Tight and loose metal binding sites in the apoalkaline phosphatase of E. coli. Reconstitution of the Ca2+--phosphatase from the apoenzyme EPR study of the Mn2+-phosphatase

Dimer asymmetry and the catalytic cycle of alkaline phosphatase fromEscherichia coli

Although alkaline phosphatase (APase) from Escherichia coli crystallizes as a symmetric dimer, it displays deviations from Michaelis-Menten kinetics, supported by a model describing a dimeric enzyme with unequal subunits [Orhanović S., Pavela-Vrancic M. and Flogel-Mrsić M. (1994) Acta. Pharm.44, 87-95]. The possibility, that the observed asymmetry could be attributed to negative cooperativity in Mg2+ binding, has been examined. The influence of the metal ion content on the catalytic properties of APase from E. coli has been examined by kinetic analyses. An activation study has indicated that Mg2+ enhances APase activity by a mechanism that involves interactions between subunits. The observed deviations from Michaelis-Menten kinetics are independent of saturation with Zn2+ or Mg2+ ions, suggesting that asymmetry is an intrinsic property of the dimeric enzyme. In accordance with the experimental data, a model describing the mechanism of substrate hydrolysis by APase has been proposed. The release of the product is enhanced by a conformational change generating a subunit with lower affinity for both the substrate and the product. In the course of the catalytic cycle the conformation of the subunits alternates between two states in order to enable substrate binding and product release. APase displays higher activity in the presence of Mg2+, as binding of Mg2+ increases the rate of conformational change. A conformationally controlled and Mg2+-assisted dissociation of the reaction product (Pi) could serve as a kinetic switch preventing loss of Pi into the environment.

Zinc stoichiometry in Escherichia coli alkaline phosphatase. Studies by 31P NMR and ion-exchange chromatography

31P nuclear magnetic resonance spectra and enzymatic activities are compared for alkaline phosphatase (orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1) species with different zinc contents. The enzyme containing two Zn2+ per protein dimer exists in two forms; one, prepared by dialysis of native enzyme, has full enzymatic activity and a 31P magnetic resonance spectrum similar to but distinguishable from that of the native enzyme containing four or more Zn2+. The other form, prepared by restoring two Zn2+ to apoenzyme, has low enzymatic activity and a 31P magnetic resonance spectrum that indicates stoichiometric binding of phosphate, but otherwise altered properties. Reconstituted enzyme with four Zn2+ is similar to but distinguishable from native enzyme with four Zn2+. Chromatography on DEAE-cellulose can separate apoenzyme and enzyme containing two Zn2+ and suggests that the binding of a pair of Zn2+ is cooperative.

Fluorine-19 nuclear magnetic resonance study of fluorotyrosine alkaline phosphatase: the influence of zinc on protein structure and a conformational change induced by phosphate binding

19F nuclear magnetic resonance (NMR) spectroscopy has been used to study a fully active E. coli fluorotyrosine alkaline phosphatase. The fluorotyrosine resonances provide sensitive probes of the conformational states of the protein. They were used to follow the addition of zinc or cobalt to the apoprotein, and the titration of the protein with inorganic phosphate or the inhibitor 2-hydroxy-5-nitrobenzylphosphonate. The results indicate that 2 molecules of inorganic phosphate per dimer of alkaline phosphatase are required to complete a general conformational change in the protein involving perturbations to the environment of several tyrosines. Spectra of the cobalt enzyme indicate that on specific tyrosine per subunit may be near the metal site. The 19F NMR results, combined with the 31P NMR results in the accompanying paper, lead directly to the conclusion that dissociation of noncovalently bound inorganic phosphate from the enzyme is the rate-limiting process in enzyme catalysis at high pH. The local environment of the individual fluorotyrosines is also discussed.

Nitrogen ligands at the active site of alkaline phosphatase

The two Zn(II) ions of native Escherichia coli alkaline phosphatase (EC 3.1.3.1) that are necessary for activity have been replaced by (63)Cu(II). Titration of apoenzyme with up to 2 eq of Cu(II) gives a homogeneous species with an electron spin resonance typical for Cu(II) in an axially symmetric environment, with A(z) = 496 MHz, g(z) = g = 2.27, and g(x) = g(y) = 2.05. At least seven nitrogen hyperfine lines, spaced 11 G apart, are clearly resolved on the M = +[unk] Cu(II) hyperfine peak in the parallel region. When more than 2 eq of Cu(II) are added, the electron spin resonance spectrum shows at least two types of Cu(II) binding sites; the additional site, or sites, are characterized by lower g and higher A(z) values. When Cu(II) is added to native Zn(II) alkaline phosphatase or to apoenzyme incubated with 2 eq of Zn(II), the electron spin resonance spectrum shows little or no trace of the species with higher g values and nitrogen splitting. These results indicate that the species with higher g represents copper bound at the site normally occupied by the 2 Zn (II) ions necessary for enzyme activity, and that the metal ion at this site has at least 3 equivalent nitrogen ligands, probably histidyl side chains.