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Figure 2
Structural organization of AxNiR. (a) The AxNiR trimer is viewed from the top down. The T1Cu is shown on the surface of the molecule, where it accepts electrons from carrier proteins. The T2Cu ion can be seen at the monomer–monomer interface. (b) Details of the SRX T1Cu and T2Cu sites. Electrons donated to the T1Cu are transferred along a Cys130–His129 bridge to the T2Cu. The catalytic Asp92 and His249 residues are positioned next to the substrate as the proton supply. (c) Resting state of the AxNiR T2Cu site (SRX). The T2Cu is ligated by three histidine residues and a water ligand Wat1, which is hydrogen-bonded to two catalytic resides Asp92CAT and His249CAT, which in turn are linked to water Wat2. Wat1 is always observed in all resting-state CuNiR structures obtained at synchrotron facilities. 2Fo − Fc electron density is contoured at the 2σ level and shown as a grey mesh. Atoms are coloured by element, with a different colour scheme used for the different chains. T1Cu is shown as a dark blue sphere, T2Cu as a cyan sphere and water molecules as small red spheres. Metal-coordinating bonds are shown as red dotted lines. Selected hydrogen bonds are shown as black dotted lines.

IUCrJ
Volume 5| Part 1| January 2018| Pages 22-31
ISSN: 2052-2525
https://doi.org/10.1107/S2052252517016128