Abstract
Nitric oxide is an important molecule in all domains of life with significant biological functions in both pro- and eukaryotes. Anaerobic ammonium-oxidizing (anammox) bacteria that contribute substantially to the release of fixed nitrogen into the atmosphere use the oxidizing power of NO to activate inert ammonium into hydrazine (N2H4). Here, we describe an enzyme from the anammox bacterium Kuenenia stuttgartiensis that uses a novel pathway to make NO from hydroxylamine. This new enzyme is related to octaheme hydroxylamine oxidoreductase, a key protein in aerobic ammonium-oxidizing bacteria. By a multiphasic approach including the determination of the crystal structure of the K. stuttgartiensis enzyme at 1.8 Å resolution and refinement and reassessment of the hydroxylamine oxidoreductase structure from Nitrosomonas europaea, both in the presence and absence of their substrates, we propose a model for NO formation by the K. stuttgartiensis enzyme. Our results expand the understanding of the functions that the widespread family of octaheme proteins have.
Highlights
Multiheme proteins have crucial roles in diverse nitrogen cycle processes
By a multiphasic approach including the determination of the crystal structure of the K. stuttgartiensis enzyme at 1.8 Aresolution and refinement and reassessment of the hydroxylamine oxidoreductase structure from Nitrosomonas europaea, both in the presence and absence of their substrates, we propose a model for NO formation by the K. stuttgartiensis enzyme
We describe an enzyme previously [8] identified as kustc1061 from the anammox bacterium Kuenenia stuttgartiensis that makes NO from hydroxylamine (NH2OH) (Equation 1)
Summary
Multiheme proteins have crucial roles in diverse nitrogen cycle processes. Results: The kustc1061 octaheme protein from anaerobic ammonium-oxidizing (anammox) bacteria oxidizes hydroxylamine to NO. In aerobic ammonium-oxidizing bacteria, HAO catalyzes the oxidation of hydroxylamine to nitrite (NO2Ϫ), the second step in the aerobic oxidation of ammonium (Equation 2) Several properties of this enzyme have been elucidated including the determination of a crystal structure of the Nitrosomonas europaea protein (NeHAO) (9 –13). Anammox bacteria, such as K. stuttgartiensis, which were shown to contribute substantially to nitrogen release into the atmosphere [14], harbor no less than. The presence of the tyrosine-dependent covalent linkage between different subunits seems to be a distinctive feature in governing reactions into an oxidative direction [17] These diverse catalytic potentials could very well make HAOlike proteins central but far overlooked catalysts in biogeochemical nitrogen cycle processes. The results of this study shed light on determinants in a widespread group of homologs about which very little is known at the biochemical level
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
