Figure 1.

Schematic illustration of the active site of red kidney bean purple acid phosphatase (rkbPAP), a representative binuclear metallohydrolase. In most (if not all) binuclear metallohydrolases the binding affinities of the two metal centers vary, with M1 representing the tight binding site and M2 the lower affinity site [8]. In rkbPAP M1 and M2 are occupied by Fe(III) and Zn(II), respectively. Combined crystallographic and spectroscopic data for PAPs indicate the presence of a bridging (hydr)oxo group and one terminal water ligand (see text). The presence of a terminal Fe(III)-bound hydroxide is currently debated with spectroscopic data suggesting its absence [8], but the crystal structure of rat PAP supporting its presence [8].