Shewanella oneidensis Cytochrome c Nitrite Reductase (ccNiR) Does Not Disproportionate Hydroxylamine to Ammonia and Nitrite, Despite a Strongly Favorable Driving Force

Authors

Matthew Youngblut, University of Wisconsin - Milwaukee
Daniel J. Pauly, University of Wisconsin - Milwaukee
Natalia Stein, University of Wisconsin - MilwaukeeFollow
Daniel Walters, University of Wisconsin - Milwaukee
John A. Conrad, University of Wisconsin - Milwaukee
Graham R. Moran, University of Wisconsin - Milwaukee
Brian Bennett, Marquette UniversityFollow
A. Andrew Pacheco, University of Wisconsin - Milwaukee

Document Type

Article

Language

eng

Format of Original

9 p.

Publication Date

4-2014

Publisher

American Chemical Society

Source Publication

Biochemistry

Source ISSN

0006-2960

Original Item ID

doi: 10.1021/bi401705d

Abstract

Cytochrome c nitrite reductase (ccNiR) from Shewanella oneidensis, which catalyzes the six-electron reduction of nitrite to ammonia in vivo, was shown to oxidize hydroxylamine in the presence of large quantities of this substrate, yielding nitrite as the sole free nitrogenous product. UV–visible stopped-flow and rapid-freeze-quench electron paramagnetic resonance data, along with product analysis, showed that the equilibrium between hydroxylamine and nitrite is fairly rapidly established in the presence of high initial concentrations of hydroxylamine, despite said equilibrium lying far to the left. By contrast, reduction of hydroxylamine to ammonia did not occur, even though disproportionation of hydroxylamine to yield both nitrite and ammonia is strongly thermodynamically favored. This suggests a kinetic barrier to the ccNiR-catalyzed reduction of hydroxylamine to ammonia. A mechanism for hydroxylamine reduction is proposed in which the hydroxide group is first protonated and released as water, leaving what is formally an NH₂⁺ moiety bound at the heme active site. This species could be a metastable intermediate or a transition state but in either case would exist only if it were stabilized by the donation of electrons from the ccNiR heme pool into the empty nitrogen p orbital. In this scenario, ccNiR does not catalyze disproportionation because the electron-donating hydroxylamine does not poise the enzyme at a sufficiently low potential to stabilize the putative dehydrated hydroxylamine; presumably, a stronger reductant is required for this.

Comments

Accepted version. Biochemistry, Vol. 53, No. 13 (April 2014): 2136-2144. DOI. © 2014 American Chemical Society Publications. Used with permission.

Brian Bennett was affiliated with Medical College of Wisconsin at the time of publication.

Recommended Citation

Youngblut, Matthew; Pauly, Daniel J.; Stein, Natalia; Walters, Daniel; Conrad, John A.; Moran, Graham R.; Bennett, Brian; and Pacheco, A. Andrew, "Shewanella oneidensis Cytochrome c Nitrite Reductase (ccNiR) Does Not Disproportionate Hydroxylamine to Ammonia and Nitrite, Despite a Strongly Favorable Driving Force" (2014). Physics Faculty Research and Publications. 7.
https://epublications.marquette.edu/physics_fac/7