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 ac id hydrolases, representatives of which have been found in animals, plants, and fungi. The goal of this study was to characterize purple acid phosphat ases from sweet potato tubers and soybean seeds and to establish their rela tionship with the only well-characterized plant purple acid phosphatase, th e FeIII-ZnII-containing red kidney bean enzyme. Metal analysis indicated th e 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 e nzyme 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 exhibi ted visible absorption maxima at 550-560 nm, with molar absorption coeffici ents of 3200 and 3300 M-1 cm(-1), respectively, very similar to the red kid ney 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 seque nces around the conserved metal ligands of the mammalian and red kidney bea n enzymes. Three sequences were obtained, one from soybean and two from swe et potato. All three showed extensive sequence identity (>66%) with red kid ney bean purple acid phosphatase, and all of the metal ligands were conserv ed. The combined results establish that these enzymes are binuclear metallo enzymes: Fe-Mn in the sweet potato enzyme and Fe-Zn in soybean. The sweet p otato enzyme is the first well defined example of an Fe-Mn binuclear center in a protein, (C) 1999 Academic Press.