Structure, regulation and evolution of Nox‐family NADPH oxidases that produce reactive oxygen species

Abstract

NADPH oxidases of the Nox family exist in various supergroups of eukaryotes but not in prokaryotes, and play crucial roles in a variety of biological processes, such as host defense, signal transduction, and hormone synthesis. In conjunction with NADPH oxidation, Nox enzymes reduce molecular oxygen to superoxide as a primary product, and this is further converted to various reactive oxygen species. The electron‐transferring system in Nox is composed of the C‐terminal cytoplasmic region homologous to the prokaryotic (and organelle) enzyme ferredoxin reductase and the N‐terminal six transmembrane segments containing two hemes, a structure similar to that of cytochrome b of the mitochondrial bc1 complex. During the course of eukaryote evolution, Nox enzymes have developed regulatory mechanisms, depending on their functions, by inserting a regulatory domain (or motif) into their own sequences or by obtaining a tightly associated protein as a regulatory subunit. For example, one to four Ca2+‐binding EF‐hand motifs are present at the N‐termini in several subfamilies, such as the respiratory burst oxidase homolog (Rboh) subfamily in land plants (the supergroup Plantae), the NoxC subfamily in social amoebae (the Amoebozoa), and the Nox5 and dual oxidase (Duox) subfamilies in animals (the Opisthokonta), whereas an SH3 domain is inserted into the ferredoxin–NADP+ reductase region of two Nox enzymes in Naegleria gruberi, a unicellular organism that belongs to the supergroup Excavata. Members of the Nox1–4 subfamily in animals form a stable heterodimer with the membrane protein p22phox, which functions as a docking site for the SH3 domain‐containing regulatory proteins p47phox, p67phox, and p40phox; the small GTPase Rac binds to p67phox (or its homologous protein), which serves as a switch for Nox activation. Similarly, Rac activates the fungal NoxA via binding to the p67phox‐like protein Nox regulator (NoxR). In plants, on the other hand, this GTPase directly interacts with the N‐terminus of Rboh, leading to superoxide production. Here I describe the regulation of Nox‐family oxidases on the basis of three‐dimensional structures and evolutionary conservation.

Epidermal growth factor-induced hydrogen peroxide production is mediated by dual oxidase 1

“…The involvement of Nox2 in EGF-induced H 2 O 2 production seems unlikely, since we did not detect Nox2 mRNA in A431 or HaCaT cells, while Duox1 mRNA was expressed at high levels in both cell lines. It is also important to note that in contrast to Duox1, Nox2 alone is not able to produce H 2 O 2 and the co-expression of four other proteins is necessary for the assembly of an active enzyme complex (25).…”

Role of Rac GTPase activating proteins in regulation of NADPH oxidase in human neutrophils

“…Upon activation, p47 phox , p67 phox and p40 phox translocate from the cytosol to the (plasma or phagosomal) membrane, where they bind to the complex of gp91 phox and p22 phox , which together form cytochrome b558. Details of the molecular interactions within the complex have been investigated and reviewed extensively [1,11,12]. The essential role of the small GTPase Rac (Rac1 in macrophages, Rac2 in neutrophilic granulocytes) was discovered soon after the identification of the Nox-related subunits [13,14] and was confirmed later in both human pathology [15] and in genetically modified mice [16].…”

Les NADPH oxydases, Nox

“…Bien qu'ayant un certain nombre de points communs, les Nox des cellules non phagocytaires chez l'homme se distinguent de la NADPH oxydase des neutrophiles, tant au plan fonctionnel que structural [12,13]. Les Nox n'ont pas encore été cristallisées.…”