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Last modified: 10 December 1998

Purple acid phosphatase
Protein Binuclear metal centre ¹ Metal ligands Formal metal oxidation states
Mammalian

2×Pentacoordinate
Fe1
Fe1
FeIIIFeII;

FeIIIFeIII
NdeltaHis;

eta2­OdeltaAsp
or
eta2­OepsilonGlu

NepsilonHis;

OetaTyr;

OH- (H2O)

µ­eta1:eta1­OdeltaAsp
or
µ­eta1:eta1­OepsilonGlu;

µ­O (µ­OH)

Plant

2×Hexacoordinate
Zn
Fe
FeIIIZnII
NdeltaHis;

NepsilonHis;

OdeltaAsn;

H2O

NepsilonHis;

OetaTyr;

OdeltaAsp;

OH-

µ­OdeltaAsp;

µ­O (µ­OH)

Purple acid phosphatases (PAPs; EC 3.1.3.2) constitute Class IV diiron-carboxylate proteins [1]. PAPs hydrolyse orthophosphate monoethers (1) under acidic conditions (optimum pH of 4.9-6.0) and are widespread in nature [2-4].

PAPs are characterised by an intense purple colour, which results from a Tyr->FeIII charge transfer [3]. The exact physiological function of PAPs is unknown. Some of the mammalian enzymes are probably involved in degradative biological processes, such as phagocytosis and active bone resorption. Two mammalian PAPs have been extensively studied, bovine spleen PAP and porcine uterus PAP (uteroferrin). These are monomeric (~35 kDa) enzymes possessing a binuclear iron centre. The enzyme exists in two forms, reduced and oxidised: FeII of reduced PAP is labile and can be substituted by other MeII ions, in particular by ZnII, retaining catalytical activity [2].

Certain plant PAPs, which are structurally related to mammalian PAPs [5], contain binuclear [FeIIIZnII] centres. These enzymes do not have an inactive oxidised form. Kidney bean PAP (kbPAP) is a homodimeric glycoprotein (111 kDa) with a disulphide bond linking two monomers. ZnII is labile and can be substituted by FeII or CoII to form a catalytically active enzyme [2, 3]. kbPAP with a [FeIIIFeII] centre shows spectroscopic and kinetic properties nearly identical with mammalian PAPs. The 3­D structure of kbPAP has been determined [6] (see Figure 1KBP). The metal ions are coordinated by three His, two Asp, one Asn and one Tyr residues in the loops at the C­terminal end of the strands in an alpha/ß sandwich. Thus, PAPs are structurally unrelated to other diiron-carboxylate proteins which have a four­alpha­helix bundle fold [1].

¹ The 3­D structure of mammalian PAP is not known. A proposed model of its binuclear iron centre is shown [2]. In the 3­D structure of kidney bean PAP, the solvent ligands have not been identified unambiguously. Based on the geometry of the coordination sphere around the metal ions, three additional ligands were added: a bridging hydroxide, a terminal water on the ZnII, and a terminal hydroxide on the FeIII [5].

Acid phosphatases in enzyme databases

ENZYME LIGAND BRENDA Official name Alternative names
3.1.3.2
3.1.3.2
3.1.3.2
 Acid phosphatase Acid monophosphatase; acid phosphohydrolase; acid phosphomonoesterase; acid phosphomonoester hydrolase; phosphomonoesterase; glycerophosphatase; uteroferrin

Purple acid phosphatase in alignment databases

Protein Superfamily Pfam LPFC 3­D
alignment
00521; tartrate­resistant acid phosphatase
04306; kidney bean purple acid phosphatase
-
-

Purple acid phosphatase in 3­D databases

Purple acid phosphatase contain one binuclear iron­zinc centre per protein subunit (see Figure 1KBP).

PDB scop BSMRELI
Base		 Header MMS Abstract ²
1kbp 1kbp 1kbp 1kbp Purple acid phosphatase (complex with N­acetyl­D­glucosamine); kidney bean (Phaseolus vulgaris) MS6CD2
3kbp 3kbp 3kbp 3kbp Purple acid phosphatase (complex with N­acetyl­D­glucosamine and tungstate) (dimer with a disulphide bond linking two chains); kidney bean (Phaseolus vulgaris)
-
4kbp 4kbp 4kbp 4kbp Purple acid phosphatase (complex with N­acetyl­D­glucosamine and phosphate) (dimer with a disulphide bond linking two chains); kidney bean (Phaseolus vulgaris)
-

² Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

  1. Nordlund, P. and Eklund, H. (1995) Di­iron-carboxylate proteins. Curr. Opin. Struct. Biol. 5, 758-766.
  2. Wilcox, D.E. (1996) Binuclear metallohydrolases. Chem. Rev. 96, 2435-2458.
  3. Sträter, N., Lipscomb, W.N., Klabunde, T. and Krebs, B. (1996) Two­metal ion catalysis in enzymatic acyl­ and phosphoryl­transfer reactions. Angew. Chem. Int. Ed. Engl. 35, 2025-2055.
  4. Lipscomb, W.N. and Sträter, N. (1996) Recent advances in zinc enzymology. Chem. Rev. 96, 2375-2433.
  5. Klabunde, T., Sträter, N., Krebs, B. and Witzel, H. (1995) Structural relationship between the mammalian Fe(III)­Fe(II) and the Fe(III)­Zn(II) plant purple acid phosphatases. FEBS Lett. 367, 56-60.
  6. Sträter, N., Klabunde, T., Tucker, P., Witzel, H. and Krebs, B. (1995) Crystal structure of a purple acid phosphatase containing a dinuclear Fe(III)­Zn(II) active site. Science 268, 1489-1492.
Bibliography on structural studies of purple acid phosphatase
Reviews on purple acid phosphatase