# Dual oxidase 1

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.

## Function

Investigations into reactive oxygen species (ROS) 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, gp91$^{phox}$. 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 gp91$^{phox}$), 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.

## Structure

Dual oxidases are characterized by a defining N-terminal, extracellular domain exhibiting considerable sequence identity with the mammalian peroxidases, 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 H$_2$O$_2$; this activity is regulated by Ca$^{2+}$ 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 Ca$^{2+}$ 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 (hDUOX1$_{1-593}$) and C. elegans DUOX1 (CeDUOX1$_{1-589}$) were made possible by expression and purification via a baculovirus system. Evaluation of these proteins demonstrated that the isolated hDUOX1$_{1-593}$ does not bind heme and has no intrinsic peroxidase activity. In contrast, CeDUOX1$_{1-589}$ 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.