TABLE 2
Thermodynamic parameters for calcium binding estimated from isothermal titration of protein solution with CaCl2
For both forms of S100A1, in accordance with stoichiometry derived from Job plots, sequential binding of Ca2+ by two binding sites is followed by binding to another two cations by yet unidentified site(s).
| Site | n | K | ΔG | ΔH | ΔS |
|---|---|---|---|---|---|
| kcal/mol | kcal/mol | cal/mol/K | |||
| apo-S100A1 | |||||
| 1 | 1 | 2.4 ± 0.8 103 m−1 | −4.6 ± 0.1 | 2.9 ± 0.4 | 25.3 |
| 2 | 1 | 1.8 ± 0.2 103 m−1 | −4.5 ± 0.1 | −6.0 ± 1.5 | −5.1 |
| 3 | 2 | 6.0 ± 1.2 103 m−1 | −5.2 ± 0.2 | 6.5 ± 1.8 | 39.2 |
| apo-S100A1-NO | |||||
| 1 | 1 | 3.1 ± 0.5 107 m−2 | −10.3 ± 0.1 | 4.5 ± 0.3 | 49.6 |
| 2 | 1 | ||||
| 3 | 2 | 1.8 ± 0.3 103 m−1 | −4.5 ± 0.2 | 1.1 ± 0.2 | 18.7 |
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