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Calcium-Binding Protein Protocols Calcium-Binding Protein Protocols

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212 Clarke and Vogel<br />

Fig. 5. 113 Cd NMR spectra of carp parvalbumin titrated with the diamagnetic<br />

lanthanide Lu 3+ . Lu 3+ displaces Cd 2+ from the EF site of parvalbumin but not the CD<br />

site. The chemical shift of the peak for the CD site depends on the occupancy of the<br />

EF-hand with either Cd 2+ or Lu 3+ (slow exchange). This resonance shifts slightly<br />

because of interactions between the free Cd 2+ and a third weak metal-ion binding site<br />

on parvalbumin (fast exchange) (16).<br />

resonances by Ellis et al. (17). Sometimes paramagnetic metal ions or nitroxide<br />

probes can be used to aid in making assignments. Finally, important liganding<br />

residues in a binding site could also be mutated to prevent metal-ion binding,<br />

causing the disappearance of the corresponding signal.<br />

3.6. Special Considerations<br />

When using substitute metal ions such as Cd2+ or Pb2+ in lieu of the native<br />

ion Ca2+ , there is always a concern that the protein does not undergo the same<br />

conformational changes upon metal-ion binding. This can be studied by proton<br />

NMR titration experiments, for Cd2+ and Ca2+ , one usually obtains the same<br />

results (3), whereas Pb2+ is often somewhat different (8). Also, highly resolved

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