Thermodynamic analysis of biotin binding to avidin. A high sensitivity titration calorimetric study.

Titration calorimetric studies on the binding of biotin to avidin were performed in phosphate buffered saline, pH 7.4. From the temperature dependence of the binding enthalpy (delta H), the delta Cp value was determined. While the delta H value of -23.4 kcal/mol at 25 degrees C is in close agreement with the previously determined value of -22.5 kcal/mol (Suurkusk, J. & Wadso, I. (1972) Eur. J. Biochem. 28, 438-441), the delta Cp value of -461 cal/mol biotin/K is significantly at variance with the value of -237 cal/mol biotin/K obtained in the previous study. A comparison of the thermodynamic data obtained for the avidin-biotin system with that of the streptavidin-biotin system revealed that the higher binding affinity of avidin for biotin is due to a smaller (negative) entropy of binding.