Fe(III) – Zn(II) Purple Acid Phosphatase

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.

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