Escherichia coli alkaline phosphatase. Kinetic studies with the tetrameric enzyme

Effects of Coupling Chemistry on the Activity of Poly(ethylene glycol)-Modified Alkaline Phosphatase

Proteins modified by covalent coupling to poly(ethylene glycol) are of interest for several biotechnical applications. In the present work we compare the effects of four commonly used coupling methods on alkaline phosphatase activi ty ; the four methods use the PEG tresylate, succinimidyl succinate, cyanuric chloride derivative, or carbonyl diimidazole derivative. All routes give active enzyme, with only the cyanuric chloride route giving significant deactivation; none the less the cyanuric chloride derivative is useful at lower degrees of modification. Examination of the Michaelis-Menten parameters for the cy anuric chloride coupling suggests that the loss of activity from this route results from intramolecular crosslinking of the protein, which in turn leads to loss of protein conformational flexibility.

Tetrameric alkaline phosphatase in human liver plasma membranes

Molecular weights of native membrane-bound alkaline phosphatase released by butanol and by nonionic detergents were more than twice that of the purified dimeric enzyme. Alkaline phosphatase released by phosphatidylinositol-specific phospholipase-C was of both high and low molecular weight: the former was a protomer of a single protein of the same molecular size as monomeric alkaline phosphatase. We conclude that the membrane-bound enzyme is probably a tetramer.

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.

Mutationally altered rate constants in the mechanism of alkaline phosphatase

The hydrolysis of phosphate esters by a mutationally altered alkaline phosphatase from Escherichia coli was studied by both steady-state and transient-kinetic methods. The difference between the catalytic-centre activities of the mutationally altered and the wild-type alkaline phosphatases was found to vary with pH and at optimal pH values the modified enzyme had the higher activity. Stopped-flow experiments at acidic pH values showed that transient product formation by the mutationally altered enzyme was faster than that with the wild-type enzyme whereas the rate of the steady state was slower. In the alkaline pH region, the transient was observed in the reaction of only the modified enzyme and not the wild type. These observations permit a fuller characterization of the individual steps in the catalytic mechanism of alkaline phosphatase than is possible by study of only the wild-type enzyme.

The role of magnesium ions in beta-galactosidase hydrolyses. Studies on charge and shape of the beta-galactopyranosyl binding site

1. beta-d-Galactopyranosyl trimethylammonium bromide is a competitive inhibitor of beta-galactosidase, K(i)=1.4+/-0.2mm at 25 degrees C. 2. Tetramethylammonium bromide is not an inhibitor (K(i)>0.2m). 3. The kinetics of deactivation of Mg(2+)-saturated, and of inhibitor-and Mg(2+)-saturated, enzyme in 10mm-EDTA are similar. 4. The apparent K(i) for the glycosylammonium salt is approx. 2.2mm in the absence of Mg(2+). 5. It is therefore concluded that Mg(2+) and the inhibitor bind independently, and that the Mg(2+) does not act as an electrophilic catalyst. 6. Complexant fluorescence measurements indicate binding of 1 Mg(2+) ion per 135000-dalton protomer. 7. This stoicheiometry is confirmed by equilibrium dialysis. 8. 1,6-Anhydrogalactopyranose is neither a substrate (k(cat.)/K(m)< 3x10(-2)m(-1).S(-1)) nor an inhibitor (K(i)>0.2m). 9. Considerations of conformations available to the cationic inhibitor and to the anhydrogalactose indicate that the substrate is bound with the pyranose ring in a conformation not greatly different from the normal chair (C1) conformation.