Nitrite reductase, copper-type <p>These enzymes are involved in nitrate assimilation in the denitrification pathway. The 3D structure of the copper-containing nitrite reductase (NIR) from <taxon tax_id="223">Achromobacter cycloclastes</taxon> has been determined to 2.3A resolution [<cite idref="PUB00005146"/>]. The enzyme is a trimer, each monomer of which contains two Greek key beta-barrel domains (similar to that of plastocyanin) and houses two copper sites. The two copper atoms in the monomer comprise a type I copper site (Cu-I: two His, one Cys and one Met ligand) that plays a crucial role for electron transfer from pseudoazurin to the type II copper site (Cu-II: three His and one solvent ligand), the catalytic centre of NIR for the reduction ofnitrite. The Cu-II site lies at the bottom of a 12A deep solvent channel and is the site to which the substrate (NO2-) binds [<cite idref="PUB00005146"/>].</p>