Trends in Biochemical Sciences
FrontlinesNovel homologs of gp91phox
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New homologs of gp91phox
The cDNAs for five new human homologs of gp91phox have been cloned, and additional homologs have been identified in rodents, C. elegans and Drosophila (Table 1). cDNA or genomic sequences, or both, for each are available in GenBank. The region in each of these proteins that is homologous to gp91phox has been aligned (http://www.biochem.emory.edu/Lambeth/gp91_homology.pdf). Evolutionary relationships based on this alignment (Fig. 1) define three groups: a gp91phox-like group (Nox1, 2 and 3),
Sequence comparisons with gp91phox
Homologous domains in the Nox and Duox groups are shown in Fig. 2a. For the Nox group proteins, the entire molecule is homologous to gp91phox. The C-terminal half of the gp91phox homology region is homologous to known flavoproteins. Highly conserved subregions generally correspond to FAD- and NADPH-binding sites (http://www.biochem.emory.edu/Lambeth/gp91_homology.pdf). Residues 410–415 in gp91phox match the canonical nucleotide-binding sequence Gly-Xaa-Gly-Xaa-Xaa-Pro. In flavoproteins that
High-molecular-weight homologs of gp91phox
Large homologs of gp91phox have been identified in human, C. elegans and Drosophila (see Table 1). These encode a C-terminal region that is homologous to gp91phox (Fig. 2a), except that they contain one additional transmembrane α helix in the transmembrane cluster. A central domain contains two regions that are predicted to be EF-hands, a motif that contains a calcium-binding site, as in calmodulin. An N-terminal region is homologous to peroxidases, including myeloperoxidase. Hence, we term
Topology model for Nox and Duox
In the topology model for Nox1–4 (Fig. 2b, left) the two hemes are bound within the transmembrane cluster, and the flavoprotein domain is inside the cell. The physical arrangement of the prosthetic groups predicts that the electron flow is as follows: cytosolic NADPH → FAD → heme1 → heme2 → oxygen, forming a superoxide transmembrane to the cytosolic flavoprotein domain. In the case of the phagocyte NADPH-oxidase, ROS are produced outside the cell or in the phagosome. The membrane location of
Tissue expression of homologs of gp91phox
In contrast to gp91phox, the Nox and Duox mRNAs that have been examined are not expressed in inflammatory cells. Instead, each shows a distinct tissue expression pattern: for example, the mRNA for Nox1 is highly expressed in colon epithelial cells with lower levels in the prostate, uterus and vascular smooth muscle cells, where it is induced by PDGF (Ref. 14). p138Tox is highly expressed in thyroid cells15.
Functions of Nox1
Nox1 functions in mitogenic regulation and cell transformation14. In vascular smooth muscle, decreased expression of endogenous Nox1 caused by antisense DNA correlated with decreased ROS level and decreased growth, indicating a role for Nox1 in normal growth. Surprisingly, fibroblasts overexpressing Nox1 showed increased mitogenic rates and a transformed appearance. These cells were highly tumorigenic in athymic mice, producing sizable tumors within three weeks. Although it is not currently
Function of Duox enzymes
p138Tox has been identified as the NADPH-oxidase that supports thyroid hormone biosynthesis15. The synthesis of thyroid hormone involves iodination of thyroglobulin, a reaction that is catalysed by thyroid peroxidase and an H2O2-generating NADPH-oxidase. The oxidase was purified based on its NADPH-dependent diaphorase activity, and peptide microsequencing provided the key to cloning p138Tox. The presence of EF-hand motifs could account for the calcium dependency of the thyroid NADPH-oxidase.
Concluding remarks
The existence of homologs of gp91phox in noninflammatory tissues implies that the generation of reactive oxygen in these tissues is not an accident of respiration but is a deliberate biological strategy. The recent cloning of multiple homologs of gp91phox, as well as intriguing recent studies regarding the biochemical and cellular functions of some of these homologs, provide a starting point for understanding the biological relevance of this new group of enzymes.
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