Biological N2O Reduction

The copper enzyme N2O reductase (N2OR) catalyses the final step in the process of bacterial denitrification (vide supra). The two-electron reduction of N2O to dinitrogen is, as mentioned above, thermodynamically a very downhill reaction (Eq. 20), but N2O is kinetically inert and a catalyst is necessary to effect this transformation. 

Recent X-ray structure determinations [213, 214] reveal the so-called Cu2 catalytic center to be a Cu4(S)(His)7 (His=histidine amino add residue, with its imidazole side-chain ligand donor) cluster the central sulfide (S ) ion bridges aU four copper ions which are arranged in a distorted tetrahedral geometry (Fig. 2.5). Binding of the copper ions to histidines connects the CU4S cluster to the protein matrix. Further, a water (or hydroxide) binds to CU v, the coppier ion which has only one (rather than two) histidine ligands. 

A reductive cleavage accompanied by hydration and elimination of N2 is represented by path iii [215]. While the active site stores four (or more) electrons with the four copper atoms and reduced sulfur moiety, only two are needed for a stoichiometric reaction the CU4S core provides a reservoir of reducing equivalents which is perhaps key in the chemistry. Once N2O is reduced, leaving some Cu(II) ions in the cluster, re-reduction of Cuz occurs by electron transfer from a neighboring dicopper center (so-called Cua), allowing enzyme turnover. 

We hope that readers will realize that while there is a reasonably long history and great deal known about NO and N2O and their interactions with metal ions, there is still much to be learned. Nitrogen oxide (NO ) chemistry is in fact quite complex and thus very rich - exciting times lie ahead for further research. 

Ignano, in Nitric Oxide Principles and Actions, J. Lancaster, Jr. (Ed.), Academic Press, San Diego, CA, 1996, p. 111. 

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