Binding of Calcium to Serum Albumin IV. Effect of Temperature and Thermodynamics of Calcium-Albumin Interaction

Abstract

Calcium binding in unbuffered serum albumin solutions at pH 7.4 and ionic strength 0.15–0.16 is temperature independent when binding results are analysed in terms of a simultaneous temperature effect on the competing calcium and hydrogen ions for imidazole groups. The energetic functions are: δ G°=–4.9 kcal/mole, δ H°=zero, and δ S°=16 cal/degree/mole. Calcium is proposed to be chelated by ionized carboxyl groups, with a critical role played by an imidazole group, whose charged or uncharged form either renders the site inaccessible to calcium ions or enhances the stability of the chelate.

Key Words:

• Binding site
• calcium ions
• chelate
• protein binding
• serum albumin
• temperature effect
• thermodynamic functions