Protein Science, Vol 3, Issue 11 2005-2014, Copyright © 1994 by Cold Spring Harbor Laboratory Press

ARTICLE

Kinetics and crystal structure of a mutant Escherichia coli alkaline phosphatase (Asp-369 -> Asn): A mechanism involving one zinc per active site

T. T. TIBBITTS, X. XU and E. R. KANTROWITZ
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02167-3860

Using site-directed mutagenesis, an aspartate side chain involved in binding metal ions in the active site of Escherichia coli alkaline phosphatase (Asp-369) was replaced, alternately, by asparagine (D369N) and by alanine (D369A). The purified mutant enzymes showed reduced turnover rates (k(cat)) and increased Michaelis constants (K(m)). The k(cat) for the D369A enzyme was 5,000-fold lower than the value for the wild-type enzyme. The D369N enzyme required Zn(2+) in millimolar concentrations to become fully active; even under these conditions the k(cat) measured for hydrolysis of p-nitrophenol phosphate was 2 orders of magnitude lower than for the wild-type enzyme. Thus the k(cat)/K(m) ratios showed that catalysis is 50 times less efficient when the carboxylate side chain of Asp-369 is replaced by the corresponding amide; and activity is reduced to near nonenzymic levels when the carboxylate is replaced by a methyl group. The crystal structure of D369N, solved to 2.5 A resolution with an R-factor of 0.189, showed vacancies at 2 of the 3 metal binding sites. On the basis of the kinetic results and the refined X-ray coordinates, a reaction mechanism is proposed for phosphate ester hydrolysis by the D369N enzyme involving only 1 metal with the possible assistance of a histidine side chain.
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