Expression, and Molecular and Enzymatic Characterization of Cu-Containing Nitrite Reductase from a Marine Ammonia-Oxidizing Gammaproteobacterium, Nitrosococcus oceani

Ammonia-oxidizing bacteria (AOB) remove intracellular nitrite to prevent its toxicity by a nitrifier denitrification pathway involving two denitrifying enzymes, nitrite reductase and nitric oxide reductase. Here, a Cu-containing nitrite reductase from Nitrosococcus oceani strain NS58, a gammaproteobacterial marine AOB, was expressed in Escherichia coli and purified to homogeneity. Sequence homology analysis indicated that the nitrite reductase from N. oceani was phylogenetically closer to its counterparts from denitrifying bacteria than that of the betaproteobacterium Nitrosomonas europaea. The recombinant enzyme was a homotrimer of a 32 kDa subunit molecule. The enzyme was green in the oxidized state with absorption peaks at 455 nm and 575 nm. EPR spectroscopy indicated the presence of type 2 Cu. Molecular activities and the affinity constant for the nitrite were determined to be 1.6×103 s−1 and 52 mM, respectively.