Binding of metal ions to apoalkaline phosphatase from E. Coli: Effect of ionic radius

Binding of metal ions to E. Coli apoalkaline phosphatase causes 1) chromophoric changes in tyrosine absorption, 2) changes in enzymatic activity and 3) the release of protons from the enzyme. Investigation of these effects for a selection of metal ions from the Group 11A, Group 11B and transition series revealed that only those ions having crystal ionic radii in the range of 0.72 - 0.99A are able to produce changes in these three properties. The hydrated ionic radii, which are in the range of 4.0 - 4.5A for the ions examined do not correlate well with the ability of the ion to affect the three properties studied here. The size requirement therefore would seem not to apply to the initial binding step which undoubtedly involves the hydrated ion. Rather, the size requirement reflects the size range of a binding site generated by the folding of the protein around the metal ion with concomitant displacement of water molecules from the coordination sphere of the metal.