Physicochemical aspects of the energetics of binding of sulphanilic acid with bovine serum albumin

• The study of interaction of SA with BSA by calorimetric, light scattering and various spectroscopic techniques.
• Two sequential binding sites, having binding constants of the order of 104 and 103 M− 1, are present on BSA for SA.
• SA binds strongly in subdomain IIIA (site II) and weakly in subdomain IIA (site I) of BSA.
• The fluorescence intensity of protein increases as SA binds.
• The secondary structure of BSA is changed slightly upon SA binding.

The thermodynamic study of the binding of sulphanilic acid with model transport protein bovine serum albumin is a promising approach in the area of synthesizing new sulfa drugs with improved therapeutic effect. Thus, such binding studies play an important role in the rational drug design process. The binding between sulphanilic acid and bovine serum albumin has been studied using calorimetry, light scattering in combination with spectroscopic and microscopic techniques. The calorimetric data reveals the presence of two sequential nature of binding sites where the first binding site has stronger affinity (~ 104 M− 1) and second binding site has weaker affinity (~ 103 M− 1). However, the spectroscopic (absorption and fluorescence) results suggest the presence of single low affinity binding site (~ 103 M− 1) on protein. The contribution of polar and non-polar interactions to the binding process has been explored in the presence of various additives. It is found that sulphanilic acid binds with high affinity at Sudlow site II and with low affinity at Sudlow site I of protein. Light scattering and circular dichroism measurements have been used to study the effect on the molecular topology and conformation of protein, respectively. Thus these studies provide important insights into the binding of sulphanilic acid with bovine serum albumin both quantitatively and qualitatively.

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