Structure of azurin from Achromobacter xylosoxidans NCIB11015 at 2.5 A resolution

The crystal structure of azurin from a denitrifying bacterium, Achromobacter xylosoxidans NCIB11015, has been refined at 2.5 A resolution using diffraction data obtained by means of synchrotron radiation at KEK. Crystals suitable for X-ray experiment were obtained by the macro-seeding method and an intensity data were obtained on imaging plates mounted on a Weissenberg camera (Rmerge = 0.09). The initial model was obtained by the molecular replacement method using the structure of azurin from Alcaligenes denitrificans NCTC8582 as a starting model. The structure was refined by molecular dynamics optimization and the restrained least-squares method to a crystallographic R-value of 0.205. However, the current model gave an electron-density of the side-chain regions of several residues close to the N-terminus quite different from those expected from the amino acid sequences reported. Very recently, two kinds of azurins (Az-I and Az-II) were isolated from this bacterium by a slightly modified purification method and have been characterized and found to have different CD spectra. On analysis of amino acid sequences around the N-terminus, the second azurin (Az-II) was proved to be a new type of azurin in this bacterium. It was consequently revealed that the current model corresponds to a new type of azurin because of the complete agreement between the electron-density and the amino acid sequence of the newly determined 20 residues from the N-terminus. Determination of the whole amino acid sequence of this azurin and further refinement are in progress.