DUOX1
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Dual oxidase 1, also known as DUOX1 or ThOX1 (for
thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...
oxidase In biochemistry, an oxidase is an enzyme that catalyzes oxidation-reduction reactions, especially one involving dioxygen (O2) as the electron acceptor. In reactions involving donation of a hydrogen atom, oxygen is reduced to water (H2O) or hydro ...
), 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. A ...
which in humans is encoded by the ''DUOX1''
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
. 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 ( ROS) in biological systems have, until recently, focused on characterization of
phagocytic Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
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 (nicotinamide adenine dinucleotide phosphate oxidase) is a membrane-bound enzyme complex that faces the extracellular space. It can be found in the plasma membrane as well as in the membranes of phagosomes used by neutrophil white ...
(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 An otolith ( grc-gre, ὠτο-, ' ear + , ', a stone), also called statoconium or otoconium or statolith, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The sa ...
formation in the inner ear (NOX3), and thyroid hormone biosynthesis (DUOX1/2). The family currently has seven members including
NOX1 NADPH oxidase 1 is an enzyme that in humans is encoded by the ''NOX1'' gene. NOX1 is a homolog of the catalytic subunit of the superoxide-generating NADPH oxidase of phagocytes, gp91phox. Two transcript variants encoding different isoforms A pr ...
, NOX2 (formerly known as gp91phox), NOX3,
NOX4 NADPH oxidase 4 is an enzyme that in humans is encoded by the ''NOX4'' gene, and is a member of the NOX family of NADPH oxidases. Function Oxygen sensing is essential for homeostasis in all aerobic organisms. A phagocyte-type oxidase, similar t ...
,
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. Nox5 interacts with c-abl, superoxide product ...
, DUOX1 (this enzyme) and DUOX2. The current model for ROS generation by ''
C. elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
'' 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.Meitzler JL, Brandman, R, Ortiz de Montellano, Perturbed heme binding is responsible for the blistering phenotype associated with mutations in the Caenorhabditis elegans dual oxidase 1 (DUOX1) peroxidase domain J. Biol. Chem. 2010, 285, 40991-41000. 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 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 ...
, extracellular domain exhibiting considerable sequence identity with the mammalian
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. Functionality Peroxidases typically ca ...
s, a transmembrane (TM) segment appended to an
EF-hand The EF hand is a helix–loop–helix structural domain or ''motif'' 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 ...
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. 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'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
'' and ''
E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'' had
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 Gr ...
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, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consisti ...
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.


References


Further reading

* * * * * * * * * * * * * * {{Thyroid hormone metabolism enzymes and transporters EF-hand-containing proteins