Magnesium in the active site of Escherichia coli alkaline phosphatase is important for both structural stabilization and catalysis.
Site-specific mutagenesis was used to explore the roles of the side chains of residues Lys-328 and Asp-153 in Escherichia coli alkaline phosphatase. The D153H enzyme exhibits a 3.5-fold decrease in activity at pH 8.0 compared to that of the wild-type enzyme, while a double mutant D153H/K328H exhibits a 16-fold decrease in activity under these conditions. However, the Km values for both enzymes, employing the substrate p-nitrophenyl phosphate, are lower than the value for the wild-type enzyme. The Ki for phosphate, which is pH- and Mg(2+)-dependent, is decreased for the D153H enzyme and increased for the D153H/K328H enzyme. Relative to the wild-type enzyme, both mutant enzymes bind Mg2+ more weakly and undergo a time-dependent activation induced by Mg2+. The half-time of the activation process is independent of the Mg2+ concentration, indicating that the activation most probably involves a conformational change. The pH versus activity profiles of both enzymes are altered relative to that of the wild-type enzyme and exhibit greatly enhanced activity, relative to that of the wild-type enzyme, at high pH values. The pre-steady-state kinetics for the D153H and D153H/K328H enzymes exhibit a transient burst of product formation at pH 8.0, under conditions at which the wild-type enzyme exhibits no transient burst, indicating that at pH 8.0 the hydrolysis of the covalent enzyme-phosphate complex is rate-determining and not the release of phosphate from the noncovalent enzyme-phosphate complex as is observed for the wild-type enzyme. Therefore, these mutations are directly influencing catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)
Janeway
C M
CM
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02167.
Xu
X
X
Murphy
J E
JE
Chaidaroglou
A
A
Kantrowitz
E R
ER
D000469:Alkaline Phosphatase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D001224:Aspartic Acid; D001665:Binding Sites; D004795:Enzyme Stability; D004926:Escherichia coli / Q000201:enzymology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008239:Lysine; D008274:Magnesium / Q000032:analysis / Q000378:metabolism*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D013816:Thermodynamics
D011994:Recombinant Proteins; D001224:Aspartic Acid; D000469:Alkaline Phosphatase; D008274:Magnesium; D008239:Lysine