Peroxidase activity of octaheme nitrite reductases from bacteria of the <Emphasis Type="Italic">Thioalkalivibrio</Emphasis> genus

Closely related penta- and octaheme nitrite reductases catalyze the reduction of nitrite, nitric oxide, and hydroxylamine to ammonium and of sulfite to sulfide. NrfA pentaheme nitrite reductase plays the key role in anaerobic nitrate respiration and the protection of bacterial cells from stresses caused by nitrogen oxides and hydrogen peroxide. Octaheme nitrite reductases from bacteria of the Thioalkalivibrio genus are less studied, and their function in the cell is unknown. In order to estimate the possible role of octaheme nitrite reductases in the cell resistance to oxidative stress, the peroxidase activity of the enzyme from T. nitratireducens (TvNiR) has been studied in detail. Comparative analysis of the active site structure of TvNiR and cytochrome c peroxidases has shown some common features, such as a five-coordinated catalytic heme and identical catalytic residues in active sites. A model of the possible productive binding of peroxide at the active site of TvNiR has been proposed. The peroxidase activity has been measured for TvNiR hexamers and trimers under different conditions (pH, buffers, the addition of CaCl₂ and EDTA). The maximum peroxidase activity of TvNiR with ABTS as a substrate (k _cat = 17 s^–1; k _cat/K _m = 855 mM^–1 s^–1) has been 100–300 times lower than the activity of natural peroxidases. The different activities of TvNiR trimers and hexamers indicate that the rate-limiting stage of the reaction is not the catalytic event at the active site but the electron transfer along the heme c electron-transport chain.