Structure of the Membrane-intrinsic Nitric Oxide Reductase
from <i>Roseobacter denitrificans</i>
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Abstract
Membrane-intrinsic
nitric oxide reductases (NORs) are key components
of bacterial denitrification pathways with a close evolutionary relationship
to the cytochrome oxidase (COX) complex found in aerobic respiratory
chains. A key distinction between COX and NOR is the identity of the
metal directly opposite heme <i>b</i><sub>3</sub> within
the active site. In NOR, this metal is iron (Fe<sub>B</sub>), whereas
in COX, it is copper (Cu<sub>B</sub>). The purified NOR of <i>Roseobacter denitrificans</i> contains copper and has modest
oxidase activity, raising the possibility that a COX-like active site
might have independently arisen within the context of a NOR-like protein
scaffold. Here we present the crystal structure of the <i>Roseobacter
denitrificans</i> NorBC complex and anomalous scattering experiments
probing the identity of each metal center. Our results refute the
hypothesis that copper occupies the active site and instead reveal
a new metal center in the small subunit not seen in any other NOR
or COX