title: Reactivity of M(II) metal-substituted derivatives of pig purple acid phosphatase (uteroferrin) with phosphate.
abstract: The Fe(II) of the binuclear Fe(II)Fe(III) active site of pig purple acid phosphatase (uteroferrin) has been replaced in turn by five M(II) ions (Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)). An uptake of 1 equiv of M(II) is observed in all cases except that of Cu(II), when a second more loosely bound Cu(II) is removed by treatment with edta. The products have been characterized by different analytical procedures and by UV-vis spectrophotometry. At 25 degrees C, I = 0.100 M (NaCl), the nonenzymatic reactions with H(2)PO(4)(-) give the mu-phosphato product, and formation constants K/M(-1) show an 8-fold spread at pH 4.9 of 740 (Mn), 165 (Fe), 190 (Co), 90 (Ni), 800 (Cu), 380 (Zn). The variations in K correlate well with stability constants for the complexing of H(2)PO(4)(-) and (CH(3)O)HPO(3)(-) with M(II) hexaaqua ions. At pH 4.9 with [H(2)PO(4)(-)] > or = 3.5 mM rate constants k(obs) decrease, and an inhibition process in which a second [H(2)PO(4)(-)] coordinates to the dinuclear center is proposed. The mechanism considered accounts for most but not all of the features displayed. Thus K(1) values for the coordination of phosphate to M(II) are in the range10-60 M(-1), whereas K(2) values for the bridging of the phosphate to Fe(III) are in the narrower range 7.8-12.4. From the fits described K(i) approximately 10(3) M(-1) for the inhibition step, which is independent of the identity of M(II). Values of k(obs) decrease with increasing pH, giving pK(a) values which are close to 3.8 and independent of M(II) (Fe(II), Zn(II), Mn(II)). The acid dissociation process is assigned to Fe(III)-OH(2) to Fe(III)-OH(-), where OH(-) is less readily displaced by phosphate.
authors.affiliation: Department of Chemistry, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K.
authors.forename: Mark B
authors.initials: MB
authors.lastname: Twitchett
authors.forename: Gerhard
authors.initials: G
authors.lastname: Schenk
authors.forename: Manuel A S
authors.initials: MA
authors.lastname: Aquino
authors.forename: Douglas T-Y
authors.initials: DT
authors.lastname: Yiu
authors.forename: Tai-Chu
authors.initials: TC
authors.lastname: Lau
authors.forename: A Geoffrey
authors.initials: AG
authors.lastname: Sykes
mesh_terms: D000135:Acid Phosphatase; D000818:Animals; D002021:Buffers; D004563:Electrochemistry; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D007527:Isoenzymes; D007700:Kinetics; D008667:Metalloproteins / Q000737:chemistry* / Q000302:isolation & purification; D008670:Metals / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013552:Swine; D000071681:Tartrate-Resistant Acid Phosphatase
chemical_list: D002021:Buffers; D007202:Indicators and Reagents; D007527:Isoenzymes; D008667:Metalloproteins; D008670:Metals; D010710:Phosphates; D000135:Acid Phosphatase; D000071681:Tartrate-Resistant Acid Phosphatase