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 193

Figure 7 Cu Z , the catalytic center of N 2 O reductase. (a) The tetranuclear Cu Z site is located at the hub of the N-terminal ʲ -propeller domain. It constitutes a
unique [4Cu:2S] cluster coordinated to seven histidine residues. (b) The spectroscopically and functionally distinct forms Cu Z (above) and Cu Z * (below) differ
in the presence of the second sulfur atom in the cluster. While Cu Z undergoes a redox transition from a [2Cu + :2Cu 2+ ] state to a [3Cu + :1Cu 2+ ] and cannot be
further reduced, Cu Z * is isolated as [3Cu + :1Cu 2+ ] and can be further reduced to a [4Cu + ] state by sodium dithionite and methyl viologen. (c) The variation of
Cu Z structures from different crystal structures of nitrous oxide reductases. The first structures (PDB ID 1QNI, 1FWX) showed mixtures of Cu Z and Cu Z *,
where a partially occupied sulfur atom between Cu 1 and Cu 4 was interpreted as a bridging water or hydroxo species. A form II structure (PDB ID 2IWF) then
had two water ligands at the Cu 1 -Cu 4 edge, but would bind the inhibitor iodide in a bridging fashion (PDB ID 1IWK). Only in the purple form I structure (PDB
3SBR), Cu Z was in the complete [4Cu:2S] state.