Nitrous oxide production via enzymatic nitroxyl from the nitrifying archaeon Nitrosopumilus maritimus.

Ammonia oxidizing archaea (AOA) are among the most abundant microorganisms on earth and are known to be a major source of nitrous oxide (N2O) emissions, although biochemical origins of this N2O remain unknown. Enzymological details of AOA nitrogen metabolism are broadly unavailable. We report the recombinant expression, purification, and characterization of a multicopper oxidase, Nmar_1354, from the AOA Nitrosopumilus maritimus. We show that Nmar_1354 selectively produces nitroxyl (HNO) by coupling the oxidation of the obligate nitrification intermediate hydroxylamine (NH2OH) to dioxygen (O2) reduction. This HNO undergoes several downstream reactions, although the major fates are production of N2 via reaction with NH2OH and dimerization with itself to yield N2O. These results afford one plausible enzymatic origin for N2O release by AOA. Moreover, these results reveal a physiologically relevant enzymatic reaction for producing HNO, an enigmatic nitrogen oxide speculated to be operative in cellular signaling and in energy transduction.