Diversity and evolution of nitric oxide reduction

Abstract

Nitrogen is an essential element for life, with the availability of fixed nitrogen limiting productivity in many ecosystems. The return of oxidized nitrogen species to the atmospheric N₂ pool is predominately catalyzed by microbial denitrification (NO₃⁻ → NO₂⁻ → NO → N₂O → N₂). Incomplete denitrification can produce N2O as a terminal product, leading to an increase in atmospheric N₂O, a potent greenhouse and ozone depleting gas. The production of N₂O is catalyzed by nitric oxide reductase (NOR) members of the heme-copper oxidoreductase (HCO) superfamily. Here we propose that a number of uncharacterized HCO families perform nitric oxide reduction and demonstrate that an enzyme from Rhodothermus marinus, belonging to one of these families does perform nitric oxide reduction. These families have novel active-site structures and several have conserved proton channels, suggesting that they might be able to couple nitric oxide reduction to energy conservation. They also exhibit broad phylogenetic and environmental distributions, expanding the diversity of microbes that can perform denitrification. Phylogenetic analyses of the HCO superfamily demonstrate that nitric oxide reductases evolved multiple times independently from oxygen reductases, suggesting that complete denitrification evolved after aerobic respiration.