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Spectroscopic studies on Tb3+ binding to S-100a protein
The Biochemical Journal
|June 15, 1987
Summary
The S-100a protein binds terbium (Tb3+) more tightly than calcium (Ca2+), with binding near tyrosine residues. Terbium binding induces conformational changes, suggesting its use as a calcium analogue in S-100a protein studies.
Area of Science:
- Biochemistry
- Biophysical Chemistry
- Protein Science
Background:
- S-100a protein is a calcium-binding protein involved in various cellular processes.
- Understanding metal ion interactions with S-100a protein is crucial for elucidating its function.
Purpose of the Study:
- To investigate the binding characteristics of terbium (Tb3+) to S-100a protein.
- To explore the conformational changes induced by Tb3+ binding.
- To assess the potential of Tb3+ as a calcium (Ca2+) analogue in spectral studies.
Main Methods:
- Direct binding assays
- Fluorescence spectroscopy (excitation and emission)
- UV-difference spectroscopy
- Circular dichroism (CD) spectroscopy (far-UV)
Main Results:
- S-100a protein binds 2 mol of Tb3+ per mole of protein at pH 6.6.
- Tb3+ binds more tightly to S-100a protein than Ca2+, with an upper limit Kd of 3.5 x 10(-6) M.
- Energy transfer from tyrosine residues indicates Tb3+ binding near tyrosine.
- Tb3+ binding induces conformational changes, decreasing helical content from ~39% to 35%.
- UV-difference spectroscopy reveals blue-shifting of tryptophan and tyrosine chromophores, suggesting solvent exposure.
Conclusions:
- Terbium (Tb3+) is a high-affinity binder to S-100a protein.
- Tb3+ binding induces significant conformational alterations in S-100a protein.
- The observed spectral and conformational changes are similar to those induced by Ca2+, validating Tb3+ as a useful Ca2+ analogue for spectral studies of S-100a protein.