Structural Aspects of a Protein-surfactant Assembly: Native and Reduced States of Human Serum Albumin

The inherently present seventeen disulfide bonds of the circulatory protein, human serum albumin (HSA) provide the necessary structural stability. Various spectroscopic approaches were used to investigate the effect of reduction of these disulfide bonds and its binding with the anionic surfactant, sodium dodecyl sulfate (SDS). Based on several spectroscopic analyses, our investigations highlight the following interesting aspects: (1) HSA on reduction loses not only its tertiary structure but also a significant amount of secondary structure as well. However, the reduced state of the protein is not like the molten-globule, (2) this structural loss of the protein due to reduction is more prominent than that caused by higher SDS concentrations alone and can certainly be attributed to the role of disulfide bonds, (3) lower surfactant concentrations provide marginal structural rigidity to the native state of the protein, whereas, higher concentrations of SDS induces secondary structure to the reduced state of HSA, (4) the binding of SDS with both the native and reduced states of HSA, occurred in three distinct stages which was followed by a saturation stage. However, the nature of such binding is different for both the states as investigated by using the Stern-Volmer equations and estimating the thermodynamic parameters. Besides, in contrast to the native state, the reduced state of HSA shows that the lone tryptophan residue gets more buried. However, there occurs a sudden decrement in the lifetime of the tryptophan and the hydrodynamic diameter increases by twofold.