The biogeochemical cycle of nitrogen is the one with the highest impact in anthropogenic activities, such as food and energy production. The overload of soils with N-fertilizers is having repercussions on both local and global nitrogen cycle, especially at the level of the microbial denitrification, a key pathway that accounts for the dissimilatory transformation of nitrate and nitrite into dinitrogen. Globally, the process relies on four reactions catalyzed by different metalloenzymes that sequentially convert nitrate to dinitrogen (with nitrite, nitric oxide and nitrous oxide as intermediates). The impact of agricultural practices in the environment is also enormous. The surplus of N-fertilizers in the fields contaminates land, surface and ground waters, causing a cascade of environmental and human health problems. In addition, although the majority of denitrified-N becomes N2 (harmless), a small portion is lost as chemically reactive nitric oxide or nitrous oxide (a greenhouse effect gas). NO is involved in several key physiological processes in mammals (including humans) and plants and is presently used together with NO releasers, in different therapeutic strategies. N2O is a potent greenhouse gas and ozone-depleting agent. Due to the recognized biological and environmental value of the denitrification pathway, in the last years there have been efforts to understand the structure of the participating enzymes and their catalytic mechanisms. In this regard, this project will address the structure/function relationship of the denitrifying enzymes that deal with NO and N2O reduction, using a toolbox of spectroscopic, kinetic, electrochemical and structural techniques, complemented with a transcriptomic approach.