A Purple Cupredoxin from Nitrosopumilus maritimus Containing a Mononuclear Type 1 Copper Center with an Open Binding Site

Mononuclear cupredoxin proteins usually contain a coordinately saturated type 1 copper (T1Cu) center and function exclusively as electron carriers. Here we report a cupredoxin isolated from the nitrifying archaeon Nitrosopumilus maritimus SCM1, called Nmar1307, that contains a T1Cu center with an open binding site containing water. It displays a deep purple color due to strong absorptions around 413 nm (1880 M–1 cm–1) and 558 nm (2290 M–1 cm–1) in the UV–vis electronic spectrum. EPR studies suggest the protein contains two Cu(II) species of nearly equal population, one nearly axial, with hyperfine constant A∥ = 98 × 10–4 cm–1, and another more rhombic, with a smaller A∥ value of 69 × 10–4 cm–1. The X-ray crystal structure at 1.6 A resolution confirms that it contains a Cu atom coordinated by two His and one Cys in a trigonal plane, with an axial H2O at 2.25 A. Both UV–vis absorption and EPR spectroscopic studies suggest that the Nmar1307 can oxidize NO to nitrite, an activity that is attributable to the h...