Three-dimensional structure of a mammalian purple acid phosphatase at 2.2 angstrom resolution with a mu-(hydr)oxo bridged di-iron center

The crystal structure of purple acid phosphatase from rat bone has been det ermined by molecular replacement and the structure has been refined to 2.2 Angstrom resolution to an R-factor of 21.3% (R-free 26.5%). The core of the enzyme consists of two seven-stranded mixed beta-sheets, with each sheet f lanked by solvent-exposed alpha-helices on one side. The two sheets pack to wards each other forming a beta-sandwich. The di-iron center, located at th e bottom of the active-site pocket at one edge of the beta-sandwich, contai ns a mu-hydroxo or mu-oxo bridge and both metal ions are observed in an alm ost perfect octahedral coordination geometry. The electron density map indi cates that a mu-(hydr)oxo bridge is found in the metal center and that at l east one solvent molecule is located in the first coordination sphere of on e of the metal ions. The crystallographic study of rat purple acid phosphat ase reveals that the mammalian enzymes are very similar in overall structur e to the plant enzymes in spite of only 18% overall sequence identity. In p articular, coordination and geometry of the iron cluster is preserved in bo th enzymes and comparison of the active-sites suggests a common mechanism f or the mammalian and plant enzymes. However, significant differences are fo und in the architecture of the substrate binding pocket. (C) 1999 Academic Press.