STRUCTURAL AND FUNCTIONAL MODELS FOR COPPER-BASED NITRITE REDUCTASE

Sian C. Davies, Marcus C. Durrant, David L. Hughes and Raymond L. Richards1 Phillip Arnold and D. Vaughan Griffiths 2

1Nitrogen Fixation Laboratory, John Innes Centre, Norwich Research Park, Colney, Norwich, NR4 7UH, UK 2Biological and Chemical Sciences, University of Essex, Colchester, Essex, CO4 3SQ, UK

Keywords: copper, tripodal ligands, nitrite reductase

Metalloenzymes frequently attain their special catalytic properties by means of subtly designed structural features, leading to precisely controlled functionality. One such example is copper-containing Nitrite Reductase (NiR). The X-ray crystal structures of two NiR's have been reported (1,2). Each contains two types of copper atom, approximately 12.5 A apart and linked via cysteine and histidine amino acid residues. One copper has Type 1, N2S2 (2His, 1Cys and 1Met) ligation; the other, active site, copper has Type 2, N3O (3His plus water) ligation. The details of the mechanism of reduction of nitrite to NO (and N2O) at the Type 2 centre are at present unknown, as is the precise role of the Type 1 copper in mediating electron transfer to the active site.

To address these questions, we have prepared new copper complexes as models for both the Type 1 and Type 2 sites. Models for the latter centre are based on the novel tripodal ligand 1,1,1-tris(2-pyridyl)methylamine, tpm, Figure 1, (3). Tpm has proved to have a very rich coordination chemistry with copper(II), illustrated by the crystal structures of the complexes [Cu(SO4)(tpm)(H2O)].3H2O, [Cu(tpm)2](BF4)2 and [Cu2Br3(tpm)2]Br. Furthermore, the properties of the copper complexes can be modulated by derivatisation of tpm at the primary amine function, as well as the introduction of a range of substitutes on the pyridine rings. In this way, we hope to obtain copper complexes whose reaction with nitrite is controlled in a similar way to the natural enzyme.

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