Resolution of Key Roles for the Distal Pocket Histidine in Cytochrome c Nitrite Reductases

Lockwood, Colin, Burlat, Benedicte, Cheesman, Myles, Kern, Melanie, Simon, Jorg, Clarke, Tom, Richardson, David and Butt, Julea (2015) Resolution of Key Roles for the Distal Pocket Histidine in Cytochrome c Nitrite Reductases. Journal of the American Chemical Society, 137 (8). 3059–3068. ISSN 0002-7863

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Abstract

Cytochrome c nitrite reductases perform a key step in the biogeochemical N-cycle by catalyzing the six-electron reduction of nitrite to ammonium. These multi-heme cytochromes contain a number of His/His ligated c-hemes for electron transfer and a structurally differentiated heme that provides the catalytic center. The catalytic heme has proximal ligation from lysine, or histidine, and an exchangeable distal ligand bound within a pocket that includes a conserved histidine. Here we describe properties of a penta-heme cytochrome c nitrite reductase in which the distal His has been substituted by Asn. The variant is unable to catalyze nitrite reduction despite retaining the ability to reduce a proposed intermediate in that process, namely, hydroxylamine. A combination of electrochemical, structural and spectroscopic studies reveals that the variant enzyme simultaneously binds nitrite and electrons at the catalytic heme. As a consequence the distal His is proposed to play a key role in orienting the nitrite for N-O bond cleavage. The electrochemical experiments also reveal that the distal His facilitates rapid nitrite binding to the catalytic heme of the native enzyme. Finally it is noted that the thermodynamic descriptions of nitrite- and electron-binding to the active site of the variant enzyme are modulated by the prevailing oxidation states of the His/His ligated hemes. This is behavior that is likely to be displayed by other multi-centered redox enzymes such that there are wide implications for considering the determinants of catalytic activity in this important and varied group of oxidoreductases.

Item Type: Article
Additional Information: ACS AuthorChoice - This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.
Faculty \ School: Faculty of Science > School of Chemistry
Faculty of Science > School of Biological Sciences

Depositing User: Pure Connector
Date Deposited: 16 Feb 2015 12:16
Last Modified: 22 Jul 2020 00:10
URI: https://ueaeprints.uea.ac.uk/id/eprint/52229
DOI: 10.1021/ja512941j

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