a wide variety of target biomolecules within a cell or tissue at a quantitative level. The technique utilizes the binding specificity of anti ...
tests, with the additional benefit of simultaneous detection of multiple autoantibodies relevant to vasculitis. Nonetheless, this method still requires further testing.


Inhibitors of MPO

Azide In chemistry, azide (, ) is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant ...
has been used traditionally as an MPO inhibitor, but 4-aminobenzoic acid hydrazide (4-ABH) is a more specific inhibitor of MPO.


See also

* Chloroma


References


External links

* {{Portal bar, Biology, border=no EC 1.11.1 Hemoproteins