No Laughing Matter: The Unmaking of the Greenhouse Gas Dinitrogen Monoxide by Nitrous Oxide Reductase - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 188

Figure 6 The electron transfer site Cu A of N 2 O reductase. Stereo representations of the two
conformations observed in structures of N 2 OR. (a) In purple N 2 OR from P. stutzeri , as isolated,
residue His583 was in most cases not a ligand to Cu 1 of the Cu A site resulting in a shift of the
copper ion into the plane formed by the two cysteine residues, Cys618 and Cys622, and Met629
(PDB ID 3SBQ). The experimental data is consistent with a sharpening of the EPR hyperfine
structure in this conformation. (b) In all instances with substrate bound to Cu Z , the side chain
of His583 was flipped back to coordinate Cu 1 of the Cu A center, linking the metal cluster to
the protein surface via residue Asp576 to allow for electron transfer from an external donor
(PDB ID 3SBR).

The two Cu ions of Cu A show a distorted tetrahedral coordination environment
that represents the classical 'entatic' state between a square-planar geometry with
hard ligands (preferred by Cu(II)) and a tetrahedral geometry with soft ligands
(preferred by Cu(I)) [ 63 ]. In Cu A ,

the two cysteine ligands are shared in a

2 -bridging fashion, and each ion in addition has a histidine ligand (Figure 6 ).
The coordination environment is complemented in each case by a further ligand
that differs between the two metals. For Cu 1 , this ligand is the oxygen atom of a
backbone carbonyl group, while for Cu 2 the first coordination sphere is completed

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