Nitric Oxide Production from Nitrite Reduction and Hydroxylamine Oxidation by Copper-containing Dissimilatory Nitrite Reductase (NirK) from the Aerobic Ammonia-oxidizing Archaeon, <i>Nitrososphaera viennensis</i>

Abstract

Aerobic ammonia-oxidizing archaea (AOA) play a crucial role in the global nitrogen cycle by oxidizing ammonia to nitrite, and nitric oxide (NO) is a key intermediate in AOA for sustaining aerobic ammonia oxidation activity. We herein heterologously expressed the NO-forming, copper-containing, dissimilatory nitrite reductase (NirK) from Nitrososphaera viennensis and investigated its enzymatic properties. The recombinant protein catalyzed the reduction of ¹⁵NO₂^- to ¹⁵NO, the oxidation of hydroxylamine (¹⁵NH₂OH) to ¹⁵NO, and the production of ^14-15N₂O from ¹⁵NH₂OH and ¹⁴NO₂^-. To the best of our knowledge, the present study is the first to document the enzymatic properties of AOA NirK.