GPX4
Encyclopedia
Glutathione peroxidase 4, also known as GPX4, is an enzyme
that in humans is encoded by the GPX4 gene
. GPX4 is a phospholipid hydroperoxidase
that protects cells against membrane lipid peroxidation
.
enzyme glutathione peroxidase 4 (GPx4) belongs to the family of glutathione peroxidase
s, which consists of 8 known mammalian isoenzymes (GPx1-8). Gpx4 catalyzes the reduction of hydrogen peroxide, organic hydroperoxides, and lipid peroxides at the expense of reduced glutathione
and functions in the protection of cells against oxidative stress
. The oxidized form of glutathione (glutathione disulfide
), which is generated during the reduction of hydroperoxides by GPx4, is recycled by glutathione reductase
and NADPH/H+. GPx4 differs from the other GPx family members in terms of its monomeric structure, a less restricted dependence on glutathione as reducing substrate, and the ability to reduce lipid-hydroperoxides inside biological membranes.
, GPx2
, GPx3, and GPx4 (this protein) have been shown to be selenium
-containing enzymes, whereas GPx6 is a selenoprotein
in humans with cysteine-containing homologues in rodents. In selenoproteins, the 21st amino acid selenocysteine
is inserted in the nascent polypeptide chain during the process of translational recoding of the UGA stop codon
. GPx4 shares the amino acid motif of selenocysteine, glutamine, and tryptophane (catalytic triad
) with other glutathione peroxidases.
This reaction occurs at the selenocysteine within the catalytic center of GPx4. During the catalytic cycle of GPx4, the active selenol (-SeH) is oxidized by peroxides to selenenic acid (-SeOH), which is then reduced with glutathione (GSH) to an intermediate selenodisulfide (-Se-SG). GPx4 is eventually reactivated by a second glutathione molecule, releasing glutathione disulfide (GS-SG).
and male fertility. In humans, experimental evidence for alternative splicing exists; alternative transcription initiation and the cleavage sites of the mitochondrial and nuclear transit peptides need to be experimentally verified.
of GPX4 die at embryonic day 8
and conditional inducible deletion in adult mice (neurons) results in degeneration and death in less than a month. Targeted disruption of the mitochondrial GPx4 isoform (mGPx4) caused infertility in male mice and disruption of the nuclear GPx4 isoform (nGPx4) reduced the structural stability of sperm chromatin, yet both knockout mouse models (for mGPx4 and nGPx4) were fully viable. Further knockout studies with GPx1, GPx2, or GPx3 deficient mice showed that cytosolic GPx4 is so far the only glutathione peroxidase that is indispensable for embryonic development and cell survival. This indicates that unlike the disposal of hydrogen peroxide
, the disposal of lipid hydroperoxides is essential for mammalian life.
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 in humans is encoded by the GPX4 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
. GPX4 is a phospholipid hydroperoxidase
Phospholipid-hydroperoxide glutathione peroxidase
In enzymology, a phospholipid-hydroperoxide glutathione peroxidase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are glutathione and lipid hydroperoxide, whereas its 3 products are glutathione disulfide, lipid, and H2O.This enzyme belongs to the family of...
that protects cells against membrane lipid peroxidation
Lipid peroxidation
Lipid peroxidation refers to the oxidative degradation of lipids. It is the process in which free radicals "steal" electrons from the lipids in cell membranes, resulting in cell damage. This process proceeds by a free radical chain reaction mechanism...
.
Function
The antioxidantAntioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...
enzyme glutathione peroxidase 4 (GPx4) belongs to the family of glutathione peroxidase
Glutathione peroxidase
Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage...
s, which consists of 8 known mammalian isoenzymes (GPx1-8). Gpx4 catalyzes the reduction of hydrogen peroxide, organic hydroperoxides, and lipid peroxides at the expense of reduced glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
and functions in the protection of cells against oxidative stress
Oxidative stress
Oxidative stress represents an imbalance between the production and manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage...
. The oxidized form of glutathione (glutathione disulfide
Glutathione disulfide
Glutathione disulfide is a disulfide derived from two glutathione molecules.In living cells, glutathione disulfide is reduced into two molecules of glutathione with reducing equivalents from the coenzyme NADPH. This reaction is catalyzed by the enzyme glutathione reductase...
), which is generated during the reduction of hydroperoxides by GPx4, is recycled by glutathione reductase
Glutathione reductase
Glutathione reductase, also known as GSR or GR, is an enzyme that reduces glutathione disulfide to the sulfhydryl form GSH, which is an important cellular antioxidant....
and NADPH/H+. GPx4 differs from the other GPx family members in terms of its monomeric structure, a less restricted dependence on glutathione as reducing substrate, and the ability to reduce lipid-hydroperoxides inside biological membranes.
Structure
Mammalian GPx1GPX1
Glutathione peroxidase 1 is an enzyme that in humans is encoded by the GPX1 gene.-Further reading:...
, GPx2
GPX2 (gene)
Glutathione peroxidase 2 is an enzyme that in humans is encoded by the GPX2 gene.The antioxidant enzyme glutathione peroxidase 2 is one out of eight known glutathione peroxidases in humans...
, GPx3, and GPx4 (this protein) have been shown to be selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
-containing enzymes, whereas GPx6 is a selenoprotein
Selenoprotein
In molecular biology a selenoprotein is any protein that includes a selenocysteine amino acid residue. Among functionally characterized selenoproteins are five glutathione peroxidases and three thioredoxin reductases, which both contain only one Se-Cys. Selenoprotein P is the most common...
in humans with cysteine-containing homologues in rodents. In selenoproteins, the 21st amino acid selenocysteine
Selenocysteine
Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...
is inserted in the nascent polypeptide chain during the process of translational recoding of the UGA stop codon
Stop codon
In the genetic code, a stop codon is a nucleotide triplet within messenger RNA that signals a termination of translation. Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide...
. GPx4 shares the amino acid motif of selenocysteine, glutamine, and tryptophane (catalytic triad
Catalytic triad
A catalytic triad refers to the three amino acid residues found inside the active site of certain protease enzymes: serine , aspartate , and histidine . They work together to break peptide bonds on polypeptides. In general terms, catalytic triad can refer to any set of three residues that function...
) with other glutathione peroxidases.
Reaction mechanism
GPx4 catalyzes the following reaction:- 2 glutathioneGlutathioneGlutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
+ lipidLipidLipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
–hydroperoxideOrganic peroxideOrganic peroxides are organic compounds containing the peroxide functional group . If the R' is hydrogen, the compound is called an organic hydroperoxide. Peresters have general structure RCOOR. The O-O bond easily breaks and forms free radicals of the form RO·...
→ glutathione disulfideGlutathione disulfideGlutathione disulfide is a disulfide derived from two glutathione molecules.In living cells, glutathione disulfide is reduced into two molecules of glutathione with reducing equivalents from the coenzyme NADPH. This reaction is catalyzed by the enzyme glutathione reductase...
+ lipid–alcohol + H2O
This reaction occurs at the selenocysteine within the catalytic center of GPx4. During the catalytic cycle of GPx4, the active selenol (-SeH) is oxidized by peroxides to selenenic acid (-SeOH), which is then reduced with glutathione (GSH) to an intermediate selenodisulfide (-Se-SG). GPx4 is eventually reactivated by a second glutathione molecule, releasing glutathione disulfide (GS-SG).
Subcellular distribution of isoforms
In mouse and rat, three distinct GPx4 isoforms with different subcellular localization are produced through alternative splicing and transcription initiation; cytosolic GPx4, mitochondrial GPx4 (mGPx4), and nuclear GPx4 (nGPx4). Cytosolic GPx4 has been identified as the only GPx4 isoform being essential for embryonic development and cell survival. The GPx4 isoforms mGPx4 and nGPx4 have been implicated in spermatogenesisSpermatogenesis
Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived. Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two...
and male fertility. In humans, experimental evidence for alternative splicing exists; alternative transcription initiation and the cleavage sites of the mitochondrial and nuclear transit peptides need to be experimentally verified.
Animal models
Knockout miceKnockout mouse
A knockout mouse is a genetically engineered mouse in which researchers have inactivated, or "knocked out," an existing gene by replacing it or disrupting it with an artificial piece of DNA...
of GPX4 die at embryonic day 8
and conditional inducible deletion in adult mice (neurons) results in degeneration and death in less than a month. Targeted disruption of the mitochondrial GPx4 isoform (mGPx4) caused infertility in male mice and disruption of the nuclear GPx4 isoform (nGPx4) reduced the structural stability of sperm chromatin, yet both knockout mouse models (for mGPx4 and nGPx4) were fully viable. Further knockout studies with GPx1, GPx2, or GPx3 deficient mice showed that cytosolic GPx4 is so far the only glutathione peroxidase that is indispensable for embryonic development and cell survival. This indicates that unlike the disposal of hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
, the disposal of lipid hydroperoxides is essential for mammalian life.