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Resolution of Key Roles for the Distal Pocket Histidine in Cytochrome

DOI: 10.1021/ja512941j DOI Help
PMID: 25658043 PMID Help

Authors: Colin Lockwood (University of East Anglia) , Bénédicte Burlat (University of East Anglia) , Myles R. Cheesman (University of East Anglia) , Melanie Kern (Technische Universität) , Jörg Simon (Technische Universität) , Tom Clarke (University of East Anglia) , David J. Richardson (University of East Anglia) , Julea N. Butt (University of East Anglia)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society , VOL 137 (8) , PAGES 3059 - 3068

State: Published (Approved)
Published: March 2015
Diamond Proposal Number(s): 7641

Open Access Open Access

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 multiheme 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 behavior is likely to be displayed by other multicentered redox enzymes such that there are wide implications for considering the determinants of catalytic activity in this important and varied group of oxidoreductases.

Subject Areas: Biology and Bio-materials, Chemistry

Instruments: I24-Microfocus Macromolecular Crystallography

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