Binuclear Metal Centers in Plant Purple Acid Phosphatases: Fe–Mn in Sweet Potato and Fe–Zn in Soybean

Purple acid phosphatases comprise a family of binuclear metal-containing acid hydrolases, representatives of which have been found in animals, plants, and fungi. The goal of this study was to characterize purple acid phosphatases from sweet potato tubers and soybean seeds and to establish their relationship with the only well-characterized plant purple acid phosphatase, the FeIII–ZnII-containing red kidney bean enzyme. Metal analysis indicated the presence in the purified sweet potato enzyme of 1.0 g-atom of iron, 0.6–0.7 g-atom of manganese, and small amounts of zinc and copper. The soybean enzyme contained 0.8–0.9 g-atom of iron, 0.7–0.8 g-atom of zinc per subunit, and small amounts of manganese, copper, and magnesium. Both enzymes exhibited visible absorption maxima at 550–560 nm, with molar absorption coefficients of 3200 and 3300 M−1 cm−1, respectively, very similar to the red kidney bean enzyme. Substrate specificities were markedly different from those of the red kidney bean enzyme. A cloning strategy was developed based on N-terminal sequences of the sweet potato and soybean enzymes and short sequences around the conserved metal ligands of the mammalian and red kidney bean enzymes. Three sequences were obtained, one from soybean and two from sweet potato. All three showed extensive sequence identity (>66%) with red kidney bean purple acid phosphatase, and all of the metal ligands were conserved. The combined results establish that these enzymes are binuclear metalloenzymes: Fe–Mn in the sweet potato enzyme and Fe–Zn in soybean. The sweet potato enzyme is the first well-defined example of an Fe–Mn binuclear center in a protein.