Interaction of calcium ions with α-elastin: an equilibrium dialysis, circular dichroism, and microcalorimetric study

The interaction of Ca2+ with α-elastin, investigated by equil. dialysis, CD, and microcalorimetric techniques, was very poor in water. In trifluoroethanol, equil. dialysis expts. showed that Ca2+ binds to α-elastin with an assocn. const. of ∼250 M-1. Such a figure is not consistent with highly specific, highly selective binding. Also, the CD response is not directly proportional to the amt. of bound Ca2+ but depends on the protein concn. From microcalorimetric expts. it was found that the heat effect relative to the binding process is ∼1.9 kcal/g ion. From this figure and from the binding const., a pos. ΔS value of ∼17 entropy units was estd., suggesting that the binding process is entropy driven. From microcalorimetric measurements a ΔH of 1.5 kcal/residue was found for the Ca2+-induced conformational transition of the protein