Mutations at histidine 412 alter zinc binding and eliminate transferase activity in Escherichia coli alkaline phosphatase.

His-412 in wild-type Escherichia coli alkaline phosphatase is a direct ligand to one of the two zinc atoms critical for the function of the enzyme. To investigate the function of this residue, site-specific mutagenesis was used to substitute His-412 with asparagine and alanine, generating mutant enzymes H412N and H412A, respectively. Both mutant enzymes show a 5-fold decrease in kcat and 30-fold increase in Km when compared to the corresponding kinetic parameters for the wild-type enzyme. In contrast to the wild-type enzyme, Tris and ethanolamine inhibit both the mutant enzymes by inhibiting the hydrolysis reaction and not participating in the transferase reaction; furthermore, both mutants have lower zinc and phosphate content than the wild-type enzyme. The addition of Zn2+ to the H412N and H412A enzymes restores catalytic activity to within 2-fold of the value for the wild-type enzyme, but more importantly the presence of Zn2+ completely restores substrate affinity. The similarity in the kinetic parameters for the H412N and H412A enzymes in the absence and presence of zinc suggests that the asparagine side chain does not play a significant role in coordinating zinc. Furthermore, both the asparagine and alanine substitutions reduce the affinity of the resulting enzymes for zinc. The pH profiles for the two mutant enzymes are different than the pH profile observed for the wild-type enzyme, suggesting that the amino acid substitutions may have altered the pKa of the zinc coordinated water molecule that is critical in the second step of the mechanism. These data suggest that His-412 does not directly participate in the catalytic mechanism but is mainly involved in zinc binding, and therefore is also indirectly involved in substrate binding and product release.