Phosphate and sulphate-mediated structure and stability of bone morphogenetic protein - 2 (BMP - 2): A spectroscopy enabled investigation

Abstract

Impact of different monovalent and divalent cationic salts of sulphates and phosphates on conformation and stability of BMP - 2 was unraveled by absorbance, fluorescence and circular dichroism (CD) spectroscopy. Increase in absorbance of protein confirms the ground-state complexation between salt and BMP - 2. Phosphate salts, with the exception of sodium phosphate quenched the fluorescence intensity. The nature of quenching was static, as revealed by temperature-dependent fluorescence studies (Stern-Volmer constant (KSV) decreased with rise in temperature). Moreover, kq (bimolecular quenching constant) was in the range of 1012 M−1 s−1, confirming binding of phosphate salts with the protein. Contrary to this, sulphate salts increased the fluorescence intensity and excited-state lifetime of BMP - 2 (2.668 ns), with the maximum calculated for 300 mM sodium sulphate (3.216 ns). Phosphates reduced the lifetime of protein, with the least observed in presence of 300 mM magnesium phosphate (1.480 ns). Thermal stability of the protein (Tm = 70.66 °C) was altered significantly upon interaction with phosphate salts; however, it did not vary significantly in case of sulphates (exception - magnesium sulphate). Experimental evidences confirm the role played by anionic group on protein conformation and stability and identifies monovalent and divalent cations as insignificant contributor.