NOReduktase

NOreduktase, also known as nitric oxide reductase (NOR), is an enzyme that catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N2O) in certain bacteria and archaea. It plays a role in denitrification, a respiratory process that enables organisms to generate energy when oxygen is scarce by reducing nitrogen oxides to form nitrogen gas or nitrogenous gases. NOR enzymes help detoxify NO, a reactive and potentially damaging molecule produced during aerobic metabolism or in response to nitrosative stress.

There are two main NOR families. Cytochrome c-dependent NORs (cNORs) are membrane-associated enzymes that use heme cofactors to transfer electrons from the respiratory chain to NO. Flavorubredoxin-type NORs (NorV/NorW) couple a flavorubredoxin to a reductase partner and use flavin- and iron-sulfur-containing cofactors to deliver electrons to NO. In some organisms, NOR activity is integrated with other denitrification enzymes in operons that encode the full nitrate-to-nitrogen-gas pathway.

Regulation and distribution vary by species. NOR expression is typically induced under low-oxygen conditions or in response to NO stress. In model organisms such as Escherichia coli, NOR activity is part of a nitrosative-stress response and is controlled by NO-responsive regulators; in denitrifying bacteria, NOR is coordinated with other denitrification enzymes (nir, nirK/nirS, nos) to complete the reduction sequence. Ecologically, NOR influences the fate of NO in soils, sediments, and aquatic environments and contributes to the production of nitrous oxide, a greenhouse gas.

Overall, NOreduktase represents a key enzyme in microbial nitrogen cycling and nitrosative stress management.