DUOX1
Encyclopedia
Dual oxidase 1, also known as DUOX1 or ThOX1 (for thyroid oxidase), is an enzyme
which in humans is encoded by the DUOX1 gene
. DUOX1 was first identified in the mammalian thyroid gland. In humans, two isoforms are found; hDUOX1 and hDUOX2. Human DUOX protein localization is not exclusive to thyroid tissue; hDUOX1 is prominent in airway epithelial cells and hDUOX2 in the salivary glands and gastrointestinal tract.
) in biological systems have, until recently, focused on characterization of phagocytic cell processes. It is now well accepted that production of such species is not restricted to phagocytic cells and can occur in eukaryotic, non-phagocytic cell types via NADPH oxidase
(NOX) or dual oxidase (DUOX). This new family of proteins, termed the NOX/DUOX family or NOX family of NADPH oxidases, consists of homologs to the catalytic moiety of phagocytic NADPH-oxidase, gp91phox. Members of the NOX/DUOX family have been found throughout eukaryotic species, including invertebrates, insects, nematodes, fungi, amoeba, alga, and plants (not found in prokaryotes). These enzymes clearly demonstrate regulated production of ROS as their sole function. Genetic analyses have implicated NOX/DUOX derived ROS in biological roles and pathological conditions including hypertension (NOX1), innate immunity (NOX2/DUOX), otoconia
formation in the inner ear (NOX3), and thyroid hormone biosynthesis (DUOX1/2). The family currently has seven members including NOX1
, NOX2
(formerly known as gp91phox), NOX3
, NOX4
, NOX5
, DUOX1 (this enzyme) and DUOX2
.
The current model for ROS generation by C. elegans
DUOX1 (CeDUOX1) proposes that superoxide is generated through reduction of oxygen by two electrons extracted from oxidation of NADPH at the C-terminal NADPH oxidase domain. This unstable superoxide, generated at the extracellular surface, may rapidly convert to hydrogen peroxide and be utilized by the N-terminal peroxidase domain to facilitate tyrosine cross-linking. This model for CeDUOX1 activity was recently supported by a study of two point mutations localized within the peroxidase domain of CeDUOX1; G246D and D392N. Both mutations result in a blistering cuticle phenotype, resulting from the loss of tyrosine cross-linking activity. Neither mutant demonstrates a significant decrease in ROS production. These results suggest this peroxidase-like region is directly involved in enzymatic tyrosine cross-linking, but not responsible for ROS production.
s, a transmembrane (TM) segment appended to an EF-hand
calcium-binding cytosolic region and a NOX2 homologous structure (six TMs tethered to NADPH oxidase). Topological studies place this peroxidase domain on the opposite side of the membrane from the NADPH oxidase domain.
hDUOX1 and hDUOX2 are 83% homologous, ~190 kDa in size (after extensive glycosylation contributing ~30 kDa in mass), and require maturation factors (DUOXA1 and DUOXA2) to achieve heterologous expression in full-length, active form. Mature DUOX enzymes produce H2O2; this activity is regulated by Ca2+ concentration through triggered dissociation of NOXA1 and possibly other as yet unidentified interacting proteins. Interestingly, when sequence alignments were performed against other mammalian peroxidases, the histidine residues responsible for heme coordination were not conserved. Due to this critical disparity, much speculation has surrounded the function of the DUOX peroxidase domain(s). Proposals for functionality include: superoxide dismutase activity, instead of peroxidase activity; a novel peroxidase mechanism; a protein-protein or Ca2+ induced conformational change which subsequently allows heme binding for peroxidase activity; or simply inactivity, as a vestigial domain.
Recent in vitro investigations into the ability of the DUOX1 domain to act as a peroxidase demonstrated that cell lysate from peroxidase expression in C. elegans
and E. coli
had tyrosine
cross-linking activity. Further in vitro studies of human DUOX1 (hDUOX11-593) and C. elegans DUOX1 (CeDUOX11-589) were made possible by expression and purification via a baculovirus system. Evaluation of these proteins demonstrated that the isolated hDUOX11-593 does not bind heme and has no intrinsic peroxidase activity. In contrast, CeDUOX11-589 binds heme
covalently and exhibits a modest peroxidase activity, but does not oxidize bromide ion. Surprisingly, the heme appears to have two covalent links to the C. elegans protein despite the absence of a second conserved carboxyl group in the active site.
Two alternatively spliced transcript variants encoding the same protein have been described for this gene.
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...
which in humans is encoded by the DUOX1 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...
. DUOX1 was first identified in the mammalian thyroid gland. In humans, two isoforms are found; hDUOX1 and hDUOX2. Human DUOX protein localization is not exclusive to thyroid tissue; hDUOX1 is prominent in airway epithelial cells and hDUOX2 in the salivary glands and gastrointestinal tract.
Function
Investigations into reactive oxygen species (ROSReactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
) in biological systems have, until recently, focused on characterization of phagocytic cell processes. It is now well accepted that production of such species is not restricted to phagocytic cells and can occur in eukaryotic, non-phagocytic cell types via NADPH oxidase
NADPH oxidase
The NADPH oxidase is a membrane-bound enzyme complex. It can be found in the plasma membrane as well as in the membrane of phagosome.-Subunits:It is made up of six subunits...
(NOX) or dual oxidase (DUOX). This new family of proteins, termed the NOX/DUOX family or NOX family of NADPH oxidases, consists of homologs to the catalytic moiety of phagocytic NADPH-oxidase, gp91phox. Members of the NOX/DUOX family have been found throughout eukaryotic species, including invertebrates, insects, nematodes, fungi, amoeba, alga, and plants (not found in prokaryotes). These enzymes clearly demonstrate regulated production of ROS as their sole function. Genetic analyses have implicated NOX/DUOX derived ROS in biological roles and pathological conditions including hypertension (NOX1), innate immunity (NOX2/DUOX), otoconia
Otolith
An otolith, , also called statoconium or otoconium is a structure in the saccule or utricle of the inner ear, specifically in the vestibular labyrinth of vertebrates. The saccule and utricle, in turn, together make the otolith organs. They are sensitive to gravity and linear acceleration...
formation in the inner ear (NOX3), and thyroid hormone biosynthesis (DUOX1/2). The family currently has seven members including NOX1
NOX1
NADPH oxidase 1 is an enzyme that in humans is encoded by the NOX1 gene.-Further reading:...
, NOX2
CYBB
Cytochrome b-245, beta polypeptide -- also known as CYBB, P91-PHOX, and NOX2-- is a human gene encoding a glycoprotein.-External links:...
(formerly known as gp91phox), NOX3
NOX3
NADPH oxidase 3 is an enzyme that in humans is encoded by the NOX3 gene.-Further reading:...
, NOX4
NOX4
NADPH oxidase 4 is an enzyme that in humans is encoded by the NOX4 gene.-Further reading:...
, NOX5
NOX5
NADPH oxidase, EF-hand calcium binding domain 5, also known as NOX5, is a protein which in humans is encoded by the NOX5 gene.- Function :NOX5 is a novel NADPH oxidase that generates superoxide....
, DUOX1 (this enzyme) and DUOX2
DUOX2
Dual oxidase 2, also known as DUOX2 or ThOX2 is an enzyme which in humans is encoded by the DUOX2 gene. Dual oxidase is an enzyme that was first identified in the mammalian thyroid gland. In humans, two isoforms are found; hDUOX1 and hDUOX2...
.
The current model for ROS generation by C. elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...
DUOX1 (CeDUOX1) proposes that superoxide is generated through reduction of oxygen by two electrons extracted from oxidation of NADPH at the C-terminal NADPH oxidase domain. This unstable superoxide, generated at the extracellular surface, may rapidly convert to hydrogen peroxide and be utilized by the N-terminal peroxidase domain to facilitate tyrosine cross-linking. This model for CeDUOX1 activity was recently supported by a study of two point mutations localized within the peroxidase domain of CeDUOX1; G246D and D392N. Both mutations result in a blistering cuticle phenotype, resulting from the loss of tyrosine cross-linking activity. Neither mutant demonstrates a significant decrease in ROS production. These results suggest this peroxidase-like region is directly involved in enzymatic tyrosine cross-linking, but not responsible for ROS production.
Structure
Dual oxidases are characterized by a defining N-terminal, extracellular domain exhibiting considerable sequence identity with the mammalian peroxidasePeroxidase
Peroxidases are a large family of enzymes that typically catalyze a reaction of the form:For many of these enzymes the optimal substrate is hydrogen peroxide, but others are more active with organic hydroperoxides such as lipid peroxides...
s, a transmembrane (TM) segment appended to an EF-hand
EF hand
The EF hand is a helix-loop-helix structural domain found in a large family of calcium-binding proteins. The EF-hand motif contains a helix-loop-helix topology, much like the spread thumb and forefinger of the human hand, in which the Ca2+ ions are coordinated by ligands within the loop...
calcium-binding cytosolic region and a NOX2 homologous structure (six TMs tethered to NADPH oxidase). Topological studies place this peroxidase domain on the opposite side of the membrane from the NADPH oxidase domain.
hDUOX1 and hDUOX2 are 83% homologous, ~190 kDa in size (after extensive glycosylation contributing ~30 kDa in mass), and require maturation factors (DUOXA1 and DUOXA2) to achieve heterologous expression in full-length, active form. Mature DUOX enzymes produce H2O2; this activity is regulated by Ca2+ concentration through triggered dissociation of NOXA1 and possibly other as yet unidentified interacting proteins. Interestingly, when sequence alignments were performed against other mammalian peroxidases, the histidine residues responsible for heme coordination were not conserved. Due to this critical disparity, much speculation has surrounded the function of the DUOX peroxidase domain(s). Proposals for functionality include: superoxide dismutase activity, instead of peroxidase activity; a novel peroxidase mechanism; a protein-protein or Ca2+ induced conformational change which subsequently allows heme binding for peroxidase activity; or simply inactivity, as a vestigial domain.
Recent in vitro investigations into the ability of the DUOX1 domain to act as a peroxidase demonstrated that cell lysate from peroxidase expression in C. elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...
and E. coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
had 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...
cross-linking activity. Further in vitro studies of human DUOX1 (hDUOX11-593) and C. elegans DUOX1 (CeDUOX11-589) were made possible by expression and purification via a baculovirus system. Evaluation of these proteins demonstrated that the isolated hDUOX11-593 does not bind heme and has no intrinsic peroxidase activity. In contrast, CeDUOX11-589 binds heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
covalently and exhibits a modest peroxidase activity, but does not oxidize bromide ion. Surprisingly, the heme appears to have two covalent links to the C. elegans protein despite the absence of a second conserved carboxyl group in the active site.
Two alternatively spliced transcript variants encoding the same protein have been described for this gene.