Purple acid phosphatases

Purple acid phosphatases (PAPs) (EC number|3.1.3.2) are metalloenzymes that hydrolyse phosphate esters and anhydrides under acidic condition. [Antanaitis, B.C. and P. Aisen, Uteroferrin and the purple acid phosphatases. Adv Inorg Biochem, 1983. 5: p. 111-36.] [Schlosnagle, D.C., et al., An iron-containing phosphatase induced by progesterone in the uterine fluids of pigs. J Biol Chem, 1974. 249(23): p. 7574-9.] In their oxidised form, PAPs in solution are purple in colour. This is due to the presence of a dinuclear iron centre, [Antanaitis, B.C. and P. Aisen, Stoichiometry of iron binding by uteroferrin and its relationship to phosphate content. J Biol Chem, 1984. 259(4): p. 2066-9.] to which, a tyrosine residue is connected via a charge transfer. [Gaber, B.P., et al., Resonance Raman scattering from uteroferrin, the purple glycoprotein of the porcine uterus. J Biol Chem, 1979. 254(17): p. 8340-2.] This metallic centre is composed of Fe3+ and M, where M is Fe3+, Zn2+, Mg2+ or Mn2+. The conserved Fe3+ is stabilized in the ferric form, whereas M may undergo reduction. Upon treatment with mild reductants, PAPs are converted to their enzymatically active, pink form. Treatment with strong reducing agents dissociates the metallic ions, and renders the enzyme colourless and inactive. [Halleen, J.M., et al., Studies on the protein tyrosine phosphatase activity of tartrate-resistant acid phosphatase. Arch Biochem Biophys, 1998. 352(1): p. 97-102.]

PAPs are highly conserved within eukaryotic species, with >80% amino acid homology in mammalian PAPs, [Lord, D.K., et al., Type 5 acid phosphatase. Sequence, expression and chromosomal localization of a differentiation-associated protein of the human macrophage. Eur J Biochem, 1990. 189(2): p. 287-93.] and >70% sequence homology in PAPs of plant origin. [Schenk, G., et al., Binuclear metal centers in plant purple acid phosphatases: Fe-Mn in sweet potato and Fe-Zn in soybean. Arch Biochem Biophys, 1999. 370(2): p. 183-9.] However sequence analysis reveals that there is minimal homology between plant and mammal PAPs (<20%), except for the metal-ligating amino acid residues which are identical. [Klabunde, T., et al., Structural relationship between the mammalian Fe(III)-Fe(II) and the Fe(III)-Zn(II) plant purple acid phosphatases. FEBS Lett, 1995. 367(1): p. 56-60.] The metallic nucleus of PAPs also varies between plants and mammals. Mammalian PAPs which have been isolated and purified have, to this point, been composed exclusively of iron ions, whereas in plants the metallic nucleus is composed of Fe3+ and either Zn2+ or Mn2+. PAPs have also been isolated in fungi, and DNA sequences encoding for possible PAPs have been identified in prokaryotic organisms, such as in Cyanobacteria spp. and Mycobacteria spp. [Schenk, G., et al., Purple acid phosphatases from bacteria: similarities to mammalian and plant enzymes. Gene, 2000. 255(2): p. 419-24.]

Currently there is no defined nomenclature for this group of enzymes, and a variety of names exists. These include purple acid phosphatase (PAP), uteroferrin (Uf), type 5 acid phosphatase (Acp 5) and tartrate resistant acid phosphatase (TRAP, TRACP, TR-AP). There is, however, a consensus in the literature that purple acid phosphatase (PAP) relates to those found in non-mammalian species and tartrate resistant acid phosphatase (TRAP) to those found in mammalian species.

Uteroferrin, bovine spleen PAP and tartrate resistant acid phosphatase all refer to mammalian PAPs, whereby research on PAPs expressed in various tissues diverged. Subsequent research has proven that all of these enzymes are the same entity. [Ek-Rylander, B., et al., Cloning, sequence, and developmental expression of a type 5, tartrate-resistant, acid phosphatase of rat bone. J Biol Chem, 1991. 266(36): p. 24684-9.] [Ling, P. and R.M. Roberts, Uteroferrin and intracellular tartrate-resistant acid phosphatases are the products of the same gene. J Biol Chem, 1993. 268(10): p. 6896-902.]

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