This is the crystal structure of a purple acid phosphatase from kidney bean. Purple acid phosphatases are enzymes that catalyze the hydrolysis of phosphoric acid esters and anhydrides like ATP at pHs ranging from 4 to 7. The actual substrate of this enzyme in the kidney bean is not known.
This enzyme is a homodimer with a mass of 111 kDa and four binuclear Fe(III)-Zn(II) active centers. Mammal purple acid phosphatases have a binuclear Fe(III)-Fe(II) center. The purple color of this class of acid phosphatases results from a tyrosinate to Fe(III) charge transfer at about 560 nm.
The Fe ion is coordinated by a tyrosine, a histidine, and an aspartate; and the zinc by two histidines and an asparagine. The metal ions are bridged by the carboxylate group of aspartate. The position of the two metal ions by a central beta-alpha-beta-alpha-beta-alpha scaffold is common to some other acid phosphatases.
It is thought that through interactions with the zinc and two histidines, the phosphate group of the substrate is oriented for a nucleophilic attack from an iron-bound hydroxide ion. A histidine (296) is thought to protonate the leaving alcohol.
Highlighting FeaturesShow beta sheets as magenta, alpha helixes as cyan, and location of iron and zinc. Show ribbons with location of iron and zinc and interacting amino acids. Show iron and zinc and interacting amino acids. Show bound phosphate with iron and zinc and interacting histidines.
This structure was obtained from x-ray diffraction of the crystallized enzyme at a resolution of 2.65 Å.
Klabunde T, N Strater, R Frohlich, H Witzel, B Krebs. 1996. “Mechanism of Fe(III)-Zn(II) purple acid phosphatase based on crystal structures.” Journal of molecular biology, 259: 737-748.
Strater R, T Klabunde, P Tucker, H Witzel, B Krebs. 1995. “Crystal structure of a purple acid phosphatase obtaining a dinuclear Fe(III)-Zn(II) active site.” Science, 268:1489-1492.
Vincent, J.B., M.W. Crowder, B.A. Averil. 1992. “Hydrolysis of phosphate monoesters: a biological problem with multiple chemical solutions.” Trends in Biochemical Science 17:105-110.
This page was constructed by L. Ward Good as a class project for “Mineral Nutrition of Plants”, SoilSci/Botany/Horticulture 626, under the direction of Drs. P. Barak and E. Spalding, and was contributed to the Virtual Museum of Minerals and Molecules