Characterization of a novel Cu-containing dissimilatory nitrite reductase from the haloarchaeon Halorussus sp. YCN54.

Dissimilatory nitrite reductase, a key enzyme in the denitrification pathway, catalyzes the reduction of nitrite to NO. Bioinformatic analysis showed that the genome of a novel nitrite-degrading haloarchaeon Halorussus sp. YCN54 possessed a gene encoding the Cu-containing dissimilatory nitrite reductase (NirK). NirK was heterologously expressed and purified. Protein sequencing indicated that two isoforms of NirK monomer were produced intracellularly. UV-vis spectrum of the purified NirK showed that it belonged to the blue NirK group. NirK showed optimum activity at 4.5 M NaCl, 55 ℃ and pH 7.0, representing a halophilic, slightly thermophilic and neutral enzyme. It exhibited high stability at 30-50 ℃. NirK activity was strongly inhibited by the copper chelating agent due to removal of copper. NirK activity was activated by Mn and Sr. It displayed good tolerance to some high polarity organic solvents and nonionic surfactants, such as glycerol, DMSO, DMF and tween-20. NaSO was an effective electron donor to NirK. The K and V values of purified NirK for nitrite were 3.2 mM and 477.2 U/mg, respectively, indicating its high activity. These results indicated that NirK may have potential applications for nitrite degradation in high-salt industries, such as salted food and saline wastewater treatment.