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Calcium-Binding Protein Protocols Calcium-Binding Protein Protocols

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<strong>Calcium</strong>-43 NMR 221<br />

Fig. 1. The solid line shows the linewidth, ∆ν 1/2 for an I = 5/2 nucleus as a function<br />

of ω 0τ c. Note the striking similarity to a temperature dependence considering that a<br />

factor of 10 in ω 0τ c corresponds to approx 70°C. Redrawn from ref. 26.<br />

For a protein with high affinity for calcium, it is, in principle, possible to<br />

work under such conditions that all, or nearly all, Ca 2+ -ions are bound to<br />

the protein and then a 43 Ca NMR study can give direct information regarding<br />

the binding site(s). Determination of the relaxation rates, R 1 and R 2,<br />

will make the calculation of the correlation time, τ c, as well as the quadrupole<br />

coupling constant, χ, of the Ca 2+ -ion possible by using Eqs. 2 and 3.<br />

The quadrupole coupling constant contains information regarding the symmetry<br />

of the calcium-binding site and the obtained correlation time will<br />

indicate whether the calcium ion has any mobility inside the binding site on<br />

the ns time scale or faster. R 1 can be determined with the standard inversion<br />

recovery pulse sequence. This is exemplified in Fig. 2 for the TR 1C fragment<br />

of calmodulin (20) and R 2 is most readily obtained directly from the<br />

linewidth, R 2 = ∆ν 1/2/π. As aforementioned, it is better to determine R 2 also<br />

using pulse techniques, however, as far as I know that has not been done.<br />

Applied to calmodulin, troponin C, and parvalbumin, it was found that the<br />

correlation times obtained agreed well with the correlation time for the<br />

macromolecule itself, showing that there is no fast motion of the calcium<br />

ion inside the binding sites. The quadrupole coupling constant was similar<br />

for all three proteins, 1.1–1.3 MHz (2). For two α-lactalbumins and two<br />

lysozymes, it was also found that the correlation time agrees reasonably<br />

well with that expected for the whole protein (6). The quadrupole coupling<br />

constants were found to be significantly smaller than for the EF-hand proteins,<br />

0.7–0.8 MHz, showing that these sites are more symmetrical.

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