Kidney bean purple acid phosphatase (KBPAP) is an Fe(III)-Zn(II) metalloenzyme resembling the mammalian Fe(III)-Fe(II) purple acid phosphatases. The structure of the homodimeric III-kilodalton KBPAP was determined at a resolution of 2.9 angstroms. The enzyme contains two domains in each subunit. The active site is located in the carboxyl-terminal terminal domain at the carboxy end of two sandwiched beta alpha beta alpha beta motifs. The two metal ions are 3.1 angstroms apart and bridged monodentately by Asp(164) The iron is further coordinated by Tyr(167), His(325), and Asp(135), and the zinc by His(286), His(323), and Asn(201). The active-site structure is consistent with previous proposals regarding the mechanism of phosphate ester hydrolysis involving nucleophilic attack on the phosphate group by an Fe(III)coordinated hydroxide ion.