The superoxide-generating NADPH oxidase: structural aspects and activation mechanism

Flavocytochrome b(558) is the catalytic core of the respiratory-burst oxidase, an enzyme complex that catalyzes the NADPH-dependent reduction Of O-2 into the superoxide anion O-2 in phagocytic cells. Flavocytochrome b(558) is anchored in the plasma membrane. It is a heterodimer that consists of a large glycoprotein gp91phox (phox for phagocyte oxidase) (P subunit) and a small protein p22phox (a subunit). The other components of the respiratory-burst oxidase are water-soluble proteins of cytosolic origin, namely p67phox, p47phox, p40phox and Rac. Upon cell stimulation, they assemble with the membrane-bound flavocytochrome b(558) which becomes activated and generates O-2. A defect in any of the genes encoding gp91phox, p22phox, p67phox or p47phox results in chronic granulornatous disease, a genetic disorder characterized by severe and recurrent infections, illustrating the role of 02 and the derived metabolites H2O2 and HOCl in host defense against invading microorganisms. The electron carriers, FAD and hemes b, and the binding site for NADPH are confined to the gp91phox subunit of flavocytochrome b(558). The p22phox subunit serves as a docking site for the cytosolic phox proteins. This review provides an overview of current knowledge on the structural organization of the O-2(-)-generating flavocytochrome b(558) its kinetics, its mechanism of activation and the regulation of its biosynthesis. Homologues of gp91phox, called Nox and Duox, are present in a large variety of non-phagocytic cells. They exhibit modest O-2(-)-generating oxidase activity, and some act as proton channels. Their role in various aspects of signal transduction is currently under investigation and is briefly discussed.