Nitrite reduction in bacteria: A comprehensive view of nitrite reductases

The last years have witnessed a steady increase of social and political awareness for the need of studying, monitoring, and controlling several anthropological activities that are dramatically impacting the environment and human health. The increasing turnover rates of the nitrogen cycle across the Planet are of major concern, so the understanding of the biological, chemical, and physical processes associated with the biogeochemical nitrogen cycle has been attracting the attention of several scientific disciplines. For many years, the primary focus has been the so-called “dissimilatory reduction of nitrate”, which refers to the stepwise conversion of nitrate into molecular nitrogen, closely followed by the assimilatory nitrate reduction pathway, which allow nitrogen incorporation into biomolecules. The contribution of bioinorganic chemists to better understand the enzymology underlying these two branches of the N-cycle has been remarkable. The constant development of mechanistic, structural, and biological tools has been keeping this bioinorganic chemistry field very active, making it a highly relevant research area still today. In this paper, we review the state-of-the-art in both dissimilatory and assimilatory nitrite reducing enzymes, highlighting the structural peculiarities of the different metalloenzymes involved in this step.