On the mechanism of the Zn2+ and Co2+-alkaline phosphatase of E. coli. Number of sites and anticooperativity

Characterization of phosphate binding by alkaline phosphatase in rat kidney brush border membrane

Phosphate binding by rat renal brush border membranes occurs on a single protein, as visualized by SDS polyacrylamide gel electrophoresis. The same protein can also be specifically labelled by gamma-32P ATP at 0 degree C or in the absence of magnesium. The phosphate binding protein co-migrates with monomers of two alkaline phosphatase activity bands previously localized on acrylamide gel. Measurement of binding by TCA precipitation, ion-exchange chromatography and dialysis gave an average of 31.1 +/- 5.7 pmol phosphate bound/mg protein. Alkaline phosphatase would then represent 0.23% of total brush border membrane protein. Maximal binding activity is obtained at pH 6.5, but when membranes are phosphorylated at pH 6.5 and the pH increased to 9.4, 50% of the bound radioactivity is released. The binding of phosphate to this protein presents two different apparent Km: one at 40 microM for low and one at 390 microM for high substrate concentrations. The membrane bound phosphate is readily exchangeable with phosphate in the medium. Phosphate binding and phosphate release are complete within 5 s. Alkaline phosphatase substrates and EDTA are potent inhibitors of phosphate binding and produce over 90% inhibition. Characteristics of phosphate binding for kidney membrane bound alkaline phosphatase seem very similar to the soluble form of the enzyme from various sources.

Negative cooperativity in alkaline phosphatase from E. col: new kinetic evidence from a steady-state study

1. A study has been carried out on the steady-state kinetics followed by the alkaline phosphatase from Escherichia coli at different pH, temperatures, ionic strengths, phosphate concentrations and in the presence of the effectors such as Tris, NH4+--NH3 and CH3OH; p-nitrophenyl phosphate was used as substrate. 2. Contrary to what has generally been accepted, in most cases the enzyme follows non-Michaelian kinetics for a wide substrate concentration range, giving concave-down Lineweaver-Burk plots. Only at high phosphate concentrations (5 . 10(-3) M) and at high ionic strengths (2.0 M) is a linear Lineweaver-Burk plot obtained (Michaelian kinetics). 3. In order to analyse the kind of kinetics obtained, a non-linear regression fitting method was used to obtain rate vs substrate concentration equations as polynomial quotients of minimum degree with positive coefficients. 4. Most of the data obtained follows 2:2 degree type equations. 5. These results tend to suggest an idea of cooperativity rather than one of independence between the sites of the dimeric enzyme. A model is discussed for cooperativity between the sites with a wide concentration range giving concave-down Lineweaver-Burk plots.

Purification and characterization of specific 3-deoxy-D-manno-octulosonate 8-phosphate phosphatase from Escherichia coli B

A phosphatase specific for the hydrolysis of 3-deoxy-d-manno-octulosonate (KDO)-8-phosphate was purified approximately 400-fold from crude extracts of Escherichia coli B. The hydrolysis of KDO-8-phosphate to KDO and inorganic phosphate in crude extracts of E. coli B, grown in phosphate-containing minimal medium, could be accounted for by the enzymatic activity of this specific phosphatase. No other sugar phosphate tested was an alternate substrate or inhibitor of the purified enzyme. KDO-8-phosphate phosphatase was stimulated three- to fourfold by the addition of 1.0 mM Co(+) or Mg(2+) and to a lesser extent by 1.0 mM Ba(2+), Zn(2+), and Mn(2+). The activity was inhibited by the addition of 1.0 mM ethylenediaminetetraacetic acid, Cu(2+), Ca(2+), Cd(2+), Hg(2+), and chloride ions (50% at 0.1 M). The pH optimum was determined to be 5.5 to 6.5 in both tris(hydroxymethyl)aminomethane-acetate and HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) buffer. This specific phosphatase had an isoelectric point of 4.7 to 4.8 and a molecular weight of 80,000 +/- 6,000 as determined by molecular sieving and Ferguson analysis. The enzyme appeared to be composed of two identical subunits of 40,000 to 43,000 molecular weight. The apparent K(m) for KDO-8-phosphate was determined to be 5.8 +/- 0.9 x 10(-5) M in the presence of 1.0 mM Co(2+), 9.1 +/- 1 x 10(-5) M in the presence of 1.0 mM Mg(2+), and 1.0 +/- 0.2 x 10(-4) M in the absence of added Co(2+) or Mg(2+).

Metal ion-induced conformational changes in Escherichia coli alkaline phosphatase

Ultraviolet difference spectra are produced by the binding of divalent metal ions to metal-free alkaline phosphatase (EC 3.1.3.1). The interaction of the apoprotein with Zn2+, Mn2+, Co2+ and Cd2+, which induce the tight binding of one phosphate ion per dimer, give distinctly different ultraviolet spectra changes from Ni2+ and Hg2+ which do not induce phosphate binding. Spectrophotometric titrations at alkaline pH of various metallo-enzymes reveal a smaller number of ionizable tyrosines and a greater stability towards alkaline denaturation in the Zn2+- and Mn2+-enzymes than in the Ni2+-, Hg2+- and apoenzymes. The Zn2+- and Mn2+-enzymes have CD spectra in the region of the aromatic transitions that are different from the CD spectra of the Ni2+-, Hg2+- and apoenzymes. Modifications of arginines with 2,3-butanedione show that a smaller number of arginine residues are modified in the Zn2+-enzyme than in the Hg2+-enzyme. The presented data indicate that alkaline phosphatase from Escherichia coli must have a well-defined conformation in order to bind phosphate. Some metal ions (i.e. Zn2+, Co2+, Mn2+ and Cd2+), when interacting with the apoenzyme, alter the conformation of the protein molecule in such a way that it is able to interact with substrate molecules, while other metal ions (i.e. Ni2+ and Hg2+) are incapable of inducing the appropriate conformational change of the apoenzyme. These findings suggest an important structural function of the first two tightly bound metal ions in enzyme.

Effects of antibodies to various molecular forms of a mutationally altered Escherichia coli alkaline phosphatase on its activation by zinc

Immunogenic and antigenic properties of a Zn2+ -deficient alkaline phosphatase produced in a mutant (U-47) of Escherichia coli have been studied. The native U-47 enzyme, that exists in a monomerdimer equilibrium, was used as immunogen. From the antisera obtained, four antibody populations directed to the various molecular forms of U-47 enzyme have been purified by affinity chromatography using specific antigens coupled to glutaraldehyde-activated beads of indubiose. 70% of the total antibody obtained was directed both to the monomeric and the dimeric forms, 9% was directed to the dimer but showed a low affinity for the monomer; 10% and 11% were specifically directed respectively to the monomer and the dimer. Each antibody population purified had a specific effect on the catalytic activity of the Zn2+ -activated U-47 enzyme. The anti-monomer-dimer and the anti-dimer-monomer inhibit to the same extent whereas the specific anti-monomer does not alter the activity significantly and the specific anti-dimer causes a 30% activation. The catalytic activity of the alkaline phosphatase produced in wild-type strains was also reduced by these anti-U-47 enzyme antibodies. However, whereas the anti-monomer had again very little effect, the anti-dimer-monomer and the anti-monomer-dimer inhibited this enzyme to different extents. The specific antidimer also inhibited this wild-type alkaline phosphate. Antibodies of high affinity to the dimeric form of U-47 enzyme, i.e. specific anti-dimer or anti-dimer-monomer, caused a 30% activation when they were added prior to the reactivation process by Zn2+. Specific anti-monomer strongly inhibited this reactivation process. The Fab fragment of the anti-wild-type phosphatase antibody, under the same conditions, caused a 300% activation. The extents of interactions of the various molecular forms of U-47 enzyme and of the wild-type enzyme with the anti-monomer-dimer and with the anti-dimer have been determined. U-47 enzyme monomeric form has three determinants exposed and the dimeric form has five determinants exposed for interacting with the anti-monomer-dimer antibody, the free wild-type enzyme has only two determinants exposed to this antibody. These determinants might be close to the active site or in another critical location since this antibody can reduce the catalytic activity of the wild-type enzyme to half the original value. The anti-dimer antibodies can interact with three determinants exposed at the surface of the free Zn2+ -reactivated U-47 enzyme and the non-covalent binding of one mole of inorganic phosphate results in the exposure of one more antigenic determinant.

31P nuclear magnetic resonance study of alkaline phosphatase: the role of inorganic phosphate in limiting the enzyme turnover rate at alkaline pH

31P nuclear magnetic resonance (NMR) was used to directly observe the binding of inorganic phosphate to alkaline phosphatase. Evidencq for the tight binding of 1.5-2.0 mol of inorganic phosphate per dimer of alkaline phosphatase is presented. Two distinct forms of bound phosphate are observed, one predominating above pH 7 and representing the non-covalent E-P1 complex and the other predominating below pH 5 and representing the covalent E-P1 complex. The 31P NMR line width of the E-P1 complex indicates that the dissociation of noncovalent phosphate is the rate-limiting step in the turnover of the enzyme at high pH.

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.