Copper complexes with N-donor ligands as models of the active centres of nitrite reductase and related enzymes

Abstract

Denitrification, the enzymatic reduction of nitrate (NO3 ‐) and nitrite (NO2 ‐ ) to gaseous NO, N2O and/or N2, is part of the nitrogen cycle which is essential to life. Studies of denitrification have gained impetus in recent years because of environmental concerns over the widespread agricultural use of potentially polluting nitrate and nitrite, and the damage to the atmosphere from NO and N2O that can be produced by their degradation. 1 Denitrification is mediated by a number of metalloenzymes; in particular we concentrate in this article on the reduction of nitrite, which is catalysed by two distinct types of nitrite reductase enzyme. One type involves a multi-haeme system and the second, upon which we shall focus, is a multicopper enzyme (CuNiR). 2 X-ray studies of CuNiR from a number of sources have shown that the enzyme is a homotrimer in which each monomer contains a so-called Type 1 copper, with a (His)2(Cys)(Met) ligand set, linked by sequential protein residues (CysHys) to a Type 2 copper, bound at the interface between subunits of the protein by a facial array of three histidine residues. The two copper centres are around 12.6 A apart. A fourth ligand, water or hydroxide, completes the pseudo-tetrahedral coordination of the Type 2 copper which is situated in a hydrophobic pocket, some 12 A from the protein surface (Fig. 1).3‐6