The Nox Family of NADPH-Oxidases


In 1997, we became interested in the observation that reactive oxygen is generated in a variety of tissues and cell types in addition to inflammatory cells. Interestingly, this included cancer cells, and several lines of evidence pointed to a role for reactive oxygen in cell growth. Although the ROS generation was generally thought to be a byproduct of mitochondrial respiration, inhibitor specificity pointed to an enzyme system similar to that of the phagocyte respiratory burst oxidase rather than mitochondria. Based upon homology with gp91phox the catalytic subunit of the phagocyte oxidase, we cloned the first new member of the Nox family, Nox1 (originally called Mox1 or p65Mox) in 1998. The family now consists of 7 members in human, with orthologs in mouse, rat, Drosophila and C. elegans. A family tree constructed by comparing the sequences of the gp91phox-homology domain reveals three subfamilies: a gp91phox-like group, a Duox group, and a more distant homolog consisting of a single member, Nox5. Size comparisons reveal that the gp91phox group and a small splice form of Nox 5 are similar in size to gp91phox, approximately 65 kDa protein. Duox enzymes and a large splice variant of Nox5 are larger and contain additional predicted domains. Sequence alignment of the gp91phox regions demonstrates the presence of an N-terminal hydrophobic region comprising about half of the molecule, and a C-terminal flavoprotein domain, consistent of predicted binding sites for FAD (yellow) and for NADPH (green), the latter containing the canonical GXGXXP nucleotide binding motif. The N-terminus consists of 6 hydrophobic regions that are apparent in the hydropathy plots of these proteins. These regions are predicted to be transmembrane alpha helices, looping back and forth across the membrane 6 times. This hydrophobic region contains five absolutely conserved histidines (indicated by dots), four of which are predicted to bind to the two heme irons.


Nox1 and perhaps other members of the Nox family function to regulate cell Growth and angiogenesis by producing reactive oxygen species that function as intracellular signal molecules. Specifically, hydrogen peroxide functions in this manner, regulating the expression of a number of growth-regulating genes.

 

 

 

The family now consists of 7 members in human, with orthologs in mouse, rat, Drosophila and C. elegans. A family tree constructed by comparing the sequences of the gp91phox-homology domain reveals three subfamilies: a gp91phox-like group, a Duox group, and a more distant homolog consisting of a single member, Nox5

 

 

 

 

Sequence alignment of the gp91phox regions demonstrates the presence of an N-terminal hydrophobic region comprising about half of the molecule, and a C-terminal flavoprotein domain, consistent of predicted binding sites for FAD (yellow) and for NADPH (green), the latter containing the canonical GXGXXP nucleotide binding motif. The N-terminus consists of 6 hydrophobic regions that are apparent in the hydropathy plots of these proteins. These regions are predicted to be transmembrane alpha helices, looping back and forth across the membrane 6 times. This hydrophobic region contains five absolutely conserved histidines (indicated by dots), four of which are predicted to bind to the two heme irons.

 

 

 

 

 

The N-terminus consists of 6 hydrophobic regions that are apparent in the hydropathy plots of these proteins. These regions are predicted to be transmembrane alpha helices, looping back and forth across the membrane 6 times.