The addition of Tb3+ to apoalkaline phosphatase at pH 8.0 results in the formation of a metalloprotein with an enhanced Tb3+ fluorescence at 492, 545, and 580 nm. The Tb3+ excitation spectrum is most consistent with a process in which energy is transferred from one or more tyrosyl chromophores to the bound lanthanide. An analysis of the fluorescence data under equilibrium conditions yields one Tb3+ binding site per enzyme dimer with a Kn = 0.16 ± 0.02 μm. The Tb3+-alkaline phosphatase complex is not catalytically active nor does it incorporate covalently bound phosphate, but the specific activity of Zn2+-alkaline phosphatase is significantly enhanced in the presence of Tb3+ indicating that this lanthanide mimics Mg2+ in stabilizing the structure of alkaline phosphatase. The fluorescence of the Tb3+-enzyme is found to be quite sensitive to conformational changes which occur upon addition of Zn2+ or substrates.
ASJC Scopus subject areas
- Molecular Biology