Phagocytic cytochrome b558 is a membrane-bound heme-containing enzyme that catalyzes one-electron reduction of O
2 to produce superoxide anion. This cytochrome b has a low-spin 6-coordinate heme iron, and its catalytic activity is not inhibited by respiratory inhibitors such as CO. In this paper, the recent progress is reviewed focusing on the structure and the function of the cytochrome. Our spectroscopic and kinetic data suggest that cytochrome b558 catalyzes the O
2- production without ligation of O
2 to the heme iron during the catalytic cycle. The mechanisms of oxygen reduction and regulation of the electron transfer reactions are discussed.
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