Cloning, purification and characterization of novel Cu-containing nitrite reductase from the Bacillus firmus GY-49.

Nitrite is generated from the nitrogen cycle and its accumulation is harmful to environment and it can be reduced to nitric oxid by nitrite reductase. A novel gene from Bacillus firmus GY-49 is identified as a nirK gene encoding Cu-containing nitrite reductase by genome sequence. The full-length protein included a putative signal peptide of 26 amino acids and shown 72.73% similarity with other Cu-containing nitrite reductase whose function was verified. The 993-bp fragment encoding the mature peptide of NirK was cloned into pET-28a (+) vector and overexpressed as an active protein of 36.41 kDa in the E.coli system. The purified enzyme was green in the oxidized state and displayed double gentle peaks at 456 and 608 nm. The specific activity of purified enzyme was 98.4 U/mg toward sodium nitrite around pH 6.5 and 35 °C. The K m and K cat of NirK on sodium nitrite were 0.27 mM and 0.36 × 103 s-1, respectively. Finally, homology model analysis of NirK indicated that the enzyme was a homotrimer structure and well conserved in Cu-binding sites for enzymatic functions. This is a first report for nitrite reductase from Bacillus firmus, which augment the acquaintance of nitrite reductase.