Biological Roles for the NOX Family NADPH Oxidases

Abstract

Linking the phagocyte defect in patients with chronic granulomatous disease (CGD) 2The abbreviations used are: CGD, chronic granulomatous disease; ROS, reactive oxygen species; NOX, NADPH oxidase. with the biochemical basis for oxygen consumption by stimulated neutrophils (1Baehner R.L. Nathan D.G. Science. 1967; 155: 835-836Crossref PubMed Scopus (257) Google Scholar) represented a seminal advance in understanding the molecular basis for a key component of innate immunity. In subsequent decades, the catalytic and regulatory elements of the “respiratory burst oxidase” were elucidated, tacitly assuming all along that the NADPH-dependent oxidase under study represented a system uniquely expressed in phagocytic cells and dedicated to generating relatively large amounts of reactive oxygen species (ROS) destined to destroy invading microbes. Development of more sensitive analytical systems for ROS detection revealed that some physiological and pathophysiological events in non-phagocytic cells were associated with ROS generation, although the subcellular source of oxidants remained uncertain. With the identification of mox1 in 1999 (2Suh Y.-A. Arnold R.S. Lassegue B. Shi J. Xu X. Sorescu D. Chung A.B. Griendling K.K. Lambeth J.D. Nature. 1999; 401: 79-82Crossref PubMed Scopus (1278) Google Scholar) as a homolog of gp91phox, the catalytic component of the phagocyte oxidase, came the birth of the NADPH oxidase (NOX) protein family and the eventual identification of its seven members. With remarkable rapidity, many features of the structure, activity, cell biology, and physiology of the NOX proteins have been described, as summarized in several excellent and comprehensive recent reviews (3Lambeth J.D. Kawahara T. Diebold B. Free Radic. Biol. Med. 2007; 43: 319-331Crossref PubMed Scopus (409) Google Scholar, 4Geiszt M. Cardiovasc. Res. 2006; 71: 289-299Crossref PubMed Scopus (179) Google Scholar, 5Bedard K. Krause K.-H. Physiol. Rev. 2007; 87: 245-313Crossref PubMed Scopus (4958) Google Scholar). This more circumscribed minireview provides an overview of the organizing features of the protein family, a summary of the physiology and pathophysiology in which NOX proteins participate (or might participate), and identification of some of the remaining unanswered questions in the field. The patriarch of the NOX protein family is gp91phox (NOX2), the heavy subunit of flavocytochrome b558, the catalytic component of the phagocyte NADPH oxidase (6Babior B.M. Curr. Opin. Immunol. 2004; 16: 42-47Crossref PubMed Scopus (655) Google Scholar). Associated with p22phox in plasma membrane and membranes of selected intracellular compartments, gp91phox operates as an electron transferase, shuttling electrons from NADPH in the phagocyte cytoplasm, across two nonequivalent hemes (7Cross A.R. Rae J. Curnutte J.T. J. Biol. Chem. 1995; 270: 17075-17077Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar) buried in the membrane, to O2, the electron acceptor, thereby generating superoxide anion. Substantial evidence suggests a stepwise flow of electrons during oxidase activity: a single electron transfers sequentially from cytosolic NADPH to NOX2-associated FAD, to the more proximal heme in the membrane (Em7 = –225 mV) (7Cross A.R. Rae J. Curnutte J.T. J. Biol. Chem. 1995; 270: 17075-17077Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar), to the distal heme (Em7 = –265 mV), and finally to molecular O2. The univalent nature of the reaction with O2 produces superoxide anion as the immediate product of the oxidase, with hydrogen peroxide subsequently generated by dismutation of the superoxide. As discussed below, some of the non-phagocyte NOX proteins display unanticipated and unexplained reactivity. Although the crystal structure of flavocytochrome b558 has not been solved, mutagenesis data suggest that the NOX2 subunit contains both hemes, where they are bishistidine-ligated in parallel transmembrane helices, coordinated with His101– His115 and His209–His222, respectively (8Biberstine-Kinkade K.J. DeLeo F.R. Epstein R.I. LeRoy B.A. Nauseef W.M. Dinauer M.C. J. Biol. Chem. 2001; 276: 31105-31112Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar). The currently accepted model resembles that suggested for FRE1, the iron reductase of Saccharomyces cerevisiae (9Finegold A.A. Shatwell K.P. Segal A.W. Klausner R.D. Dancis A. J. Biol. Chem. 1996; 271: 31021-31024Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar), with the paired hemes stacked between two α-helices, perpendicular to the plane of the membranes, and coordinated with histidines 12–13 amino acids apart in the linear sequence (10Taylor R.M. Baniulis D. Burritt J.B. Gripentrog J.M. Lord C.I. Riesselman M.H. Maaty W. Bothner B.P. Angel T.E. Dratz E.A. Linton G.F. Malech H.L. Jesaitis A.J. J. Biol. Chem. 2006; 281: 37045-37056Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar). Although gp91phox is the catalyt