The 1.2 Å structure of the human sulfite oxidase cytochrome b₅ domain

The molybdenum- and iron-containing enzyme sulfite oxidase catalyzes the physiologically vital oxidation of sulfite to sulfate. Sulfite oxidase contains three domains: an N-terminal cytochrome b₅ domain, a central domain harboring the molybdenum cofactor (Moco) and a C-terminal dimerization domain. Oxidation of the substrate sulfite is coupled to the transfer of two electrons to the molybdenum cofactor. Subsequently, these electrons are passed on, one at a time, to the b₅ heme of sulfite oxidase and from there to the soluble electron carrier cytochrome c. The crystal structure of the oxidized human sulfite oxidase cytochrome b₅ domain has been determined at 1.2 Å resolution and has been refined to a crystallographic R factor of 0.107 (R_free = 0.137). A comparison of this structure with other b₅-type cytochromes reveals distinct structural features present in the sulfite oxidase b₅ domain which promote optimal electron transport between the Moco of sulfite oxidase and the heme of cytochrome c.