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Calcium-Binding Protein Protocols Calcium-Binding Protein Protocols

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<strong>Calcium</strong>-43 NMR 225<br />

Fig. 4. Dependence of the 43 Ca NMR linewidth on pH at 23°C; (A) 0.98 mM<br />

phospholipae A 2 and 1.1 mM 43 Ca 2+ ; (B) 1.1 mM phospholipase A 2 and 11.5 mM<br />

43 Ca 2+ . The dashed curves are calculated using Eq. 6 and the solid curves are calculated<br />

taking two binding sites into account. Reproduced with permission from ref. 17.<br />

where<br />

[E] = [Etot]/(1 + K[Ca2+ ] + 10pKa-pH )<br />

and<br />

[Ca2+ ] = [Catot]/(1 + K[E])<br />

Equation 6 can also be extended to take more than one binding site or pKa into<br />

account (Fig. 4) (17).<br />

43Ca NMR has also been used to study the effect of drug binding on the<br />

calcium-binding properties of proteins (3,29,30). This is a clear example for<br />

the need of care in the interpretation, because the reduced line broadening on<br />

TFP addition was initially interpreted as a competition between Ca2+ and TFP<br />

(30). It was later shown that the reduced line broadening is a result of a reduced<br />

exchange rate between free and bound Ca2+ -ions (29). This was shown quite<br />

elegantly with the use of shift reagents, whereby the 43Ca NMR resonances<br />

from free and protein bound ions could be shifted apart, Fig. 5.

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