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J. Biol. Chem., Vol. 262, Issue 14, 6539-6545, May, 1987
MD Gross, GL Nelsestuen and R Kumar
The binding of calcium and terbium to purified chick vitamin D- dependent
intestinal calcium-binding protein was studied by terbium fluorescence,
circular dichroism, and intrinsic protein fluorescence techniques.
Calcium-binding protein bound, with high affinity, at least 3 mol of
terbium/mol of protein; numerous low affinity terbium-binding sites were
also noted. The three highest affinity sites were resolved into one very
high affinity site (site A) and two other sites (sites B and C) with
slightly lower affinity. Resonance energy transfer from tryptophan residues
to terbium occurred only with site A. This site was filled before sites B
and C. Competition experiments in which calcium was used to displace
terbium bound to the protein showed that larger amounts of calcium were
needed to displace terbium from site A than from sites B and C. Energy
transfer from terbium to holmium indicated that the terbium-binding sites
(B and C) were located close to each other (about 7-12 A) but were distant
(greater than 12 A) from site A. The addition of EDTA to calcium-binding
protein resulted in a 25% decrease in intrinsic protein fluorescence,
suggesting a conformational change in the protein. The titration of
EDTA-treated calcium-binding protein with calcium resulted in recovery of
intrinsic protein fluorescence. A reversible calcium-dependent change in
the ellipticity of calcium-binding protein in circular dichroism
experiments was also seen. These observed properties suggest that vitamin
D-dependent chick intestinal calcium-binding protein behaves in a manner
similar to other well-known calcium-binding regulatory proteins.
Observations on the binding of lanthanides and calcium to vitamin D- dependent chick intestinal calcium-binding protein. Implications regarding calcium-binding protein function
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