Insight into Catalysis of Nitrous Oxide Reductase from High-resolution Structures of Resting and Inhibitor-bound Enzyme from Achromobacter cycloclastes.
The difficult chemistry of nitrous oxide (N(2)O) reduction to gaseous nitrogen (N(2)) in biology is catalysed by the novel mu(4)-sulphide-bridged tetranuclear Cu(z) cluster of the N(2)O reductases (N(2)OR). Two spectroscopically distinct forms of this cluster have been identified as Cu(Z) and Cu(Z)*. We have obtained a 1.86 A resolution crystal structure of the pink-purple species of N(2)OR from Achromobacter cycloclastes (AcN(2)OR) isolated under aerobic conditions. This structure reveals a previously unobserved ligation with two oxygen atoms from H(2)O/OH(-) coordinated to Cu1 and Cu4 of the catalytic centre. We ascribe this structure to be that of the Cu(Z) form of the cluster, since the previously reported structures of two blue species of N(2)ORs, also isolated aerobically, have characterised the redox inactive Cu(Z)* form, revealing a single water molecule at Cu4. Exposure of the as-isolated AcN(2)OR to sodium iodide led to reduction of the electron-donating Cu(A) site and the formation of a blue species. Structure determination of this adduct at 1.7 A resolution showed that iodide was bound at the Cu(Z) site bridging the Cu1 and Cu4 ions. This structure represents the first observation of an inhibitor bound to the Cu1-Cu4 edge of the catalytic cluster, providing clear evidence for this being the catalytic edge in N(2)ORs. These structures, together with the published structural and spectroscopic data, give fresh insight into the mode of substrate binding, reduction and catalysis.[1]References
- Insight into Catalysis of Nitrous Oxide Reductase from High-resolution Structures of Resting and Inhibitor-bound Enzyme from Achromobacter cycloclastes. Paraskevopoulos, K., Antonyuk, S.V., Sawers, R.G., Eady, R.R., Hasnain, S.S. J. Mol. Biol. (2006) [Pubmed]
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