STRUCTURE OF AZURIN FROM ACHROMOBACTER-XYLOSOXIDANS NCIB11015 AT 2.5 ANGSTROM RESOLUTION

The crystal structure of azurin from a denitrifying bacterium, Achromo bacter xylosoxidans NCIE11015, has been refined at 2.5 Angstrom resolu tion using diffraction data obtained by means of synchrotron radiation at KEK. Crystals suitable for X-ray experiment were obtained by the m acro-seeding method and an intensity data were obtained on imaging pla tes mounted on a Weissenberg camera (R(merge) = 0.09). The initial mod el was obtained by the molecular replacement method using the structur e of azurin from Alcaligenes denitrificans NCTC8582 as a starting mode l. The structure was refined by molecular dynamics optimization and th e restrained least-squares method to a crystallographic R-value of 0.2 05. However, the current model gave an electron-density of the side-ch ain regions of several residues close to the N-terminus quite differen t from those expected from the amino acid sequences reported. Very rec ently, two kinds of azurins (Az-I and At-II) were isolated from this b acterium by a slightly modified purification method and have been char acterized and found to have different CD spectra. On analysis of amino acid sequences around the N-terminus, the second azurin (At-II) was p roved to be a new type of azurin in this bacterium. It was consequentl y 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-te rminus. Determination of the whole amino acid sequence of this azurin and further refinement are in progress.