BINDING OF CA2+ AND MG2+ TO HUMAN-SERUM ALBUMIN - CALORIMETRIC STUDY

The binding of calcium and magnesium to human serum albumin has been studied in the pH region 2.5-8.0 by a calorimetric procedure. Both metal ions bind to the carboxylate groups of albumin. 36 and 44 carboxylate groups appear to be involved in the binding of Ca2+ and Mg2+, respectively. Based on previously reported results that twelve Ca2+ ions are the maximum which can bind to albumin, the results given here support previous X-ray crystallographic evidence that three carboxylate groups can be involved in the binding of a Ca2+ by a protein. The data also confirm that Ca2+ and Mg2+ binding is competitive. Binding of the cations to the carboxylate groups appears to involve the breaking of carboxylate-imidazole hydrogen bonds in the protein. Log K, ΔH and ΔS values obtained for the binding of metal ions to albumin in aqueous solution at 25°C are 2.72 ± 0.02, 0.0 ± 0.1 kcal/mole, and 12.4 ± 0.3 cal/mole K for Ca2+ and 1.12 ± 0.05, -0.2 ± 0.1 kcal/mole, and 4.5 ± 0.3 cal/mole K for Mg2+, respectively. © 1978.