The covalent structure of the blue copper-containing nitrite reductase from Achromobacter xylosoxidans

The complete amino acid sequence of the blue copper-containing nitrite reductase enzyme (NiR) from Achromobacter xylosoxidans has been determined by chemical analysis, supported by high precision mass analysis. The polypeptide chain contains 336 residues with an overall charge of 0, including the +2 state of each of the copper ions. The two NiR enzymes for which the three-dimensional structures have been solved are green in color and have different absorption spectra than those of the blue-colored protein from A. xylosoxidans. The ligands to the two copper atoms are conserved. Therefore, the difference between the blue and the green NiR must depend on subtle changes in the geometry of the type I copper-sulfur bond. Both overall protein charge and active site charge are different in A. xylosoxidans NiR which may reflect the use of azurin as electron donor as opposed to the other enzymes that use pseudoazurin.