Tryptophan phosphorescence as a monitor of the structural role of metal ions in alkaline phosphatase

The phosphorescence properties of Trp109 in alkaline phosphatase from Escherichia coli have been utilized to probe the conformation of the polypeptide following the removal of metal ions, reconstitution with Zn2+ and Cd2+ and phosphorylation. The complete removal of metal ions induces a drastic loosening of the protein structure that extends to the inner core of the macromolecule. While binding of a single metal ion/subunit (A‐site occupancy) restores the holoconformer, practically no structuring effect is observed upon B‐site occupancy by the second incoming metal ion. An exception to this rule occurs at alkaline pH and when the adjacent subunit in the dimer is metal‐free. Under these circumstances a conformation of the subunit more compact than that of the fully saturated dimer manifests some degree of communication across the subunit interface. The binding of more than two metal ions/monomer generally destabilizes the protein, the effect being more pronounced at acid pH. Finally, the binding of inorganic phosphate restores the native‐like configuration abolishing any destabilization induced by excess metal ions and acid pH. If the negative cooperativity towards metal binding to A sites in doubly metalated forms at pH 8 is in substantial agreement with 113Cd‐NMR data, the equivalence in conformation between Zn2+‐ and Cd2+‐reconstituted alkaline phosphatase emphasizes that no serious structural changes are introduced by the metal replacement.